ORIGINAL: Mr Cox
To me they appear to be more of a danger device than a safety device. They will not seal properly and this can trick you into increasing the compression so that when/if it fires it is over compressed and that damages the engine.
If plastic seals are so great, then why don't all diesel conversions have them?
Iwhole heartedly agree mate,the time and effort that goes into a lapped contra piston's manufacture is repaid with years of trouble free use.
Ihave an 'O' ring head and the fact that it was supplied with many spares tells me to expect that I will have to use them at some stage.
This will involve pulling the head off engine the engine and maybe disturbing the liner and then having to lap in the piston fit again.
And good point with the plastic seal fooling you into upping the comp - I never thought of it that way.
Cheers.