Imp Site logo - imps4ever!
  Type Overhead camshaft
inclined 45°
  Stroke 2.377 in. (60,37 mm)
  Bore 2.6770 in. (68,00 mm)
  Capacity   875 cc (53.4 cubic inches)
  Compression ratio
   normal compression  
   low compression
10 : 1
8 : 1   (9 : 1 ??)
   standard Imp  
   standard, low compr.
   Imp Sport
39 @ 5000 rpm
37 @ 4900 rmp
51 @ 61000 rpm

The Imp Site

The Imp Engine

Die-cast at Linwood, machined and assembled at the Stoke plant.

The 875cc Hillman Imp, introduced in 1963, has a four cylinder all aluminium engine of very light weight. The whole engine and transmission weighs 176 lb. (around half of what it would weigh in cast iron).

drawing of Hillman Imp engine BW
artist: 'Bennett'; RL 796 /C

    The engine incl. fittings
Transaxle with oil
Water Pump Assembly
Exhaust Manifold
Starter Motor
75 kg
30 kg
14 kg
7 kg
10 kg
7 kg
10 kg
3 kg
6 kg
4 kg

The block and head are both of alloy (75% Al; 23% Si; 1% Cu; 1% Mn). Overhead valves are driven by a three-bearing overhead and an advanced combution chamber shape allows. Imps have beautifully engineered, highly stressed engines. They need care.

The 875 cc Chrysler engine is, to a degree, a development of the single ohc. all aluminium, Coventry Climax engine which at 1220 cc. was somewhat larger than the Chrysler version. By working on and developing an already proven design, the Rootes engineers (now Chrysler) were able to produce a tough, reliable engine. While being adapted for mass production use, the engine lost little of its original Coventry Climax character - the only thing it seems to have lost which is of any consequence is the extra capacity which the Climax unit had over the Chrysler. The engineers working on the Imp project in the early days needed, if the Imp was to be a rear-engined car, a very light engine. This resulted in an abundant use of aluminium alloys on the engine. In fact all major parts are aluminium, and this has resulted in a very light engine.

The engine was first introduced as a motive power for the Imp in May 1963. In its original concept, it had a single carb and it produced a power output of 39 bhp net at 5000 rpm. In 1965 a Mk2 version was introduced to alleviate some of the shortcomings and problems found in the Mk1. The Mk2 benefited from larger valves and ports plus - and this is very important - a much stiffer block and head casting. About a year afterwards a Sport engine was introduced. By using larger inlet valves in the head, a hotter cam, a tubular exhaust manifold, together with twin carbs, the Imp Sport engine power output was brought up to 51 bhp net at 6100 rpm.
Towards the end of 1965, the factory offered a 998 cc version of this engine as a conversion for 875 units. The extra capacity was achieved by using a larger bore although at one time Chrysler considered increasing the capacity to 1000 cc by using a longer stroke. This, however, was decided against.
Since 1965/66 these engines have changed little, in fact only detail changes have been made since this date.


The rear-mounted engine meant an evenly distributed weight. The consequences were very good road-holding and (compared to an engine in the front) superior climbing and safer braking.

Engine; 875cc

  Hillman Imp single OHC engine
       MaterialAluminium alloy
with cast in iron liners
   Block height (sump face to deck)   10.309 in./ 10.313 in.
  Max standard oversize rebore+0.030 in.

The standard 875 cc block is an aluminium casting with ribbed liners cast in position. Unlike many blocks, the Imp block is not split at the main bearing centre line, but is of the deep skirt configuration, where the sump flange of the block drops well below the crankshaft centreline. The block has three main bearings and the caps, which are themselves aluminium, are secured to the block hy high tensile steel bolts. Early blocks suffered from lack of stiffness, and were not ideal for tuning. Later blocks, from 1965 on, were much stiffer, and could be used for competition.

There are differences to the Mark 1 and the Mark 2 engine blocks. On Mk1 (or curly edge) blocks there is room for 'movement' due to the top deck being a little flimsy. This can lead to gasket failure.
On later blocks (and heads) the castings were thicker and generally the quality was better. For tuning, use the Mk2 engine. The largest size valve that can safely be fitted into the Mk1 head are those fitted standard to the Mk2.
Retooling for the straight edge block must have been very expensive, and Rootes would not have spend money like that without reason. The Mk1 block assembly is too thin in all the wrong places to convert to 1000cc.

