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Special cylinder heads

Power is made in the combustion chamber. With the side valve engine the chamber shape is important as during the induction and exhaust strokes the chamber forms part of the gas route, restrictions etc affect the engines breathing. Also the position of the spark plug affects the speed of combustion. Finally the volume of the chamber sets the compression ratio of the engine, the higher being the better. However these factors don’t combine comfortably in a side valve engine and the design is always a compromise between them, usually to the advantage of two at the cost of the other.

It seems to be accepted that a spark plug located over the exhaust valve will give the best combustion but somewhere between the centre of the chamber and over the exhaust valve will be a good compromise. Although I’ve seen a few un-machined castings about, this parameter is normally set by the original design and casting of the head. Pat Stephens in ‘Building and racing my 750’ asked Colin Chapman of Lotus to machine his spark plug position in a raw casting as close to the exhaust valve as the casting would allow.

 

For the other two factors, breathing and compression there seems to be two camps. One suggests that breathing is everything and compression should be secondary. Not surprisingly the other camp goes for a reasonable compression keeping the combustion chamber as tight and as small as possible.

With a both pre and post war racing history not surprisingly there was quite a bit of development of Austin Sevens’ cylinder heads. Other side valve engines had a similar problem and there was much cross fertilisation of ideas. Jack French suggested we look at a modern side valve to see what the latest thinking on design is; good idea, but my lawn mower is reliable (currently) and I don’t want to take it to bits just to look. Still, I can see where the plug is positioned and, being air cooled, can infer some of the shape inside. Many racing engine builders just plumped for a certain special head—perhaps the one they already just happened to have, but there were some discussions at the time on what shapes, compression ratios etc to use. Of course a road special may need slightly different things to a racing A7 as well. Books and articles worth a read are;

· 750 Companion—the ‘green’ book. This in the ‘Getting the best from your 7’ section has a few things from the past notably the Jack French Simplicity series (that were also in The Special Builders Guide) and In Sheeps Clothing. Also the Bill Cowley engine tuning principles and the Holly Birkett inlet valves articles suggest ways on how to get the most from the head.

· Not A7 but the Speedsport book, Tuning side valve Fords by Bill Cooper has good information and photographs on this subject for the Ford 10 engine.

· 1989A A7CA magazine had a collection of pictures some of which are copied here where I haven’t my own.

Pre war special heads

Along with the Austin sports heads a number of companies sold go faster goody heads. I’m not sure of the relative chronology of all of these but some of them are definitely pre 1930 and thus could be claimed to be VSCC compatible.

Post war heads—750 Formula

In the early years of the 750 formula many of the heads above were used on the traditionally styled cars with varying degrees of success. Some new aluminium castings must have been available, the Lotus 3 and Pat Stephens Stoneham special used a special head in the early fifties. The growth in popularity of special building and the racing formula brought specialist companies into the market developing new heads as part of their range of goodies. Cambridge Engineering run by Bill Williams had been in existence pre war and their early head may have started then. They were followed by Sporting Motoring agency—Dante, Speedex, Super Accessories—Supaloy as well as a number of smaller lesser known concerns into the sixties.

Looking at some typical examples and what they were intended for, the breathing heads are predominantly intended for a racing engine that revs to 5 or 6+ thou rpm. Breathing is needed to get some air with petrol into the engine. There’s no point in having a high compression if there’s nothing to compress. Compression heads work well on a slower revving engine that doesn’t need such good breathing. So, in simple terms racing engines work well with a breathing head and a road engine gets the best from a compression head.

In the mid to late sixties the final developments of the side valve head were made on the Seven and its successor the side valve Reliant 750 engines. Final stages were two piece heads to make easy manufacture, see similar examples below. Others machined the bottom half of a cylinder head including the combustion chamber from a slab of 5/8” aluminium, made a lid and then glued the whole lot together with four sides and two tubes of Araldite. Low drag cars needed engines inclined over, the heads had water outlets on the side to match. The ultimate A7 750 was probably the Cowley developed and raced in the late sixties and early seventies. This not only had an eight port block with inclined inlet ports but a twin plug head that probably is the optimum side valve design. Note how this twin plug has them both over the valves—the works head had them across the chamber, see above. Reprinted with permission from the 750 MC Bulletin.

