Imp Site logo - imps4ever!

The Imp Site

Andy Dawson

Dawson, A.
Dawson's Dodges / by Andy Dawson. - Cars & Car Conversions 1976, May. - p.67-69,71
How the experts prepare competition Imps, by the man who probably knows more about it than anyone else. This month Andy Dawson looks at the basics of screwing together 875cc and 998cc engines.


Dawson's Dodges

WHETHER it is because I began my rally career in an Imp, or whether it is because the Imp is so much fun to drive, I don't know, but I have an affinity towards the little beast. Colin Malkin, Andrew Cowan and Robin Eyre-Maunsell all have the same relationship, so I guess it must be because they are such fun to drive.

I have in fact been involved with the Imp since 1965, when, as a penniless student, I fell into some of the pitfalls that this article is intended to help you avoid. Since then I have been involved with George Bevan's incredible Group 2 racers, Peter Harper's mid-engined rallycross car, various rallycars and that most successful Imp variant, the Clan Crusader. So I hope that in the next few thousand words I will be able to pass on some of the experience that I have been lucky enough to obtain.

I'll begin with two basic but vital rules, both of which I will come back to later.

  1. Air cleaner
    The first is that due to the rear engine and the bodyshape, it is vital 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.
  2. New bolts
    The second rule is to always fit new big-end and flywheel bolts.
    Never use them twice, unless you want to risk a ventilated sump and a useless motor. For the same reason always use a good quality torque wrench.

I'll begin with the engine and describe the mods that I would do to the various components, and then run through the specifications that I have found work well.

There are two types of cylinder head: Mk1/2 and Sport. They differ only in that 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 Mk1/ 2 head we are limited to the type of cam we can use. The strongest single valve springs available being Terrys 451.596 (which cost £3.22 at the time of writing). When fitting these stronger valve springs, you should also fit the Sport type valve collars; the standard ones have a tapered outside to the collet fitting and the Sport ones are parallel. For use with strong, double valve springs there is a competition valve collar which is of the same design as the Sport, but is made of a better material. It can be recognised visually as the material is of a much darker colour.

cylinder headMk.1 / Mk.2Sport
difference head drain:
an oil drain cast into the back (thermostat end)
head drain
oil has to flow to the front of the cambox and down the timing chestto allow oil to run down an external pipe into the sump
head drain
better not use double valve springs
using those would mean excessive oil consumption for a road engine
(about 200 miles per pint)
valve springs
limitation in the type of cam you can use
For any of the 0.36 or higher lift cams it is necessary to use hard double springs if the engine is to rev.

Valve springs

The strong single valve springs are about as hard as the Imp sport doubles, and can thus be used with the Imp Sport camshaft, but for any of the 0.36 or higher lift cams it is necessary to use hard double springs if the engine is to rev. The Chrysler comps springs (CTS 1018) are good to 10,000 rpm with the heaviest of valves and the hairiest of cams (but beware of Terry's double springs as they can become coil bound).

Cams, their uses and timing are in the adjacent table.
My own personal preference for almost every use is the R20 as it is only marginally less tractable than the R17, gives a very wide power band and yet appears to be very kind to the valve gear.

 Type  Lift Theoretical
 valve timing 
Ideal inlet
 opening at 
Std.2470, 30, 30, 0-5 & 11 
Sport.31212, 50, 50, 120.040"7 & 14Road
R17.36030, 60, 60, 300.107"8 & 13Road
R20.36032, 68, 68, 320.115"8 & 13Rally/ Rallycross
R22.36040, 70, 65, 350.162"8 & 13Stage rally
R23.36048, 76, 76, 480.178"8 & 13Full Race
Cam curv graph
Showing camshaft lift against angle
    multi-hole timing sprocket; competition followers; cam carrier; R23
 Imp cam gear
At left is a multi-hole timing sprocket.
From the top are: competition followers a cam carrier modified for a 0.38 1ift cam and an R23 camshaft.