Van / Husky

According to the manual, you can't remove the engine without the gearbox. But you can by raising the back of the car and lovering the engine. Lift the back of the car slightly and support the gearbox.


    Type Solid skirt
Lo-ex aluminium alloy
  Valve pockets
   normal compression  
   low compression
10 : 1
8 : 1 (?)
  Length 2.054 in.
  Compression height standard Imp: 1.050 / 1.055 in.
Imp Sport: 1.055 / 1.060 in.
  No. of rings
Ring gap in bore
0.013/0.008 (all three)
  Piston skirt clearance
   (measured at right angles  
   to gudgeon pin hole)
0.0011 in./ 0.0017 in.
  Gudgeon pin dia 0.6248/ 0.6250 in.
(graded to piston)
  Gudgeon pin type Fully floating
  Mode of retention Circlip in piston

Pistons are cast in LO-EX aluminium alloy and have two compression rings and one oil scraper ring. The piston runs in a nominal bore size of 2.6770 in. dia and the piston-to-bore clearance, measured at the skirt is, depending on tolerances, between 0.0017 in and 0.0010 in. These measurements are taken at right angles to the gudgeon pin hole.

On all except very, very early engines, of which only a few were built, the pistons have two cutouts machined in the top to provide clearance for the valves. Two compression ratios are available as far as the pistons are concerned. The one for use in the cars gives 10 : 1 cr, whilst the low compression for commercial vehicles such as the Imp van give 8 : 1 cr.

Pistons come in 2 types: small pin and large pin. Low or higher cost depends on what quality you are looking for.

Reboring of an 875cc engine up to 30 thou oversize is considered to be the maximum. But many people do go to 40 thou.
60 thou pistons can be obtained.
Talbot only did (do?) 15 and 30 thou pistons.
JP Pistons are manufactured in Australia.


    Type Steel forging with counter weights
    Main bearing type and no. 3 steel-backed lead bronze/ lead indium overlay
    Main bearing journals dia 1.875 in.
    Big end journals dia 1.625 in.
    End thrust taken on : Centre main bearing
    End float : 0.003/ 0.010 in.
    Main bearing clearance nominal 0.0018 in.
    Big end bearing clearance nominal   0.0018 in.

of forged steel and fully counterweighted,
capable of revving to 10,000 - making it ideal for tuned engines

The crankshaft is a counter-weighted steel forging with three main bearings. The main bearings are of a bronze lead indium overlay type and end thrust is taken up on the centre main bearing, the end float being between 0.001 and 0.002 in. The main bearing journal diameters are 1.875 in. and the big end journals are 1.625 in. Bearing clearance for both big ends and mains should be between 0.001 in and 0ยท0027 in. The stroke of the standard crankshaft is 2.377in (60.37mm). Compared with some crankshafts, the Imp crankshaft can look a little frail. Appearances do however, belie its abilities. When suitably prepared, the Imp crankshaft is capable of being run to about 10,000rpm and staying in one piece.

Connecting rods

    Material Steel forging
  Big end bearings Steel-backed lead bronze/lead indium overlay
  Big end housing dia 1.7705 in/ 1.7710 in.
  Big end, side float 0.005 in/ 0.009 in.
  Small end type
(with bearing fitted if applicable)  
Bronze bush
   for fully floating gudgeon pin
  Small end bore
(with bearing fitted if applicable)
0.6250 in./ 0.6252 in.
  Centre distance 4.124 in.
4.126 in.

short, sturdy and reliable

Imp connecting rods are of a stiff H-section steel forging and they can, by any standards, be considered to be very short - even for an engine of this size. The bronze/lead indium overlay type big ends fitted as standard are housed in a bore in the rod of 1.771Oin/ 1.7705 in. dia.
A 0.625 in. dia gudgeon pin runs in the bronze bush of the little end with a clearance of between 0.0001 in. and 0.0002 in. This sort of fit means that the gudgeon pin is a slide fit in the little end, but has no apparent slop.

For racing purposes the engine was much easier to modify than the Mini's as most of the major components could be used even in a full race engine. Little Ford and BMC engines could not be tuned to racing specification without a change of crankshaft and rods.
The block was exchanged for a wet liner 998cc block, the crankshaft and conrods were balanced and (optionally) 'Tuftrided'. The oil pump replaced by a competion department one with bronze drive gears. Upgrade oil pumps which were converted to use Hillman Hunter 1725 pump rotors were also sometimes used.
To improve oil return from the upper reaches the cylinder block and head were modified to accept a sport type external oil drain and the inlet valve oil seal discarded (to accomodated high lift cams).

