With love from the Service Department

The Imp Site

Any consequences of use or mis-use of these bits of information are all for yourself. In most cases I do not know at all what I am talking about (being not the least bit technical) and in any case I take no responsibility for anything on this page (or site for that matter) - Franka

Imp tips

Haute Couture Mechanics & Parts suppliers

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    Letters to the Technical editor. - Motor Racing, August 1965

Question & Answer


Q.     I wish to improve the circuit performance of my standard 1965 Hillman Imp without affecting its day-to-day reliability. At present, I can afford to take part in half a dozen sprints a year on the Mallory Park and Brands Hatch club circuits, and have to limit myself to a strict budget of £10 for any improvements. I have considered the following ideas:

  1. Remove air filter and convert the standard silencer to a straight-through type using a perforated tube and glass wool packing (10 shillings).
  2. it a Lucas sports coil and open up the plug gaps to 0.030 inch (41s 6d).
  3. Fit Stage 1 camshaft on exchange basis (£7 109).
  4. Fit non-fade brake linings all round (£5).
  5. Lower the suspension all round by fitting shortened standard springs on exchange, and adjust front wheel toe-in to suit (£2).
  6. Fit Singer Chamois wide-rim wheels at rear only (£5).

I should like your opinion as to which combination of these ideas would be the most suitable for my purpose. Is there anything that can be done to raise the performance of the standard carburettor? Would the fitting of harder plugs make any difference? Is there anything else that could be done within my £10 limit?
I am able to do fitting and adjusting myself, but have no access to any machining facilities.

F. C. Wykbs, Caldecote, Warwicks.

A.     I don't know what opposition you visualise, but I am afraid that a ten quid tune will not get you far! However, my comments on your numbered suggestions are as follows:

1 and 2. Only marginal improvement, if any is discernible at all.
3. No use without more comprehensive modifications to the induction system.
4. You will, of course, stop quicker, but normal linings are designed for additional things which you may lose, such as long life, smoothness, light application, silence, etc.
5. I have no opinions on these things, as only practical experience shows the value-and the snags. You'll have to ask another racer!
6. As for 5. The same applies to your query regarding tyres and pressures.

There is little you can do to improve the standard carburettor, assuming it is already in proper tune. Unless you are having plug trouble, the same applies to plugs.


General advice
• Make it a religion to check your oil, water and tyres before you leave home each day and a visual check of wiper blades, hoses and brake lines every time you wash the car.
• If it ain't fixed continuously, it's broke.   - unknown American
• Before working on Imp, switch on brain
• Braille: if you can reach a number in relief or imprinted with you fingertips, but you can't see it as it is behind something, use chewing gum or plasticene to press onto / into it and read the relief/ impression. (e.g. the mastercylinder behind the petrol tank)

To obtain spares or spares advice

Uprating an Imp's engine and handling, according to CCC in 1976
Suspension (the basics)



Smith, P.H.
The theory of tuning / P.H. Smith . - Foulis, 1960, 1972
ISBN 0-85429-150-4


separate file. (file also deals with: Cylinder heads; Head bolts; Head gasket (replacing a head gasket); Flywheel bolts; camshafts; Oils; Thermostat cover)

Active engine mount for a large amplitude of idling vibration

Return to Index

Fuel & Carburation


A worn carburettor wastes petrol.

Piping cold air to the carburettors would have a noticable effect.

For all fuel connections use thick walled tube, with an inside diameter such that it needs to be forced on the spigot. Use a hose clip at all connection points. Secure the tube at intervals.

If you buy a carburettion kit, find out how complete it is. Buying separate fittings may be expensive.

Air filter

Imp Sport air filters (part no. 5044639 / VFA 106) can be obtained from Ryco Filters, NZ or similar small filter companies in the UK. If they are given the old filters, they can strip and clean the metal chassis and replace the paper filter.

I use K&N cones. They are meant for a motorcycle, I think, and they are quite tall. The rear one fits only just (there is thick sound insulation against the engine bay wall above it). I like the look of them.
Every year when my Imp goes for its MoT, the original airbox is put on. (The elbows are the same.)

A K&N (gauze) airfilter for a Saab 99 is nearly the same as the paper filter of an Imp

According to Andy Dawson rule no. 1 for Imps is that the engine is fitted with a paper element air cleaner. "In Chrysler Comps we used to call the lack of an air cleaner an 'instant rebore kit'. Fifty miles on a dry day and the piston rings are as good as non-existent."

A Clan benefits even more from fitting K&N air filters to Strombergs than the Imp. Clan carbs don't get too much air, due to the fact that the standard Sport air box 'spout' is only one inch away from the ABS Clan engine lid, which has no cooling slats. With the K&Ns fitted, there is a noticeable difference - and you don't have any clearance problems either. - Grahame Pearson. - Impressions 1983, Dec.