Developments ‘down under’

Similar special building and racing was going on in Australia and New Zealand. Heads were also developed along the same lines. I can’t add any more to the history of these various heads, I’ve included them for interest and completeness. Some do illustrate the split head with a separate top and bottom. This design simplified casting as no cores were needed. All bar the Ludgate, which is definitely fifties and sixties, they seem to be based on the earlier gasket shape and may therefore be pre war.

Cylinder head miscellaneous

· In terms of how many were sold or which ones you’re more likely to come across; the Supaloy and Speedex heads seem the most numerous now. After that the Dante and Cambridge heads still keep popping up with an occasional Alta or LRM. The original pre war heads, Coldwell, etc seem very scarce.

· Your compression ratio increases by approximately 0.1 for every 0.010” taken from the head face.

· Standard thickness of the normal A7 cast iron heads is 1.500” measure how thick yours is before grinding it—it may have been done previously or you could select a higher compression one if you have a few to choose from.

· There seems to be a traditional opinion that head gaskets can be reused. It’s never worked for me, they’re usually a bit distorted when you get them off and I’d rather not take the chance. OK maybe to get you home.

· Use of a torque wrench is not something often seen with Austin Seven’s. That’s surprising as it is the smaller threads prevalent on the A7 that are prone to over tightening. I would recommend that a controlled tighten is performed on the head studs.

· Make sure with an aluminium head you’ve got some good thick washers under the nuts to spread the load into the head.

· Start off by hand and gently tighten, pulling a short spanner with just the end of your little finger, then torque in the correct sequence to 10 ft.lb, 15, then 20 and 20 again. Let things settle into place for a while and then 20 again. Keep doing this until none of the nuts move further. Note that over tightening doesn’t help make a better seal, it just stretches the studs making them more liable to fail and locally distorts the head around the studs rather than pulling it down squarely.

· Head nuts should be followed up once the engine has been run up to working temperature. Try to do this hot for a cast iron head but for an aluminium one let it fully cool and retighten.

· All these heads may require different spark plugs. Watch out if the plug is located over the valves, make sure it has good clearance at full valve lift. Normal reach plugs are preferable as extended nose ones obviously need more clearance—more clearance gives less compression. However avoid retracted racing plugs, the heat ranges rarely work consistently. See the page on plugs.

· One thing to watch out for with the later 1937—39 cast iron head and similar aluminium heads. The two front studs (6 & 13) also retain the aluminium water outlet. A number of years ago rubber gaskets were available for this like the side water branch ones. The two studs have to pull down the head, they can’t do that with a soft gasket in the sandwich. You end up either pulling it down hard enough but extruding the gasket or not tightening it enough. Only use a very thin paper gasket with some of your favourite gasket sealant (Hylomar in my case).

· Head nuts can be hard to tighten with the ribbed or finned types of head as the cast ribs and water branch interfere with the spanner or socket. I have found the Austin ‘barrel nuts’ normally holding the block down make good head nuts as they lift the hex away from the ribs.

· When fitting the gasket, they rarely drop into place, don’t try and work it down onto the block with your fingers, it’s easy to bend or distort it. Enter it over the studs and then use the head to push everything into place in one flat movement.

· Before fitting the head very lightly countersink the stud holes so that if the gasket edge does ride up slightly its got somewhere to go without holding up the head. The threaded stud holes in the block also benefit from countersinking.

· Speedex and Supaloy heads used a special water branch based on the standard Austin side branch. This was recast to allow the water to come straight up and out of the head rearwards towards a bulkhead mounted header tank. The branch shown in the centre one of the picture below is a Supaloy one—it comes straight up. The Speedex one comes up and bends slightly to the left, note the Speedex logo cast into it. The third is a standard side branch, a ribbed Reliant one in this case. SPEEDEX were the first with the reversed water branch. Raeburn’s used to advertise replacement Supaloy branches if you haven’t got one—many have lost theirs.

· Finally here’s the tightening sequence—don’t overtighten!

Another good source is information from the USA. Searching the web on ‘tuning side valves’ brings back very little. Once I’d got the terminology right — ’hopping up flat heads’ - there was quite a bit on 45 Harley’s and the Ford V8 flathead. There’s a few books on Ford V8 tuning (search Amazon with ‘flathead’), the one that seems to me the best to extrapolate into the A7 is ‘Flathead Fever’ by Mike Davison (Australian not American). Being for Hot Rods they do tend to use shiny new proprietary parts — but there is information that helps with A7 ideas. The V8 has domed pistons, valves at 5 degrees to the bore centre line and they are pocketed into the block. The engine is tuned in the same way to maximise breathing and compression.