The heads were also machined to accept 'Wills Rings' hollow internally pressised seals which replaced the cylinder head gasket.

  Wills rings are hollow rings that fit into machine grooves in the cylinder head around each combustion chamber. They are gas filled so when the temperature increases the gas expands, increasing the sealing pressure when a conventional head gasket would be giving up the ghost.

The works specification cylinder head used larger inlet (1.4") and exhaust valves combined with a 0.360" lift camshaft using a converted sport cam carrier. This type of engine could produce about 115bhp.

Late in the 1970's the late Andy Chessman developed heads which used smaller inlet valves (1.375") and a higher lift camshaft (0.420") which gave a better spread of torque and more power (123 bhp). These camshafts required very special cam carriers with bronze liners.

Another optional modification was the use of a a girdle plate between the sump and cylinder block to support the centre main bearing and reduce flexing. The stiffening plates used by some racing Imps between the bock and sump are much like the alloy crankcase ladders that are now popular to use for more stiffness. If it can be avoided then avoid putting high tensile (pull out) loads unto weak aluminium blocks.



I still need to convert these to MP3...

  • The sound (19sec., wav, 427kB) of a Sunbeam Imp Sport (JYX 484K) on some snow (1991) - send in by Dave Hutchy, Tue, 3 Feb 1998

  • The sound (10sec., wav, 116kB) of an Imp - send to Onelist by Murray Mitchell, Wed, 5 Jan 2000

Engines; 998cc

Early 998 cc blocks from the factory were of the dry liner type, i.e. they had liners fitted in a similar manner to the standard blocks. This block in 998 cc form was not too reliable and led to the development of a wet liner block. These liners should be fitted only to the later, stiffer, blocks.

In earlier years you could buy an 998cc performance engine, built to Imp Sport specification but with 998 block and pistons; complete with clutch, distributor, plugs, manifolds and carburettor. Power output: 65bhp @ 6000rpm.

Chrysler Comps and later Talbot Special Tuning sold 998cc Sport engines, both direct from the factory as well as via Rootes (later: Talbot) dealers.
In 1981 the outright price was £450.
They would also convert an existing engine into a 998 if you sent them the block.

Reboring of a 998 engine up to 20 thou oversize should be ok for road use. Getting pistons may be a problem. Sheepbridge Engineering used to make them.
For competition use it would be better to put in new liners.

998cc Hepolite pistons

Engines; Sunbeam Talbot 930cc

The 930 engines exist in abundance, as Chrysler made quite a lot of spare engines for the Sunbeam 1.0, and at the moment (summer 1997) there are some around at good prices.
The main stockist is Grimes (Surrey), or Speedy Spares (Sussex) or Reg Patten (Surrey). Others may be able to supply, too. They want about £120 for a bottom half of the engine, the crankshaft is different so if you're buying it for your Imp you have to get your own slightly modified, or buy a new one from them for £30 which is quite cheap, then modify it with a lathe.

Swapping your Imp engine for a new Sunbeam B1 one is reputed to have advantages:

  1. Better oil sealing due to a rear crank seal, as opposed to the Imp's (inefficient) oil return scroll.
  2. Stronger than the Imp due to the B1 blocks additional strengthening ribs.
  3. Conrods stronger 11/16" pin.
  4. Crankshaft material higher grade (EN16U) (Imp E9T) and 5 bolt flywheel.

The B1 motor / Robert Allen / Impressions 1990 Spring, pp. 35-36. (A how-to.)
Fitting a 928 Sunbeam motor in an Imp / Dion Fluttert; tech. assistance: Arnoud Malherbe Impressions Aug. 1986

There is more than one type of clutch friction plate available. The one from Boch(?) has a more hollow centre than some other clutch plates. Those don't fit on the input shaft on some boxes. The main problem is the length of the 930 crank.

Engines; V8

From: Nick Cleak
Date: Wed, 11 Aug 1999 14:35:42 EDT
Subject: Re: [imps] V8 Imp

I have seen in the flesh a V8 Imp engine. It was made a long time ago for an old formula 1 car of the period. Rather cheaper than the genuine article!

