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Any thoughts on this (mystery to me) cylinder head?

Cast iron
Deeper than normal sparking plug wells
Differently positioned core plugs
Far from standard combustion chambers

?????????????

Mike

Mike,

what numbers are cast into the top of the head, difficult to see in picture??

Ruairidh

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Ruairidh

Thanks for your interest.

Unfortunately the machining of the top face has removed most of the characters. The best I can come up with - and this is prone to considerable guesswork - is ............

M653?1
?? ?1?4

Mike

Mike,

Have you tried measuring the combustion chamber volume so you can calculate compression ratio?

Fit a sparkplug set the head level and fill with water measured from a measuring cylinder / burette.

Comparing this to a standard head of similar date will give you some idea what its meant for. A lower compression ratio and i would hazard a guess at it being a blown engine head. If its higher then some kind of unblown sports. Is the combustion chamber shaped to clear high lift valves?

More importantly if you bolt it on your car what does it go like?

I had a look at a blown ulster head a while back, the deep recesses round the plugs were similar but it didnt have the brass plugs.

Regards

Rob

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Rob,

I hadn’t tried measuring, but I have now – good game, good game!!

Having furtled around with a measuring glass, an eye dropper, and a spirit level, and then giving due regard to surface tension and wetting out, I came to the conclusion that a standard head was 41cc whilst the ‘mystery’ head was 42cc. Thus both about the same allowing for experimental error. By my calculation that is a compression ratio of approximately 5.5:1.

Measuring the valve space there is a marked difference in depth. Standard is 0.384” whilst the mystery is 0.738”.

Not in a position to bolt it onto my Chummy as the head is not mine

Thanks for your thoughts.

Mike

When I first looked at the head it was obvious that the combustion chambers were sort of half way between a high compression head and a low compression head. On a low compression head the combustion chamber cutaway covers the whole of the cylinder beneath, with a high compression head half of the cylinder cover has no cutaway into the head. I hope you are following me so far !

With Mikes head the spark plugs are in a similar position as the standard low compression head ( over the cylinder ) but the head has no cutaway on the side away from the valves. My guess is that the head has been modified ( at the factory ) to have deeper cutaway in the head for high lift valves but they have maintained the relatively low compression ratio by filling in the head chamber on the opposite side.

Why would they want to do this. If you look at the outside of the head it has the brass blanking plugs where the priming cocks were originally fitted on the very early engines. This would make the head somewhere in the region of 1927 1928 and therefore the crankshaft would be 1 1/8".

My guess is that the head was produced to permit high lift valves, keeping the low compression for use on the 1 1/8 crank ( which may not have liked a high compression head ! ) possibly as an experiment at the factory for competition work.

On the other hand I could be totally wrong !

If you are intending to measure the volume as suggested by Rob, you may have trouble with the meniscus in getting an accurate figure. Tip:- Clamp a piece of clear perspex, with two 6mm holes in it over the upturned head. Set the head dead level. Fill the burette with paraffin, which has a much lower surface tension, and fill through one of the holes. The other hole is to let the air out and, eventually, to help chase out the last bubble.
Ron

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Ron,

Thanks for the tip. Actually I had another go using Turps Substitute as there was a bottle handy. It gave more or less the same result but as you say the meniscus is much less with the spirit, thus taking away some of the guess work.

Mike

I have come across an identical cylinder head which has SP 702 cast on top where the usual XL or 1A numbers are situated. Is this any help?