'Filter socks' from K&N are a good option. They don't flow much more air, but they do keep that sand and dust out of the engine. They keep the insides of the carburettors clean.

Fitting pancake filters to an Imp is not recommended because of the position of the exhaust and the chance of drawing hot air in. Pancake filters get overhot in traffic and cause over rich fuel mixture due to very low air pressure. David Vizard wrote that he found that the standard air box gives more power than pancake filters.
Avoid the 'Speedograph' pancakes with a foam filling and the wire mesh surrounds. They don't protect. K&N pancakes are better quality, but may not be successful, because of the heat etc.
If you insist on using them, at least fit a heat shield fitted.
In a Husky engine bay there are fewer options as to the airfilters that can be used. An adaption of the Imp Sport air cleaner box to clear the rear deck of a Husky: Cut the top off the lid and weld a flat plate over the top; Drill holes at regular intervals around the lid to let air in.

Andy Dawson: silicone sealant
One of our readers, Guy Nelmes of Trampus Electronics in Windsor, tells me that another marine product, silicone sealant, can be used to prolong the life of spherical bearings and anything else that needs protecting from water or dust, presumably even air cleaner joints. The sealant comes in a tube and the parts to be sealed are covered with the sealant, which takes an hour to become a hard rubbery substance and protects the parts.

Fuel Pump

If either of the press-fitted inlet or outlet pipes works loose, it's a nuisance. It may be realistic to use the appropriate loctite or similar glue when tapping it back. Or to take the pump to an engineering shop and have the body threaded.
An AC threaded version does apparently exist.

If the rocker arm spring is not springy enough, and the arm is moving about on its shaft with loads of sideways play, then the pump may get noisy.

Throttle cable

Throttle linkage
If the throttle movement in relation to the pedal is too violent, the car won't be easy to drive in traffic. Plus the engine mounts and transmission will take a hammering.
If the relation is too slow, the car will feel sluggish, even if it is perfectly tuned. You may have to experiment with various lengths of throttle lever to arrive at the ideal result.

The throttle cable needs to bend 90° to activate the carb. I found a 90° curved tubing off a motorbike throttle twist grip proved invaluable - saves the cable from snapping.
M. Parris. - Impressions Oct 1986

There are throttle cables available made of an inner cable of stainless steel, and Teflon (PTFE) outer.
A stiff throttle operation is not economical.


The freeflow exhaust system, used on the Sport, does make a difference to the performance and is well worth fitting if you can get a Sport exhaust manifold. But the exhaust is about twice the price of a standard system.


If the car stalls for no apparent reason, and later on will start and run with no problem,

This pipe isn't just for overflow, it is to let the tank breathe.


Shelf life of unleaded fuel seems to be very short.
Leaded eventually goes off, but petrol that's a year old can still be used. Unleaded can be as short as a month when the components separate or break down and don't respond to a spark as readily.
You can buy anti-oxidant additive for chainsaws...

If you change over from leaded to unleaded or from unleaded to unleaded with an additive, the timing needs to be reset.

With their high compression engines and cast iron valve seats Imps are not suitable for lead-free fuel. Less lead in 4-star petrol will reduce the life expectancy of every Imp engine. Shell has stated that they do not intend to produce leaded petrol from the year 2000 on.
How to deal with this

Lead substitutes

The Federation of British Historic Vehicle Clubs (FBHVC) is now endorsing four Lead substitute products as being adequate for normal driving. (press release) Twelve out of the forty or so manufacturers that make such additives put forward their products for extensive testing by the Motor Industry Research Association (MIRA).

Each product completed a 70 hour test programme including a 20 hour period of accelerated wear testing at full throttle and full load. Tests were also conducted with normal leaded, unleaded and low-lead petrol for reference purposes.
The four products that passed testing are:

These products must be used at every re-fuelling and used strictly in accordance with the manufacturers instructions. They must not be mixed - once a product has been chosen stick with it.

Millers VSP:
Trade price £3.49 +VAT = £4.10 for a 250 ml bottle for 50 litres of petrol.
There are apparently two versions of the additive: the regular which gives the lead-substitute valve protection and raises the octane rating of regular unleaded from 91 to 97, and a competition version, which raises octane ratings maybe as high as 101.


During the year 2000 Leaded petrol will start to disappear and Lead Replacement Petrol (LRP) will become available in Australia. It will be phased in.
Leaded Super (96 RON) will be replaced with LRP (also 96 RON).
There will be no LRP with 91 RON or 95 RON (premium ULP).