The block and heads are Mk1.
One head has had the timing chain end cut off and welded on the other end and the head has been reversed. It is a 90 degree V ...
The connecting rods have been slimmed down to fit on one crank pin The capacity is around 1150cc with standard bore so the stroke is miniscule.
It revs a lot apparantly ...

From: Andrew W MacFadyen
Date: Thu, 20 May 1999 08:30:03 +0100
Subject: Re: [imps] Imps Lotuses and Climax engines

One of the heads off a 1.5 litre F1 V8 Climax engine would have made the basis fo a nice twin cam Imp unit particularly the 16 valve version made only for Team Lotus :-)

a display of The Imp Club

Engines; Lynton

The famous Imp twin cam engine

Lynton Racing built two twin-cam, 16-valves prototypes in 1966-1967. It apparently produced 110bhp @ 10,000rpm.
The project never got of the ground as the engine was too expensive to make.

Motor 7/1/67, page 52
Motor 14/1/67

From: RDH
To: imps at
Date: Wed, 19 May 1999 19:27:18 +0100
Subject: Re: [imps] Imps Lotuses and Climax engines

Hi Andrew

The Group 5 Fraser engines that I am rebuilding both have the Coventry Climax designation FWH, but you are right they share no common components with any other climax engine to my knowledge. Designs also exist for a Climax 1 litre twin cam engine (sounds familiar), I dont know why it was never built but I do know that the Nathan twin cam engine (later the Lynton twin cam) was designed by two Climax engineers

From: RDH
Date: Wed, 26 May 1999 23:21:45 +0100
Subject: Re: [imps] Imp Specials & Twin Cams

Hi Edward

I read your note about Twin Cam imp engines with interest as I have been researching these for some time. I think the company you refer to was the Nerus Engineering Co Ltd who were based in Rye in Sussex, they were quite active in Imp tuning in the mid to late 60's. I dont think they produced their own Twin cam head, I think they would have purchased it from one of two companies, either Roger Nathan or Lynton Engineering. Both these companies were producing T/C heads at the same time using patterns and drawings 'pinched' from each other.

Lyntons didn't develop the head to it's full potential because Rootes/Chrysler decided not to pursue the Twin Cam Stilleto as their answer to the Lotus Cortina. But they did consider it fairly seriously as they donated two cars and tested them exhaustively for two months.

Roger Nathan developed the heads to a much better degree, he realised the design of the 'pent dome roof' combustion chamber (the designs for the heads were by two Coventry Climax engineers) was not ideal, so in stages he increased the angle of the roof and improved the performance of the heads to a significant degree he later re-designed the valve train using titanium, but more about that at a later date

Cheerio for now


Replacing the timing chain
The manual says that the cylinder head should be removed before the cover of the timing chain is removed.
But the timing chain cover can be removed just as easily if the two studs, which join it to the head, are carefully unscrewed from their seating in the head, using a pair of pliers. If you do this with care, the threads will not be damaged and the studs may be re-used. Even if they are spoiled, the cost of new studs is less than a new head gasket. Plus you will have avoided unnecessarry work and you won't have disturbed the head (which might have encouraged head gasket trouble).

Cylinder head

see also separate file

The standard castings were made by the Aeroplane and Motor Company of Birmingham, and castings numbers have 'AM' in front of them. Aero Engineers 'AE'.

Wait until it's cold
If you want to remove the cylinder head, make sure the engine is stone cold. Otherwise you may distort it when you unscrew the headbolts.

Heads to blocks
It is possible to fit any 875 head to any 875 block. The only modification needed to to do to earlier blocks is to drill and tap the boss (near the oil filter) on the block, which accepts a pipe from the Sport head. The reason the Sport head has this pipe is to allow oil to drain back into the block, rather than being sucked down the valve guides. (With the Sport cam less efficient valve seals were fitted, because of clearance problems).

Cylinder head types
There are two types of cylinder head: Mark 1 or 2 and Sport. The Sport has an oil drain cast into the back (thermostat end) to allow oil to run down an external pipe into the sump instead of having to flow to the front of the cambox and down the timing chest. Without the head drain, the use of double valve springs (which necessitates the removal of the top hat oil seals on the inlet valve guides) means excessive oil consumption for a road engine (about 200 miles per pint). Thus with a Mk 1/2 head we are limited to the type of cam we can use, the strongest single valve springs available being Terrys 45 f.596 (which cost £3.22 at the time of writing
Sport cylinder heads are very expensive. If you don't want ultra large valves, then you'd be better off buying an L4 head. These have the oil drain and can easily be modified to Sport and above. Hartwell Clubman heads were usually all based upon a Mk2 head, and these had 1.35" inlets. (Sports use 1.28").

