The Imp Site

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Tappet adjustment 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)

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.

• Tape a piece of 400 wet-and-dry sanding paper onto a flat surface
• work in figure-8 motion
• turning the shim frequently (other source: grind only one side and fit that against the bucket)
• apply micrometer regularly!

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

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.

Taurus Performance Tuning Ltd.

Taurus did Imp cylinder heads. Their folder gave performance specifics.

Arden Conversions modified the Imp head for £15 (on an exchange basis of course).

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)