When fitting a hot cam, the first thing to do is to stone off the edges of the cam lobes to stop the followers getting scratched. Fit the cam into the carrier, which will need relieving with a small rotary cutter to stop it fouling the edges of the follower guides. Then fit the carrier assembly to the head. This method is contrary to the workshop manual, but a high lift cam is very easy to break when fitting, and the carrier gives it a bit of extra support.

Ian Carter, who builds a lot of race engines, redrills the cam carriers to feed more oil to the cam followers, but I have found that either standard followers replaced regularly, or competition followers (CTS 1043) with the edges radiussed off, work perfectly well. With a very hot cam the standard followers can either crack or hollow out, but this always happens during initial running, so a thorough check after about 5 hours running will show up any problems.
When using a cam with a non-standard base circle radius, such as the many regrinds that are available, it is necessary to use either very thick tappet shims or have the head milled on its top deck to set the cam carrier lower in the head, Under no circumstances should the cam carrier itself be thinned down, as the followers will tip and lock at full cam lift, causing horrible results.

The only other tweak concerning valve gear is the use of a stop on the timing chain tensioner to prevent the chain from jumping a tooth if the engine is turned over backwards. All I do is to braze a little strip of metal across the tensioner slide, usually about ¼ inch down the slide, although on an engine which has had a lot off the head it might need to be even further down.

There is no magic in the timing of an Imp cam, you just need the right equipment. With a Sport cam it is possible to set the timing with the standard sprocket and then transfer to a multiholed sprocket if necessary. With a higher lift cam, you just set the cam at the required No 1 inlet valve lift (this is in fact the rock point on the cam, when the inlet and exhaust valves are open by the same amount) bring the engine up to top dead centre, No 4 firing, and find the hole on the sprocket which gives the best chain fit. Then check the valve opening when the engine is again turned onto TDC. As long as the valve opening is within 5 thou then don't worry, unless of course you are looking for the last half a bhp. Make sure that you don't fit the sprocket the wrong way round, as there is no chamfer on the front and the sprocket doesn't sit properly on the end of the cam. Also make sure that the sprocket bolt is torqued to 20 lb.ft., as the dowel shouldn't have to do any work when turning the cam - it should be driven by the friction between the sprocket and the cam.


As a basic stage 1 spec, the head should be cleaned up in the throat area, but not polished as this gives a poor tickover, and the back of the standard valves should be polished (Emery cloth and an electric drill are sufficient). Next step is to open up the exhaust port to 15/16 inch diameter, open the throat to the same diameter and blend the two together. Give the inlet port a thorough clean up, but again not a polish, although the exhaust port can be polished. Next is larger valves, for an 875cc motor, 1.3125 in inlet valves are as large as can be sensibly fitted, and the standard exhaust valves suffice. With the bigger inlet valves goes the opening up of the inlet ports, to 31/32 in. with the throats being cut to 1.2 in.

For big bore engines it is possible to go to even bigger valves as it is also possible to open up the combustion chamber to give clearance round the bigger valve heads. The 1.4 inch inlet valves need bigger valve seats, and the valve throats can be cut to 1.25 in, with the ports opened out to just over an inch. On full race units it is worth fitting 1.125 in exhaust valves and opening the exhaust ports up to 1.0 in.

My own feeling is that anything over the third stage of head is best left to the professionals, as it is very easy to go through a port wall and scrap a good head casting. Andy Chesman at Greetham Engineering, Edgewick Road, Coventry, offers the best value for money on Imp heads, as well as producing the best for performance. Personally I would like to see a bit more swirl from his ports, but the heads give a lot of power so I'm not complaining.

Combustion chamber shape

Ports and valves are the main power producers, but the engine's characteristics can be altered with the combustion chamber shape. For torque on a rally engine I leave the chamber fairly angular, just opening it out round the inlet valve and flaring it back slightly at the plug hole. Racing engines seem to like a far more flared chamber, almost hemispherical, with the flare coming from the edge of the valve seat right back to the liner diameter. It is a case of maximum gas flow for the racers, and some squish for the rally units.