Avgas will not be legally available to car owners (due to the reduction in taxes). At least one company (Shell, perhaps) will be selling exactly the same fuel as 'Racing Fuel' in 20 litre drums with the extra tax added to the price. 'Racing Fuel' will be rated at 100 RON and has been recognised by CAMS as 'pump fuel' and therefore usable in normal racing.

Petrol treatment 'engine formula', John Rhodes (who is on the Imps mailing list) is an agent.

Fuel pipe
Clear a blockage of the fuel line by blowing down it with a foot pump, or if that is insufficient, use a piano string.

Classical Gas (Instrumental) / Mason Williams. - 1968
354 k, RealPlayer file

Return to Index
Table 1: comparing some production engines
and their individual advance requirements
CarMax advance require
crank degs.
RPM Note
850 Mini
3500 highest advance requirements at 42° but at quite low RPM. This indicates
  • a poor combustion efficiency
  • a tendency to drop off volumetric efficiency
  • 1000 Mini
    490032° and at higher revs. With the same port sizes, valve lift and compression ratio it shows that just an increase in bore area will improve both volumetric and combustion efficiency by virtue of increased piston area and effect of unmasking the valves slightly, thus helping them to breathe better.
    1275 Mini GT
    4000The 1275 GT changes the 'A series' character altogether with a big increase in bore, higher compression ratio, big ports and valves. Because of these changes the volumetric and combustion efficiency has taken a sharp upward jump, but with combustion conditions improving slightly more than breathing conditions, so allowing the RPM for max advance to drop slightly.
    Maxi 1500
    6000 with just a change in stroke and a slight change in compression ratio (9:1 to 8.75:1) the Maxi needs no apparent change in its advance at all. So this would indicate that bore size is a controller of ignition advance
    Maxi 1750
    5200 the Imp and Imp Sport figures show the effect of changing from one carb to two, without improving the internals at all
    Imp Sport
    Avenger 1600
    4500comparing the Avengers shows the opposite requirement, when cam and valves are changed in addition to an extra carb
    Avenger GT
    Dolomite Sprint

    Ignition System

    "The ignition is always critical on these high-revving engines and we always recommend using the Lumenition Kit - we swear by that."
                -- Ray Payne, Hartwell

    Ignition timing for modified engines / Dave Andrews [mirror]

    Successful ignition relies on high voltage. Wire cored ignition cable helps a little in this way, but it is not essential. Suppressed lead in good condition can be adequate. The real key lies with a good coil and good contact breaker assembly. Well maintained four cylinder assemblies can be O.K. up to 9000 RPM.

    An Imp is sensitive to maladjusted ignition timing.
    The correct advance curve is as important as the right mixture or the optimum cam timing. It can be the key to tying up a complete tuning package and producing a smooth power curve, from what otherwise feels like a series of jumps and jerks until the rev counter is at 6000.

    If maximum combustion pressure occurs too late then effective available power is wasted
     too early then it is resisting the natural motion of the moving parts
    it will set up alarming stresses in pistons, con-rods and crankshaft.

    Object: produce maximum combustion pressure as the piston is starting to descend.
    The mixture of fuel and air does not explode when ignited, but micro-seconds later. So ignition advance is necessary to compensate for the 'burn time' between ignition and the point of maximum pressure build-up.

    The advance curve should be modified if the engine is modified, because the modification influences the 'burn time'. This burn time varies according to engine design characteristics - see table 1.

    Volumetric Efficiency is a measure of the engine's breathing power, compared to its cylinder volume. It is more difficult to achieve an efficiency as the RPM rise and the breathing time become shorter.

    The Dolomite and Dolomite Sprint figures probably show the nearest thing in production engine change to what happens when we start modifying for ultimate performance. The bore changes from 87 to 90 mm.; compression ratio from 9 to 9.5: 1, and two valves per cylinder to four valves per cylinder. Power jumps from 68 bhp at 5200 rpm to 95 at 5700 and torque from 105 lb/H at 3500 rpm to 122 at 4500. Only one more degree of, advance needed by the Sprint and at much lower RPM - why?

    Because the four valve layout is so efficient, giving good breathing over the whole rev range and also creating a quick burn due to the central spark plug in a compact combustion chamber. The object of good modification is to achieve performance improvements like these without going to the expense of fitting a four valve head but in the final analysis the four valver will always be on top as power RPM gets higher and higher.

        advance curve
    850 Mini40001042
    BLMC 1275 GT4500536
    Imp 87545001040
    Imp 998 1046
    Rover 3.5 V.830001032

    This table is baed on general tuning stage about ST.2. Individual engine requirements could be different but this will usually be a reasonable starting point.

