Over the years i have read literally hundreds of people asking why Cosworth specified a maximum head depth of 138.68mm and can they risk going thinner?

Lots of people reply that it is solely to do with compression ratio, and whilst that is certainly an issue, it is not the most important one. In my opinion (Although i am not an engineer) it is also largely to do with structural rigidity, heat distribution and valve seat security in the head.

I very recently had cause to dismantle a YB which has suffered a catastrophic engine failure due to the loss of a valve seat whilst running and it happens to be a very thin head so i can use pictures of it to illustrate teh difference between a new head and a heavily skimmed one. (Not necessarily the cause of a seat or any other type of failure of course, but good for visual education)

Hopefully, these two pictures will tell a thousand words about how to tell if the heads very skimmed or not without measuring it. This will save my tired old fingers.

New unskimmed head direct from Cosworth:





New head cropped around inlet valves:
(Please look at the material to the right of the valve seat)






Over Skimmed head cropped around inlet valves.
(Please look at the LACK OF material to the right of the valve seat)





Actual size of the over skimmed head measured face to face at the front.



As you can see, the skimmed head has very little, if any, material to the right of the valve seats, this means when it gets hot the retaining tension around the valve seat could be somewhat uneven and the hammering of the valve against the seat at high speed could cause it to come free, thus destroying the engine! It also of course means that the compression ratio is higher and the valves are extending further into the cylinder than normal, and they catch pistons as standard as it is if you over rev them.

Hopefully, this will help people in the future when they are buying expensive heads that have been over skimmed by unscrupulous traders because you dont know how skimmed they are until you have seen an unskimmed one as per my pictures. Slight skimming is fine, as Cosworth naturally allow for this, but beware how far you go.

Please, if you know of people who have had such damage recently, dont bring them up in this topic as it may affect any ongoing legal cases if they were to think this was their head...