All the sport heads have the oil drain (from head to crankcase) and they all use the same valve sizes.
Sport head casting numbers (not a Rootes part number) with last digits
....107 : the very early sport head, possibly for use with competition cars and the dry-liner 998 Rally Imp from the end of 1965
....162 : 162 fitted to early production Sports up to 1968; the Rallye head casting, basically the same as the 180 cylinder casting but ported and chambered for 150 CD Strombergs and 998cc.
....180 : the most common. It was the later sport head, used until the end of production. It is a thicker casting than the 107, giving more scope for modification.

As there is only one cylinder head listed for Sport (one Rootes part number), it may be assumed that the cylinder head would not change where there is a related part involved. Differences may occur where there is not a part involved (e.g. port size, gas flow).

Later (basic) Imps had the Sport camshaft, tappet block, double valve springs and oil drain tube. This applies to all models with chassis number prefix L4, L5 or L6 (approx. from 1974 on).
But it isn't the Sport head. The main difference is the size of the valve seats. It could be converted to Sport specifications by fitting Sport seats and valves.

Minor modifications
A few minor head mods can add something at almost no extra cost. Clean up the ports, but do not polish them. Match up the ports to the inlet and exhaust manifolds. Equalize the combustion chambers volumes to within 0.2cc. You can turn the head upside down to do this, with the valves in. Place a sheet of perspex over the head face with a hole drilled over each chamber. With the head dead level (use a spirit level) you can measure how much parafin -or something similar- can be placed in each chamber. Use a graduated burette for the purpose and remove metal a little at the time.
The head can be skimmed to increase the compression ratio, but this is not a good idea if the head has previously been skimmed, and should always be done with caution anyway - if reliability is not to suffer.
As far as major mods go, the only one which is relevant here is the fitting of Sport inlet valves. The ports can also be enlarged to Sport spec, which is essential anyway, if a Sport carb set-up is planned. Ready modified head are available, so be guided by your wallet. You get what you pay for.
Imp tuning : tuning on a budget / Richard Freeman. - Impressions 1983, Dec.
Compression Ratio Calculation By Bowling

A vernier sprocket can be used to increase compression ratio without the losses of inaccurate valve timing. At Imp 09 they were for sale (Shrigley Engineering) and I photographed the instructions.

If an Imp overheats, the cylinder head could warp. A warped head probably has to be replaced.

After a cracked cylinder head has been repaired well, it's stronger than original.

Date: Wed, 19 Jan 2000
From: Si Trickett
Subject: Re: [imps] nheys heads wills rings

Wills ringed sport head are more prone to cracking...

What is the theory behind the thought that a Sport cylinder head with Wills rings is more prone to cracking? Does the proces of making the grooves affect the metal? Damage its stability? Do the grooves mean that, since there is less material, make the head more vulnerable?

Is it possible to use a Wills ring with another one? Extra grooves per cylinder? If yes, would it be a good idea? In which sizes are Wills rings available? Rings for a 998 are smaller than for a 1040.


  Do not use Copper Slip for the head bolts. Copper reacts with aluminium, slowly and even at room temperature.
The handbook says to use 'Shell Ensis'.
Use something anhydrous, perhaps an emulsion of lanolin (woolfat) and petrol. Dip the thread and lower shank in it, put a drop of oil between the head and the washer and insert in engine.
Or you could use Loctite antiseize. (It's like the copper-loaded brews but silver in colour - tin or aluminium?). It doesn't misbehave on alloy bits. Always gives a clean, smooth re-torque of head-bolts even after a couple of thousand miles. This Loctite Antiseize has anti-corrosion properties. (Some antiseize products only have anti-gall properties.)

Head bolts

There are two different bolt lengths. The long ones go under the camshaft. They should be undamaged and have the correct washer to prevent the heads digging into the soft aluminium (and then working loose). Coat them with an anti-seize compound, so you can remove them more easily when you need to - but also to make sure it won't fool you into thinking it has reached correct torque when it's just stuck.