    The stage comprises multiple carbs, 9.5-10 : 1 C.R. camshaft with more than 70° of overlap, oversize ports and valves, modified combustion chamber, multi-branch exhaust manifold.

    Crank degrees = distributor degrees x 2
    Crank RPM = distributor RPM x 2

     advance curve
    A = RPM below which no advance occurs and will always be 800-1000 RPM
    B = RPM over which total advance occurs
    C = Static advance setting
    D = total advance

    Advance curves usually look something like Diagram 1: a two step advance slope. The shape of this slope is controlled in several ways. The starting point is represented by a static advance setting of 10° and an RPM figure, under which no centrifugal advance occurs. Advancing starts when engine RPM increases and is governed by two springs of different rates, the lighter of which allows a fairly rapid advance, in this case from 10° to 25° between 800 and 2000 RPM and then the heavy spring asserts itself and a further 7° of advance is allowed over the next 1700 RPM from 2000 to 3700. At 32 total advance, the curve stays flat throughout the rest of the rev range. This typical curve might apply to an average 1600 cc OHV engine running on a single carb, with a CR of 8:1 and valve timing around 15-50 50-15, Let's say we increase the CR to 10:1 , re-work the combustion chamber, fit larger valves, polish the ports, fit a cam giving a timing of 30-60/60-30 and twin carbs with a decent exhaust manifold - what would happen to the advance curve?

    Due to the increased overlap, large valves and ports, the incoming gas is sluggish and its swirling capacity is poor - so the burn time tends to be long. But to offset this, the compression ratio is higher and the modified combustion chamber tends to help the burn. But overall more advance is wanted than standard and therefore the static setting is higher - generally about 25%-30% more than standard so our 10° static will shift to 12½ or 13°.
    As the engine starts to increase RPM so the mods really start to pile in the mixture. Even at relatively low RPM the charge weight necessitates additional advance over standard, until at the point where the volumetric efficiency of the standard engine starts falter (and the advance needs to start tapering off), the deep breathing equipment is really starting to work well. Thus a lot more advance is required, generally 25% more and also at an earlier point in the rev range.
    The actual amount of advance of course depends on the particular engine and its modifications, but usually the modified advance curve finishes up with a linear form as in diagram 1. This means that springs of different weights no longer have to be used. Quite often, a safe first step is to replace the heavy spring with an additional light one from the same distributor.
    The centrifugal advance stops will then have to be modified to increase the overall duration (see picture). Remember when carrying out this operation that distributor degrees are half crankshaft degrees, so very little metal removal is needed to achieve several degrees of correction.

    Ultimate timing figures should be checked using an ignition strobe and an accurate rev counter. Be suspicious if very small or very large amounts of advance are needed. It's telling you that something is wrong. Almost all engines should require between 28° and 42° total advance, unless working at very high RPM, i.e. consistently over 9000. Static figures should not ideally be more than 15° BTOC or difficult starting is likely to occur.

    Vacuum advance - what does it do? is it necessary?
    Due to the poor burning characteristics of production engines at idling RPM and just above the long burn time has to be compensated by excessive advance to produce a smooth idle. This is done with a vacuum operated diaphragm, which allows intake manifold depression at closed throttle to pull the contact mounting plate in the distributor against the direction of rotation thus advancing the ignition independently of the centrifugal advance mechanism. As the throttle is opened for acceleration so manifold depression drops and the vacuum advance falls back allowing control by the centrifugal weights.

    The vacuum advance diaphragm should be disconnected at any stage of tune above a mild stage one. It is undesirable to retain it for the following reasons:

    1. Imagine the condition where, with a nicely modified engine and a correctly modified ignition slope, you have accelerated to 70 mph and being a responsible citizen, you back the throttle off until cruising at 70 on part throttle. Because the throttle is partially closed, manifold depression can build up and start to draw over the vacuum advance diaphragm, thus starting to over advance the ignition, cause detonation, overheat valves and burn holes in pistons.
    2. The other reason is a mechanical one in that the loose rotating plate on which the contacts are mounted can tend to vibrate and promote contact bounce at high RPM, so it is best to rivet or screw this plate to the distributor base plate and remove the diaphragm housing altogether.

    So much for advance curve importance, table 2 is laid out to assist in choice of correct slope, but it can only be a generalisation on the basis that in theory even a jet change should involve an ignition advance correction.


