Whilst the head is basically of similar design to the F.W.A Coventry Climax, certain limiting factors on power output are imposed by the canting over of the engine at 45°, necessitating long inlet ports, the smaller bore and size of the combustion chamber, restricting the size of the valves.
unsorted bits & pieces
If performance is your goal, then cylinder head selection and preparation is the most important part of your engine block. Cylinder head preparation is also the most direct and rewarding way to improve your engine. Ask the best engine tuners and they'll tell you that cylinder heads are the key to building horsepower.
Exhaustive tests reveal that polishing the heads gives no increase in flow. In Vizard's opinion overall performance may decrease due to the smooth walls.
Standard heads were made for Imps with single carburettors, Sport heads for twin carb models. The Sport heads do not have a '2' on the casting.
|Standard cylinder heads|
|7010031||Fitted to the early Mk I|
On the very early versions of these, the area between the spark plug holes and the top of the head was filled in. On the later heads there is a recess here, tapering from one end to the other and with strengthening ribs running across it.
|7010128||Fitted to the later Mk I|
Some of the later models of this head had the large valves as fitted to the Mk II (prior to its introduction in September 1965).
|7010133||Fitted to the early Mk II models up until the middle of 1968. On the rear end of the head (at the back of the car), the area from the cam box to the block does not taper, but has a step.
Richard McIntosh, 25 July 2013:
|7010171||The later Mk II head|
No core plug at the end of the exhaust manifold (next to no. 4 exhaust port), but it is solid and machined flat as a continuation of the manifold area. Also on the rear end of the head (at the back of the car), the area from the cam box to the block tapers.
Both this one and the earlier have a large '2' cast on the top face of the timing chain aperture.
Richard McIntosh, 25 July 2013:
|71981533||From late 1973 on.|
Normal Mk II size valves and a '2' on the casings, but it has an oil drain like the Sport heads, and it does not use inlet valve seals. This was made necessary by the fitting of the Sport cam, tappet block and double valve springs to all cars from 1974 on.
It can be distinguised from the Sport head by having a thinner casting.
|Sport cylinder heads|
|7010107||Very early Sport, may even be part of an initial run, prior to production of the 'Sport'. Destined for use on competition cars, including the dry liner 998cc 'Rallye Imps', produced from the end of 1965.|
|7010162||Fitted to the early production models, until the Mk III, mid 1968|
Richard McIntosh, 25 July 2013: I have owned one of these early Sport heads, number 7010162. But they are rare. I think the later and much more common 7010180 head was introduced well before the MkIII cars and therefore before mid 1968. For example my MkII green Sunbeam Imp Sport was manufactured in November 1967. As far as I can tell, this car is wholly original. It has the 7010180 head.
|7010180||No core plug at the end of the exhaust manifold (next to no. 4 exhaust port), but it is solid and machined flat as a continuation of the manifold area. Also on the rear end of the head (at the back of the car), the area from the cam box to the block tapers.||7050112|
|Head||Inlet||Outlet||head can be modified to take|
|Mk II size valves|
1.40" and 1.25" valves
|The ports can also be modified proportionally.
The Mk I obviously has the least scope for alteration,
as the ports become dangerously thin
if enlarged more than a very small amount.
|Chamois valves. - Hot Car, September 1968. - [Technical info]
I have a Singer Chamois - serial number B 431012508 HHSO. Can you tell me please whether it is fitted with the smaller or the larger valves? If the smaller valves are fitted, what is involved in fitting the larger ones? Can I also fit the Imp Sport oil cooler? - Is it available in kit form? Where also do you suggest tapping the inlet manifild for fitting a vacuum gauge?
B. Inskip, Eltham, SE9
Your Chamois appears to be one of the earlier ones, which were fitted with the smaller valves.
Your inlet manifold can be tapped. But the metal is rather thin and it's essential to get as close as possible to one of the juctions.
|Imp Part No.||inches||mm||BL Maxi Part||TR7 Part|
|Imp Part No.||inches||mm||BL Maxi Part||TR7 Part|
Shims / Valve adjustment / Setting the tappets
As Kuzmicki said: "The valve running clearances are adjusted by means of graduated shims and once correctly set-in the setting does not alter." Normally there is little wear on the cam and shims, even after many miles. So generally it is a job left until the head has to be dismantled. Unless the clearances are greater than 0.008" for inlet or 0.014" exhaust, you might be tempted to leave well enough alone. (If you can live with the noise...)