The bolts on the chain housing are loosened first (before the ten headbolts) and tightened last (15 lbs ft).
Factory setting is 36lb/ft torque. Use for the initial setting up to 40lb/ft, to be on the safe side... if the threads in your block are good then it will be worth torquing the head to 40 ft./lbs. This was recommended by Chrysler in one of their Service Bulletins.
Only use clean bolts that you can fit and remove by hand.
To prevent corosion or force welding between the threads of the bolts and cylinder block, coat them with gear oil, (eg. EP, which has been designed to prevent force welding).
To retorque the cylinder head bolts you must slacken each one (about 1/2 a turn) before re-torquing. Do this to each one singly and don't loosen them all at once. Heat, expansion and dust etc. are enough to cause the bolt to bind, preventing further straight re-torquing. To help the task, put oil between the bolt and the thick washer. It may seem a mere detail, but it could make the difference between a motor lasting 25,000 miles rather than 5,000.
Use something to prevent corrosion.

Torque wrench
Always use a good quality torque wrench. Check it against another from a friend. Having it calibrated costs £15 - £18.

Torque Wrench Settings
Cylinder head Stage 1 Stage 2 Stage 3 Stage 4
Fixings 25 lbs.ft. 30 lbs.ft. 34 lbs.ft. 36 lbs.ft
Camshaft bearding cap nuts 6 lbs.ft.
Camshaft sprocket bolt 19 lbs.ft.
Camshaft carrier nuts 15 lbs.ft.
source for above info:
Haynes - Hillman Imp DCF 020
other source 20 lb/ft 30 lb/ft. 38 or 40 lb/ft.

Head bolt tightening sequence

Start from the middle and work outwards in a spiral. Don't torque the middle ones up to max torque when the others are still loose, but gradually build up the torque on all bolts, using the spiral method.
Torque the head down nice and gradually and let it rest before re-torquing. Start with 20 lb/ft, then 30 lb/ft. finish with 38 or 40 lb/ft.
Retighten all nuts and bolts when the engine is totally cold again after the initial warm-up, and again after 100 miles (when totally cold), and then again after 500 miles.


The genuine Chrysler headgasket for the Imp is made by Reinz in Germany. The Reinz head-gasket has been praised; it will even cope with some head and block distortion. Cheaper after-market types are better avoided.

If you run a 998cc it is well worth having the head machined for the Hartwell raised lip gasket (if you can find one) or Wills rings.
If the block is converted to accept 'wet' cylinder liners, use Wills-type gas-filled sodium rings as the sealing medium. They rarely fail. (Wills ring will dig into the standard dry liners.)

A leaking head gasket will put (a hint of) an oil film or bubbles in the coolant. If a head gasket shows signs of blowing, examine the block and head faces for distortion.
Also check for cracks between the bolt holes. With sport engines the biggest chance for a crack is between cylinder 3 and 4; with ordinary engines it's probably between 2 and 3.

Reasons for headgaskets to blow

One cause of repeated head-gasket failures on Imps is the sequence: initial water loss - overheat - headgasket - fit a new one - fail again - skim head & fit a new one - etc etc. The problem is usually that the bores have sunk down into the block a little, due to the first cook-up. The bore-grommets of the new gasket don't see their designed compression loading when the head is re-tightened. Shaving the block to bring the periphery level with the bores stops this cycle.

If the headgasket blows, it is usually around the waterway between cylinders two and three.

If the headgasket blows

Shaving the head

One opinion next to another...

Maximum distortion for the cylinder head: 0.003"
Maximum distortion for the block: Nil
All distortion should be removed by resurfacing. Skimming the head without skimming the block might even be unhelpful - the head might have adapted its shape to the block.

    If the distortion is less than 0.003-0.004" over the length and breadth of each casting, skimming is an unnecessary expense.
Don't automatically assume that they'll both need skimming. In case of the head: it could raise the compression ratio too high. If the head had already been skimmed... 10.0 : 1 is high already - don't strive for detonation.

Replacing an Imp head gasket

Check all bolts for damaged threads and stretching.
Tap out bolt holes (always) and blow them out (a bicycle pump will do). If any block thread is damaged, get it helicoiled (get thread inserted).

On fitting gaskets, check even for simple things such as human hair on a mating surface. Make sure that the faces of the block and the head are perfect.