Compression ratio

        Compression ratio graph
Compression ratio graph

The last aspect of heads is compression ratio. As standard the Imp is high for a road engine at 10.0:1, but on even mildly tuned road engines it is possible to go up to 11.0: 1 without the need for 5 star fuel. For rally engines I use between 11.5 and 12.0:1, and on race engines I aim for 12.5: 1, although this is sometimes difficult to achieve with a flared combustion chamber.

To calculate your own compression ratio it is necessary to build up the volume from the dimensions below and then check the ratio on the graph.

Reinz 875 gasket3.3 cc      
Three layer gasket4.0 cc
Wills Rings1.9 cc

Std piston cut outs0.7 cc
998 piston cut outs1.6 cc
Large 998 piston cut outs2.1 cc

Pistons down bore per thou0.1 cc

Make sure that you add in one type of head gasket, one of the piston cut out sizes and the distance the pistons are down the bore.

Whilst on this subject, the optimum distance for squish is 5 thou if you are using a standard type of combustion chamber, less if it is a very flared chamber.

Bottom ends

The standard 875 block in Mk 2 or Sport form (the only difference is the tapping for the oil drain) can be bored out to plus 60 thou which gives 915cc. There is a slight danger that the liners might not be thick enough to allow for 60 thou being removed, but in my experience the chance of going through the liner is 2%. There are both BHB (Chrysler Part) and Powermax pistons available at plus 60, the latter being more expensive but lighter.

The next step from 915, and in my opinion a better step, is to 998cc. The cast-in liners are machined out and a spigot left in the bottom of the block into which the semiwet liners are Loctited. The spigot bore has to be machined accurately to give a three thou clearance to the liner, and then thoroughly cleaned before the liner is fitted in with Green Loctite and then clamped in place for 24 hours to allow the adhesive to harden. The final step is to skim the top of the liners to leave them 2 thou proud of the water jacket, and hone the bores to size. If the fitting has been properly done then it takes 18,000 pounds to push the liners out, so anybody who worries about the reliability of the wet liner 998 blocks had better consider how the liners might be moved.

For the 998 there are three types of piston available, BHB (Chrysler Comps) with either large or small valve cut outs, the large ones being needed for 1.4 and/or 1.125 valves; and the Powermax type. The BHBs are available in three diameters to give the required clearance. As a base rule, they should be used grade for grade for a road engine, one grade out for a rally engine and two grades out for a race engine, ie: A pistons in C bore, this giving the same clearance as a Powermax in an A bore. The Powermax are slightly lighter and thus better for a race motor, but they are also 35% more expensive.


The blocks are plenty strong enough for anything other than sustained revs of over 8,000, when the centre main bearing has a tendency to fall out due to panting of the crankcase. The remedy is to fit a strengthening plate to the bottom of the crankcase, and connect this up to the main bearings. Chrysler market a kit, which consists of long studs to replace the main bolts and pass through a ¼ inch thick steel plate sandwiched between sump and block, with a nut fitted to the studs either side of the plate. Setting this up is a bit of a hassle, as it is necessary to bolt the plate to the block without the sump and then preload the upper nuts onto the plate and tighten the lower nuts before removing the fixings to the block and fitting the sump.

The alternative type of strengthening uses the same plate, but has aluminium blocks which fit between the main bearing caps and the plate. The thickness of these blocks is such that it is possible to bolt the main caps on with nuts on the underside of the plate. In practice, this type is better, as the studs break with use on the Chrysler type. But the aluminium blocks must be machined very accurately to give the right nip on the sump gasket.

Con rod

 Big end bolts and con rods
Far to the left a Cosworth bolt next to a standard type; the left rod is modified and polished, that on the right being standerd.

The Mark 2 type of con rod is just good enough for all the hammer that a 998 can give it, so long as it is properly prepared. Chrysler market a lightened, balanced set of rods, and for all but the most powerful engines these are OK, the lightening being just the milling off of the balance weight on the cap. For a full house engine I machine the caps so that they are U section all the way round, remove the balance weight at the little end and lightly polish all over. Then, having crack tested them, I have them shot peened and the bolt holes opened up to 3/8 in so that Cosworth bolts can be fitted.