    Separate file: tech\engine\ignition\distributor

    Ion-gap sensing for engine control / J. Auzins; H. Johansson; J. Nyomt. - Automotive Engineering (US) vol.103 (1993) DV31293, no.9 (Sep), pp.65-68. (Combustion will produce ions.
    If a spark-plug can detect these (ion-gap sensing) and use them (the currents of 'em) the information can be used for engine management - misfire detection, knock and pre-ignition detection)

    The Electronic ignition was then retro-fitted by the dealers.
    Some of the later 930 Sunbeams used Bosch as part of dual sourcing. Bosch only supplied for Hall Effect type ignitions on Linwood built Avenger/Sunbeams.
    Hall Effect ignitions were introduced about 18 months into production to replace the ballast type electronic ignition, which had proved a major cause of roadside breakdowns (due to ballast resistors burning out). Hall effect cars tended to give a lot of trouble with poor starting due to bad coils (mainly made in Spain by Femsa). Fitting a Lucas coil seemed to cure it.

    Nick Cleak has recently been using the 'Emerald Cams' M3d system, which is fully programmable with a Personal Computer. This ignition system will also run programmable fuel injection.
    Basicly that means you can set any degree of advance or retard at any rpm in 500 rpm steps, at 8 different throttle positions. It is has temperature compensation too, so it will, say advance 5° if the coolant temp goes over 100°C to cool the combustion process.
    It is not cheap, but when it does the fuel as well, it does become quite economic. (He got gains of 12% in fuel consumption on his Clan with the ignition system alone - over a comp distributor which has no vacuum advance).
    The system is triggered by a fixed electronic sensor in the distributor, either the hall effect type or the inductive type as on the Sunbeam 930 as standard. The Sunbeam type will also trigger most of the electronic ignition modules on newer cars.

    The Lucas M35G was much better than the later M35J!
    The M35J has aluminium stator windings, and a thin pressed-steel commutator-end plate. Less torque, more current and the endplate doesn't conduct heat away from the bronze bearing, so these chop out, leading to (at first) a funny noise when the starter disengages (a sort of whee-ow sound) - because the shaft precesses around in the worn bush. The M35G (at least the ones up to about 1965 anyway) have all-copper windings, and a die-cast endplate at the commutator end. So the bearing-bush lasts more or less forever.
    Pre '65 Rootes mobiles used the M35G.

    Only one advantage for the M35J - it's much easier to reassemble, with its faceplate commutator - no need to wrestle with the brushes!

    Spark plugs

    Rewarding for careful attention to detail. The spark which is one of the most vita1 functions in the engine, is the culmination of a series of carefully calculated events starting with the battery and finishing at the plug. The correct or incorrect operation of any of these events can mean the difference of plus or minus five horsepower in every fifty hp.

    Combustion chamber temperatures greatly affect spark plug efficiency. These temperatures are influenced by air/fuel ratios, exhaust back pressure, ignition advance and compression ratio. Plugs should run between 400°C (750°F) and 955°C (1750°F), colder than this and they will foul up; hotter and they'll overheat and cause pre-ignition.

    Best way to check for correct plug heat is to select a long hill that will hold your speed to 70 mph at full throttle. Hold it for as long as possible and then flick it into neutral and switch off ignition (mind you don't lock the steering!) Remove plugs and check visually: the side electrode should be slightly blue and the insulator tip should be milk chocolate colour.

    It should be emphasised that mixture has nothing to do with plug selection. If the mixture is right, combustion should be clean and plug temperature will depend on compression and spark advance. Air/fuel ratios should be chosen by specific fuel consumption figures - don't try and correct a plug that is running hot by richening the mixture or weaken off for one that's running cold.

    The spark intensity depends on good sharp edged electrodes and gaps as small as can be reasonably used. Average gaps should be reduced to .020", but racing plugs can be as small as .016".

    Do you ever see a pale green deposit on your spark plugs? This can occure if the flame within the cylinder is slightly too hot. I've seen it many times but I could never work out the cause. This could be used as an early warning of cylinder overheating. You will still need to find the exact cause but it may prevent long term perminant damage. To allow the deposit to form you will need to read the spark plug when the engine is hot and just after a good run. Please respond stating under what conditions this green deposit occures and the basic engine specification
    Please send your comments to Gary Harding

    Speedy Spares sell a kit with an electronic ignition module of the points-assited type.

    Be cautious about guessing spark plug suitability for your Imp. Even when they may appear to be working fine, over a period of time you may be damaging the pistons and valves if the plugs are the wrong heat rating for the engine.

    A full page advert in Autocar, 8 May 1964. - p. 898:
    KLG Spark Plugs - chosen & fitted exclusively to the Hillman Imp.
    KLG - chosen by Rootes for all their cars, after extensive and exhaustive tests. Fitted to Humber, Hillman, Sunbeam, Singer and now, exclusive initial equipment on the Hillman Imp. KLG the all British Spark Plug - chosen for top performance under all conditions.
    Rootes choose KLG
    S. Smith & son (England) Ltd., Spark Plug & Ceramics division. St. Peters Road, Rugby, Warwickshire.