Degrease the mating faces with much rubbing using a rag soaked with meths, to make sure the varnish on the gasket can stick.
Some use Curil. It covers the varnish and provides its own adhesive layer. The varnish acts as a sealant, too. If you want to try re-using a gasket (not recommended), try Curil. You'll probably be able to lift off the gasket intact. (Clean it with petrol).

Date: Wed, 19 Jan 2000
From: Cam Johnson
Subject: [imps] nheys heads wills rings

I too believe in letting a head get to know a block by clamping the two together without gasket for a bit before you get your torque wrench out for the final assembly.
I used grease to fit Wills rings to the grooves, because they are prone to corrosion.
To misquote TimMo, a skimmed head is more likely to crack if it is fitted back onto the warped block it has known for years. Favourite place in my experience for block cracks is a terminally major one, running across the bottom of the water jacket on the right hand (oil filter side) between 2 and 3, due to silt and fossils preventing the water from heating that side of the jacket and expand it as much as the increased expansion of the liners due to silt preventing cooling of the liners which try to push the head off. Silt on this side of the water jacket makes the liners expand more and the water jacket expand less, hence the crack; clean it out!

Flywheel bolts
Always fit new big-end and flywheel bolts; never use them twice unless you want to risk a ventilated sump and a useless motor.

engine plus gearbox (worked open)

The engine in 875cc Hillman-Imp-form weighs 170 lbs (77 kg), that includes the accessoires.

see: camshaft.html

Unusual bits

A pre-engaged starter. No more chewed-up ring gear, no more churning and flying out of engagement when trying to start a hot and vapour-locked engine it's a brilliant idea. It comes from an Austin Maxi.


    There are plenty of books on lubricating oils.
mentioning these titles does not imply recommendation.

Engine Oils : Rheology and Tribology (Sp-1069). - Publ: Soc. Automotive Engineers, Feb. 1995
ISBN: 1560916192 (Hardcover)
Price: $29.00

Automotive Lubricants Reference Book / Caines & Haycock. - Publ: Soc. Automotive Engineers, Dec. 1996
ISBN: 1560915250 (Hardcover)
Price: $125.00

Engine Lubricants (S P 1121). - Publ: Soc. Automotive Engineers, Oct. 1995
ISBN: 1560917059 (Paperback)

Engine Oils and Automotive Lubrication (Mechanical Engineering, No 80) / WJ Bartz (Ed.). - Publ: Dekker, Jan 1993. -
ISBN: 0824788079 (Hardcover)
Price: $199.00
Review by Booknews, Inc., Jan.'93:
A comprehensive presentation of all major aspects of automotive and engine lubrication. Experts from the mineral oil, additive, and automobile industries, as well as from research institutes, address topics including film thickness in engine bearings, base oils for automotive lubricants, additives and mechanism of effectiveness, engine oils and their evaluation, sludge deposits in gasoline engines, special aspects of engine lubrication, two-stroke-engine oils, tractor lubrication, gear lubrication, and lubricant influence on ceramic and seal materials.

Fuels and Lubricants Primer for Automotive Engineers/Pbn Sp-671. - Publ: Soc. Automotive Engineers, Dec 1986. - 2nd Ed.
ISBN: 0898839424 (Paperback)

Multicylinder Test Sequences for Evaluating Automotive Engine Oils, Part 1 : Sequence Iid (ASTM Special Technical Publication, No 3151). - Publ: Am. Soc. for Testing & ..., Apr 1993. - 10th Ed.
ISBN: 0803118805 (Paperback)
Price: $30.00
Review by Booknews, Inc., Jan.'93:
Details test sequences that make possible a quantitative description both of the operating conditions for determining the performance of crankcase oils, and of the oil properties needed for satisfactory performance in modern passenger cars and light trucks. Updated frequently since 1961. The A.S.T.M. has not evaluated the test, and does not recommend it.

