The Imp Site

Cylinder Head

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.

Standard cylinder head types
Standard valve sizes
Shims
Greetham Engineering
Literature

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 cylinder head types

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
part no.:	details:
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).	7050001
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.
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.	7050119
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.	77054265
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
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

Standard valve sizes

Valve Sizes
Head		Inlet		Outlet		head can be modified to take
Mk I Mk II Sport		1.06" 1.20" 1.28"		1.01" 1.06" 1.06"		Mk II size valves Sport inlets 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.

Shims

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)

Saab 99: 1.78mm (0.070") to 2.85mm (0.112")
Triumph Dolly 1850: start at 0.090"
Triumph Dolomite sprint 2000
Triumph TR7
Austin Maestro 1600
Jaguar 6 cyl (Mk II; XJS; ...) (diameter the same?)
Lotus Elite (also a FeatherWeight)
Lotus 900 series
Lotus supply these shims from 60 to 150 thou. A specialist Lotus garage may overcharge. Part numbers e.g:
A026E6181Z (81 thou)
A026E6182Z (82 thou), etc.
Rover 2000, SDI
Audi 5 cylinder.

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. Laborious, but it might still be quicker than chasing all over for that elusive exact size.

Tape a piece of 400 wet-and-dry sanding paper onto a flat surface
work in figure-8 motion
turning the shim frequently
apply micrometer regularly!

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.

Greetham Engineering

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
The inlet ports were slightly enlarged and reshaped, but they were left fairly rough so as to aid fuel build-up. The inlet valve remained at the standard Imp Sport size. The exhaust ports on the other hand were enlarged quite considerably over the standard and the exhaust valve itself was increased from 1.06" to 1.125". that is a 12% increase in area. The general idea of this is to cut down pumping losses on the exhaust stroke. This will make a small contribution to bhp but more important it will also help the economy.
David Vizard used this combination and tried different carburettors with it on an 875cc: Small bore power

Wedge head
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.

Derrington

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

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.
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.

Literature

Vizard, D.
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)

see also the part on cylinder heads on the page on engines
see also the mailing list archive: apr. 98
Imp Anatomy
The Imp Site