Roger, again thank you for your insights. Without opening a separate thread, could give a (short) answer what difference in machining you suspect?
Martin
I have a vague recollection that 3.8 heads had a larger diameter combustion chamber to match the larger bore, but am prepared to be corrected on that.
No they did not change it, I have both 3.4 and a 3.8 engine.
Even on the 4.2 with quite a difference offset of the cylinders, they did not change it.
2 Likes
Sorry for delay in responding Martin, but I was looking for photos and detail I had done, but cant find…
So top of my head and incomplete…
C12500 was the first ‘B’ type head, and was only fitted to 3.4 litre blocks.
C12600 was the first ‘straight port head as fitted to only 3.4 litre blocks fitted to early XK150S’
C13799 was the second ‘B’ type head, and was only fitted to 3.8 litre blocks fitted to early Mark IX.
C14956 was third ‘B’ type head, and fitted to only to 3.4 litre blocks.
C14957 was the second ‘straight port head’ and was fitted to both 3.4 and 3.8 blocks
C14958 was the fourth ‘B’ type head, and fitted only to 3.8 litre blocks - from 1st standard 3.8 XK150, Mark IX and 1st 3.8 Mark 2
So there was a rationalization of part numbers/application interchangeability, with examples of early C.14956 castings being used for C.14956 applications, merely by deleting the cast 6 last number, stamping an ‘8’ over it, and I think machining a bevel around the circumference of the combustion chamber - see pic showing the C.14956 overstamped 8, and bevel on combustion chamber
A huge more work to be done on this, and indeed reliable photos of both C.14956 and C.14958 heads, and advise of associated Engine No…
Hello Peter,
I also have a 3.4 and 3.8 head. There is definitely a difference in combustion diameter- wider camfer to optimize the burning front. If the casting is different I would have to look. The bigger camfer is machining related and clearly visible in comparison.
Best regards Thomas