Crankshafts are made from EN 9T and so, although strong, they cannot be nitrided. Tuftriding has to suffice, but you should be careful who you get round to it as I've seen many cranks which have either bent due to being supported at the ends in the tuftriding bath or on which the tuftride hasn't taken. Polish the tuftrided journals with used 400 grit Emery tape, and then strap them with a leather belt and Brasso to give a mirror-like finish. If you have to have a crank ground, then make sure that the radiusses at the ends of the journals are smooth and at least 0.1 in dia, or the crank stands a chance of breaking due to stress concentration.

Lubrication system modifications

The standard lubrication system is adequate for all engines, and despite some people advocating the use of one type of bearing shell or the other, I have seen all three common types of shell used without any trouble. The oil pump should have a second groove cut in its location ring to spray more oil onto the drive gear, and the clearances should be reduced to a minimum by lapping the open end on emery on plate glass. The oil pump drive gears on the Imp Sport are tuftrided, and can be identified by a T on the crank gear and a spot of blue paint on the driven gear; these should be used on any engine which is going to be revved. If the crank nose has over one thou run out (and most tuftrided cranks have), then a bronze driven gear should be used as this is less affected by run out than a steel gear.

The only other mod to the lubrication system, that is worth doing, is the uprating of the oil pressure relief valve. George Bevan makes his valves adjustable with a bolt behind the spring through the top of the valve. The danger of this is that the bolt might vibrate loose. The alternative is to file the end off the valve to free the plunger. You can then fit 0.1 in of packing behind the spring, refit the plunger and peen the end over. The result should be 70 psi oil pressure obtainable at anything over 4,000 rpm if the lubrication system is in good order.

Oil sump

The oil sump should have its capacity increased for competition engines, if only as a safeguard against running low on oil and getting surge. The way to increase the capacity is to weld an extension over the usual flat back plate, the same shape as the normal sump, and then cut inch holes in the plate and a slot along the lower edge to allow the oil to run into the extension. For racers it is worth fitting a 2 in. extension to the pick-up pipe to stop surge under braking, and also a baffle plate above the open part of the sump.

Head gasket

    Head gaskets: standard; Reinz; Wills rings
left: a standard 875cc;   middle: Reinz;   right: 4 Wills Rings & their surrounding water gasket

Having dealt with the bottom end and the head, we had better join them up.
For engines of up to 75 bhp, a gasket is good enough, as long as it is fitted dry and with new bolts torqued to 40 lb/ft. For both 875 and 998cc, the Reinz type of gasket is best.
For engines of over 75 brake, it is best to fit either Wills Rings or a raised lip gasket and for both it is necessary to machine a groove in the head. The raised lip gasket has a raised sealing ring which sits in the groove in the head and this is a good, easy way of sealing, if not quite as good as Wills Rings.

The Wills Rings are made from mild steel filled with Argon at 1,000 psi. They sit in grooves in the head (3.0875 inch o/d, 0.098 inch wide, 0.058 inch deep). There is an outer gasket to stop water leaking out from round the block, and this compresses to 17 thou, which gives 20 thou nip on the rings.
I have found this system 100 per cent reliable, the only slight problems arising when water does seep out past the gasket.

The way to stop leaks is to fit the gasket with a hard setting, jointing compound, such as Green Hermetite or Silicon RTV, and to pull the head down very carefully - I do it in 10 lb/ft steps up to 40 lb/ft.


oo - 00 - OO - 00 - oo

Next month:
in part two of his epic tome on Imp preparation, Dawson moves onto the subject of engines over 998cc, carburation, exhausts and other exciting matters.



The Imp Site
   Imp Anatomy (index)
   Imp technicalities (assorted bits of knowledge)
   Tuners & Builders
      Andy Dawson



© Franka
Version of: 7 Jan. 2013
File started: 2 Jan. 2014