Passenger Car Engine Lubricants (Sp-939). - Publ: Soc. Automotive Engineers, Oct 1992
ISBN: 1560913037 (Paperback)
Price: $53.00

Recommended engine oil for the Imp
°F °C grade oil
Below 5°F
0 - 32°F
20 - 68°F
Above 68°F
Below -15°C
-18 -  0°C
-7 - 27°C
Above 20°C
10 W or 5W/20
Shell X 100 Multigrade 5W/20
Shell X 100 10 W or Multigrade 5W/20
Shell X 100 20 W or Multigrade 10W/30
Shell X 100 30 W or Multigrade 20W/40
Continuous high speed20W/40 or 100W/40Shell X 100 Multigrade 20W/40
or Shell X 100/40

Change the oil and filter frequently.
When you seem to be burning oil, use thicker oil. A consumption rate of 500 miles/pint seems to be normal for a Sport engine... even a new one. An old rally block may be so thirsty as to guzzle a pint per 170 miles.
Valve guides wear and valve stem seals become brittle, allowing oil to seep into the combustion chamber. It's what you get with OHC engines. If you need to cure a high oil consumption, fit oil saver rings and new valve guides. But perhaps a rebore and new pistons are the only remedy.

A possible cure for an oil leak:
Keep the wire mesh in the small base of the air filter, along with the pipe to the air filter, clean from thick soapy sludge.

When the engine is running in, use a cheap oil.
"Straight and running-in oils don't do modern rings any good. They also cause premature wear on OHC cam gear. Far better to use a normal multigrade and stir the engine along a little when running it in."
This is what the specialists at the engine reconditioning shops say these days, according to Brian Bracken, who uses Shell 15w-50)

M. Jones, Impromptu February 1989:

I thought I'd treat my Stiletto to a sump full of high-tech oil, Gemini in this case. I first made sure that it was a 15W50 and not 10W40.
I don't think I am imagining it, but the car is definitely smoother running. What I didn't imagine was the 2 P.S.I. increase in oil pressure and the beautifully clean condition of the valve gear when I recently checked it. With the price of Imp engines these days, it seems to make sense to use a first class oil. I think I'm right in saying that Gemini is a semi synthetic oil. If you've got more money to spend, you could use a 100% synthetic oil such as Mobil 1 (see Anthony Shelton's letter Oct '88 Impromptu. This is supposed to be superb or so I'm told.

If the oil level in your sump goes up, it might be a petrol leak. Maybe a ruptured diaphragm in the mechanical petrol pump - just a small pinhole would already cause trouble. Or maybe the float needle valve is sticking or incorrectly set or the float itself is punctured. Either or all of these failings would cause petrol to trickle through the carburettor, into the inlet manifold after the engine is switched off. Possibly some of this petrol may find its way into the sump.

oil as coolant

Oil filters
There are two types:

  1. a screw-on canister type filter or
  2. the older one where you have to take it apart and change the internal element.
The normal oil filter unit doesn't have a place to fit the oil cooler pipes, the Imp Sport & Stiletto ones do ..
Combine the Sunbeam BI spin off filter block with a Volvo filter. The Volvo one is bigger than the Sunbeam one and it's a very high quality: it filters more finely. - idea: Nick Cleak
Volvo 340 pre 1985 spin-on filter with oil cooler take-off part no. 3517857-3, it's like the late Imp Sport type and has the very same fit, only a bigger diameter. It has a non-return valve which gives instant oil pressure on start-up, as no oil is allowed to drain away overnight, saving considerable engine wear. The filter only fits later Imps.

Oil pump
When building an engine, the oil pump gears should be replaced as a matter of course. The tuftrided gears are the gilding of the lily, but the standard one (provided that it's in good condition) is quite sufficient.

Oil pressure gauge
The bolt head that lives in the middle of the upper crankcase (just behind the starter motor is the blanking plug for when an oil pressure gauge is not fitted.
The Singer Chamois was the only Imp which had it as standard; for the rest it was an optional extra. For the D.I.Y. a gauge kit was obtainable, which contains the fitting for the crankcase.

Thermostat cover
How to go about removing the thermostat cover (part no. 7010105): Richard McIntosh, Impressions 8 (1980), no. 6

Active engine mount for a large amplitude of idling vibration / T. Shibayama; K. Ito. - SAE Technical Paper 1995 May, no.951298. - pp. 531-538. (Reporting the decrease in floor vibration achieved when a piezo active mount was installed. Theory; construction; test method.)
Experimental study and modeling of hydraulic mount and engine system / S. Gau; J. Cotton. - SAE Technical Paper 1995 May, no.951348, pp.927-939. (High damping over a narrow band of frequencies, useful for reducing vibrations of an engine on tickover.)

aluminium alloy engine


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Version of: 9 April 2012
File started: 10 July 1997