    See the spark plug comparison charts in the Champion guide.

    HT leads
    Copper leads with suppressors in the caps
    An HT lead gone to high-resistance (50 Ohm or so) will cause glassy growths in plug gaps. Correct HT leads will give enough energy to the spark, that these deposits will be blown away before they can build up to this level.

    An immobilizer can save pounds on your insurance premium. An article on the What & How by Ian Reeves: Impressions February 1995. - p. 21.


    Spark Plug Type  
    AC7C cold  
    AC9C standard  
    AC12C hot  
    AirC9C iridium  


    Return to Index

    Clutch and Actuating Mechanism

    For racing purposes the standard MK2 clutch was adequate although heavy duty and multi-plate racing clutches were also available. The clutch flexible hose was a major weakness sometimes being damaged in engine removal (when it supports the slave cylinder) and usually failed by allowing air to be drawn into the system. This was normally replaced by a standard Imp brake flexible hose as those were much more reliable.

    Rootes manual:
    5.5" diaphragm fitted to Mk1 up to B.429
    6.25" diaphragm ever after

    All lever-type are 'conversions' due mainly to the Morris thou 6.25" being compatible and available.

    Laycock units give a light pedal. Borg and Beck item are similarly light.

    The master cylinder should be the .325" type.

    There are roller type clutch release bearings available to replace the carbon ones. They are a step up.

    Although the carbon clutch release bearings may seem fragile, they are what is used in heavy duty industrial machinery, too.

    Mailing list items: clutch release bearing system

    Return to Index

    Rear Hubs, Drive Shafts & Universal Joints

    Rotoflex couplings

    There are two types of doughnuts.

    If a coupling has been removed, refit the coupling in exactly the same position, i.e. the same bolt for the same hole.

    When refitting driveshaft doughnuts:
    Rotate a drive flange into position where a bolt can be inserted through the flange and the doughnut (this is usually near the bottom of the circle of rotation). After just nipping up the nut, the flange is rotated until the next bolt can be fitted.
    When all the bolts are in place they can be torqued up. Torque the bolts to the recommended settings. It is the difference in angle between the drive flanges when they are rotated that seems to pull the rubber doughnut into the correct position to enable the bolt to be fitted.
    Make sure that, when fitting the bolts, thay are inserted from the right direction. (see diagram in your workshop manual). Or the next time you want to disconnect your driveshaft, you may find you can't get the bolt out, because the transaxle is in the way.

      two driveshaft assemblies
    Two Elan rear drive-shaft assemblies: from an Elan two-seater and from an Elan 2+2
      Metaflex coupling
    Lotus Pt.nr. CO50D0034Z: Rubber drive coupling with the holes formed from two curved v-shaped pieces placed face-to-face. This revised doughnut is made by Metaflex (owned by Dunlop).

    Early Lotus Elan S1 couplings are identical to Imp ones.

    Universal joint

    Trouble has been experienced with the outer Universal Joints breaking up following rust. The best answer is to install replacements with a grease nipple incorporated - ones with a long nipple that can be placed when greasing is a good idea.
    The UJs from a Volvo Amazone or 140 will fit an Imp and are of better quality - Swedish manufacturer.

    Return to Index

    Transmission Unit

    Bill West and his assistant John Lewis (aged 31 at May 1963) were responsible for the transaxle design.

    The Imp gearbox

    Repairing Imp gearboxes / by Bob Hoare. - Impressions 1986, February (2 1/2 page)

    Oil; gearbox
    Castrol Syntrax 25W90 apparently works fine.

    If there is a leak at the rear of the transaxle, where the selector rod from the gear lever enters the case:
    There is an oil seal in the transaxle end-plate around the gear-change shaft. This can be renewed after the end-plate has been removed. Put the weight of the transaxle on a jack before you remove the plate. Uncouple the flexible joint in the gearchange rod.
    In bad cases, the gearshift hole itself is worn to an oval shape. Then you will need a new end-plate.
    There is no gasket between the plate and the casing. Use some non-hardening joining goo.

    The Imp transaxle: part 1 / Dave Weedon. - Impressions 1990 Spring, pp. 37-39
    An alternative for filling Imp transaxles (in stead of using the bottles that EP80 comes in) can be found in Impressions December 1995, page 31.
    Castrol Syntrax is a modern synthetic oil which has all the properties of the thicker EP80, but it is thinner and therefore reduces friction. It makes the gear change easier too. If you have difficulty in getting into 1st gear when the gearbox oil is still cold, you may like to try it.