It is a simpler job than it appears when you read about it. The biggest problem is obtaining replacement shims of the correct sizes.
After having measured the clearances, removed the camshaft and taken out the shims, you need a micrometer to measure the existing shims.
There are other cars that use (more or less) the same shims, diameter 0.625". These have cylinderheads with the same shaped combustion chambers, the same idea for a camshaft, therefore the same shims. (AE = Aero Engineers)
If you really can't find a shim that will fit, you could find a slightly thicker one (maybe one of which the marking are now illegible anyway), and rub it down to size (slide it over grinding paper). Laborious, but it might still be quicker than chasing all over for that elusive exact size.
Maybe you could get a machine shop to make them for you. The hardness is a ROCKWELL 50 and then re-hardened ?
From: Martin Freestone, a former employee at Rootes
Date: Fri, 24 Jun 2005
I used to use Coventry Climax valve shims to adjust my valves as they were so out of spec and cov climax had a wider range of shims on the fire pump engines - shows how close they were.
Mine was a nice car (Sunbeam Imp Sport), 90mph all day on the M1, and I've never had such a well balanced handling car since.
Team Hartwell wrote [14th November, 1968] that
...to cover the range of shims we require, we purchase a large number from Coventry Climax. These are considerably more expensive than the Rootes shim, but to save operating a two-tier price structure, we average the cost out, thereby arriving at a cost of 5s.
Greetham Engineering (Andy Chessman) at Edgwick Road, Coventry had a head and camshaft combination which increased both torque and bhp throughout the normally used rev range. The head itself was done with economy in mind as much as any power increase that it may achieve.
|Toby V.S. has a graph on his site comparing a Chesman head with Mini Coopers|
Andy Chesman devised a way of machining Imp cylinder heads at an angle to help the engine make more power eg. 125bhp from a 998cc engine with twin Webers and R23 camshaft. This special head is called a WEDGE head.
In their article on "Tuning for the Imp range" V.W. Derrington say:
Engine performance can be considerably improved by suitable modifications to the ports & combustion chambers, increasing the gas flow by over 50%, as proved by Rootes Development Dept., when one of our modified heads was tested by them. Obviously, the smaller cc engine cannot possibly be improved to the same extent as the F.W.A. 1100cc engine.
The modifications include
The specia1 o/s inlet valves can be fitted without new seatings by en1arging the throats and re-cutting the valve seats, and are specially designed for maximum gas flow. These are on. the left, in comparison to the standard rough stamping, and the special exhaust valve on the right. Owing to the large selection of valve adjusting shims which may be needed to obtain the correct valve clearances with another camshaft, it is advisable to order the camshaft to be fitted modified cy1inder head. This will then be supplied set to the correct valve clearances. Prices are on exchange for standard cylinder head.
- enlarging and reshaping inlet ports to venturi form,
- enlarging valve throats,
- reducing valve guide bosses,
- fitting larger inlet valves of improved shape,
- re-shaping combustion chambers,
- machining the face to raise the compression ratio, which cannnot be overdone without affecting the timing chain adjustment and the timing of the valves.
Taurus did Imp cylinder heads. Their folder gave performance specifics.
Arden Conversions modified the Imp head for £15 (on an exchange basis of course).
Theory and practice of cylinder head modification / by David Vizard. - Croydon : Motor Racing Publications, 1971. - 152p. + 22 diagrams (drawn at actual size) without page numbers. - [MRP Speedsport]
ISBN 0 85113 066 6 (paperback) © 1973, reprints: 1988; 1991; 1995; 1999
In Pt 3 'Head designs and applications' there are a few pages (p. 132-136) on 'Modifying Imp heads'. Page 133-135 are all diagrams:
Fig. 77: Standard valved Imp Sport inlet port modification. Modification to Imp Sport chamber. Imp inlet port for full race. Imp exhaust port for full race. (4 drawings)
Fig. 78: Imp chamber shape for full race engine. (1 drawing)
The Imp Site
Imp Cylinder Head
see also the part on cylinder heads on the page on engines
see also the mailing list archive: apr. 98
File version: Sat. 28 July 2012
File since: 5 October 1997