Compression ratio ?

[Davemate]

1500 lump
Please excuse the dumb question
If a 1500 head is skimmed 40 or maybe 60 thou will the compression be to high
and if it is, can it cause a piston to "hole".
Is the compression chamber purely in the head and if so how deep
should it be and what's the min depth you can get away with on a standard(ish) engine

The reason I am asking is that I have got hold of a unleaded head,but I have been told it may have been over skimmed and that was why the engine it came from "holed a piston"
Using a ruler I have measured the depth of a 1500 and a 1300 head chamber
and my head is smack in the middle so it is a 1500 but it must have had a
heavy skim

[andy thompson]

To much compression combined with too much ignition advance can create detonation which WILL hole a piston

There are several factors contributing to the overall static compression ratio - apart from the obvious - ie Bore and Stroke

here is a spreadsheet to help with working out your CR - if using a 18/58 camshaft you don't want more than about 9:1 CR - Spit Mk3 profile will cope with 9.5 - even 10:1 but you need to make sure that you don't have to much ignition advance

1) Combustion chamber volume with valves fitted - measure with burette or syringe using parafin and perspex / vaseline
2) compressed headgasket volume - about 0.035" (0.89mm)  if using Payen
3) distance between top of block and top of piston at TDC about 0.025" (0.64mm) on most Triumphs but needs measuring - decked blocked may mean this is much less and it has a significant influence on CR
4) piston dish or dome. None on standard 1500 - USA LC engines had a dish - not sure on the CC http://www.rimmerbros.co.uk/Item--i-155167