    The gear lever movement on an Imp should be short and precise. If it isn't (due to wear) than the cure is very cheap and not terribly time consuming. It will include replacing three bushes and a spring in the linkage. (If you can lift your lever up and down, it needs attention).

    Gear balls
    The domed rubber half ball which fits at the base of the gear lever can be replaced for a hard nylon lathe-turned Captain Scarlet version. (indestructable)

    There is a difference between the right and the left gearbox support. Switching them will place the engine a bit too far forward, which can lead to problems with the gear lever. (And as the rotoflexes won't be in line with the wheel centre, they'll probably wear out sooner).

    Jack Knight gearbox

    The other major advantage was the gearbox, the Rootes/Chrysler Competition department Jack Knight of Woking produced a wide range of gearbox conversions from the 'works' semi-close ratio gear set to full racing non-synchro mesh 4 and 5 speed boxes.
    These Jack Knight Imp boxes were similar to Hewland racing gearboxes with easily changeable ratios. In the JK gearbox the bronze bush bearings were replaced with needle roller bearings allowing the use of a higher viscosity gear oil.
    Correct choice of gear ratios was essential because of the narrow power band (7000 to 9600rpm) of a full race 998cc unit.
    It was also essential that the rear drive shafts were replaced by Sport components and the rubber couplings replaced by Lotus Elan parts. The rear hubs were fitted to the splines on the outer drive shafts using 'Green Loctite' and the hub nuts properly tightened, otherwise fretting would occur between the two parts resulting in a stripped spline.

    Jack Knight also made a Pawl type LSD which was not really essential to improve the traction but did improve the handling on tight bumpy corners were the rubber drive couplings would wind up and release and cause rather strange handling.



    From an SKF bearing manual, 1973, which listed the Imp
    Front wheel inner
    Front wheel outer
    Drive shaft front
    Drive shaft rear
    Main shaft or pinion front
    Main shaft or pinion rear
    Differential right
    Differential left
    Rear wheel inner
    Rear wheel outer
        K-L 44649 / K-M 44610
    K-LM 11949 / K-LM 11910
    6204 / C3
    410425+BMN. BRH 162112
    K-21075 / K-21212
    K-LM 48548 / K-LM 48510
    K-LM 48548 / K-LM 48510
    K-LM 67047 / K-LM 67010
    6006 / 31

    In a race or sprint car or even a road car that sees uncommon sporty use, the standard differential can shear the cross pin or snap off at the root of the bearing carrier. This happens more often with non-tarmac competition Imps.
    A limited slip differential won't do this. Another (cheaper) prevention method is to heat-treat, polish and tuftride the carrier and pin.

    For corners that you would take in 2nd gear (where the rotoflex couplings tend to wind up and snatch), a limited slip differential would improve handling.
    Types of limited slip differential:
    JK pawl

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    Braking System

    Separate file: brakes/brakes.html

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    Bodywork & Interior

    For exterior tips and parts, see: Exterior


    Don't try to fit a Rally Imp dash (CTS 1502) to a Mk I Imp. It involves more drilling and cutting than you think. Fitting one to a later model is easier.

    Madadash used to make an aftermarket dashboard for the Imp, called the Impanel. A full-width walnut dash panel, made from English Walnut and finished in high gloss, with a scratch proof finish. It has two pull down locker lids. No necessity to move any switches or instruments. £7,2,6 (unknown if there are polished walnut door facias to go with it)
    Chrysler Comps./ Talbot Special Tuning did a special dash until well in the 80s.
    Imp Rootes Accessories obtainable from Rootes dealers included an Instrument Panel which fits under the heater control and consists of a tach, ammeter and oil pressure gauge.

    For a stock 1964 Sunbeam Imp Deluxe, you could choose a small Smiths tachometer, as fitted to early Avenger GT (1250 and 1500).
    It is said that M*** Cooper speedos and tachometers fit the Imp Rallye Dash.

    Steering wheel

    Andy Dawson recommends a 14" steering wheel. It will take less strength to park the car. But the real benefit is over bumpy roads and on long journeys where it is more comfortable and doesn't jarr the hands over road irregularities.


    Front interior coupe
    interior in the back of a coupe
    passenger seat relaxed
    back seat folded down
    back seats folded down


        May 1963 - ref. R45571
      Look at the two-tone seats. The door cover looks two-tone, too. And the inside dash is painted a very light colour, while the outside is maybe red or so... (No, this isn't the Lord Imp)

    Two-tone seats appearantly existed, as shown in this press photo by Clayton R45571. In handwriting on the back: Hillman Imp - May 1963

    Vinyl seats fade and go brittle with sunlight.

    Motor shops do sell paints especially for refinishing vinyls

    Possible swaps?

    DAF 66 Marathon or Volvo 66 seats fit straight on

    Mk2 Fiesta seats fit with hardly any effort. The backs fold forward, which is handy.

    Peugeot 405 seats are very similar size. Perhaps they are made by the same people as the Imp seats?

    Seat belts
    Inertia-reel seat belts. How to. Impressions December 1995, p.32. And another how-to for Huskies by D. Couldry. - Impressions 8 (1988), no. 4.

    The Imp's seat belt arrangement was updated by my father in 1974 see '07/74 seat belt adjustment' in the patents section. Brian's was not a model specific role, his engineering went across the board in this instance as part of his interest in raising safety standards.

    Three point harnesses
    A three point harness going back to the rear wheelarch tends to pull you over to one side, a four point harness is probably more comfortable.

    Four point harnesses
    Three mounting points
    Without a rear seat: Full (4 strap) saloon harness are mounted at three points with the 3rd mounting hole drilled into the load platfom (or rear seat area). An accident restraint loads on the mountings must be carried by the sheet metal in shear (as far as is possible). Normally this means using the eyebolt provided with the belts straight through the load platform with large plates sandwiching the sheet steel.
    Do not mount the rear strap to the original side mounting as this would pull unevenly.

    Four mounting points
    While you can use a four point harness by crossing over the two top straps and securing them to the lower points, this is not a really good idea. Adding the shoulder straps to to the original floor mounts (that already hold the lower harness straps) would double the load on these points. Also it would be very hard to tension correctly a belt which performed such a U-turn.
    RAC competition regulations, the Blue Book: 'The fixation points to the rear should be positioned so that the strap from the shoulder is as near horizontal as possible. It should not be located on the floor directly behind the driver/co-driver' It also says that if there is one rear mounting it should be 'symetrical for the two shoulder straps'.

    Damage repair

    When an Imp rolls, they tend to twist.

    Straightening out a bent Imp that got frontal impact damage isn't easy. I did it with mine. It cost me lots and I'm not entirely satisfied with it. It did pass the thorough RDW test it had to go through, before I could ask the papers back (it had been totalled) and put it back on the road.

    If you are really attached to the car, the best and safest method of keeping the spirit of the car alive is to re-shell it (99.9999% of competition Imps have been reshelled at least once in their careers). Unfortunately, the only shells that are available now are s/hand ones and that is something that the DVLA officially don't like. However a fair few Imps have been reshelled, and it can be done legally and safely.


    Avenger armrests


    Badges, lettering, emblems

    Door hinge screws in wheel arch
    Stainless steel ones can be had

    Wheels & Tyres


    separate file


    separate file

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    Top plates
    A how-to by G.Harden (Impressions 1988, no.5) describes how repair a damaged top plate (1st sign: holes above the petrol tank lip just above the top-plate bolt heads) and how to improve on the design.

    Heater hose
    Heater hoses on an Imp are 5/8" internal diameter rubber hose - an unusual size for repair using 15mm copper pipe. However if you solder a 15mm Yorkshire fitting on the end of the pipe, the solder rings on the fitting make the size up exactly. In fact, I am sure the elbows on the heater unit are basically Yorkshire fittings with rings on, too.
    says Nigel Frosdick in Impressions December 1997

    Check condition of heater hoses and renew any doubtful looking ones, before they start to give trouble. -service item
    Here's the How-to by Peter Nunney.

    To thaw the washing liquid in your washer system in winter, have the tubing winded around the heater hose. (mentioned by G. Pearson)
    Put the windscreen wash reservoir bag up against the passenger bulkhead (towards the offside of the heater). The bag will rest on the heater inlet hose and be supported by hooks or stiff wire from the cardboard retaining clips. In this position the bag won't freeze as easily and will thaw when the engine runs.

    Wiper spindle rubbers
    The wiper spindle rubbers are easily fixed. Remove the wiper and undo the retaining nut. The remains of the rubber can then be removed.
    Obtain some wiper spindle seals from a Volvo 340, the earlier the better, as these have a chrome seal... much nicer and a fit-and-forget part.

    Imp wiper blades were the same as for Humber Super Snipe or Hawk 1959-62.

    DL3W or a 15W
    The 15W is the one with the big cylindrical motor casing and permanent magnets. The DL3W has a wound field, and a peculiar-shaped housing and, if not used for a while, no sign of magnetism.
    The difference? The 15W needs 2-way switching to prevent it over-running the park switch. The DL3W only needs simple on-off switching.

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