Piston mystery

[handi_andi]

Brian

Yes sorry I got my unders and overs confused. Have taken your advice and orderred a plastigauge as well as a micrometer set so I can more accurately check the sizes of the crankpins. Will post the results when I have them, as have a little bird in my head saying 'measure everything, measure everything' thanks to a fellow forum member, cheers!!!

Thanks

Andy

[rgh0]

Photos of pistons look like knock damage to me. The high cylinder pressures during knock cause overloading and fatigue failures of the piston ring lands. Have a close look at the fracture surfaces to see if you can see the classic wave markings of a progressive fatigue failure through crack growth. If you crack test the other pistons you may find more cracks that have started but not yet progressed to failure. Unbalance unlikely to damage piston ring lands as shown. However it could just be age and / or poor piston quality as it appears the engine has had a fair history of questionable quality repairs

The missing bits of piston probably made it out the exhaust port without doing damage to the head or remaing piston surface if you can see none once you clean them up.

All the other data so far says a total rebuild is in order with everything checked measured and machined or replaced as needed. Crank journals should be round to within .0001 of inch with any good crank grinding and polishing machine shop.

cheers
Rohan

[handi_andi]

Crankshaft is out of the engine, only front backing plate and oil pump left to take off before measuring and rebuilding can commence. Crankshaft and flywheel are off to Elgin later for grinding and skimming. Have lucked out as it turns out the engine shop in Elgin is run by a man that has been rebuilding twincams for his cars for years and has quite a few spares and a couple of spare engines as well.

Thanks everyone for your help, no doubt there will be more questions as the rebuild continues

Cheers

Andrew

[dlbutler]

My Twin Cam, and another club member's TC as well, had the Distributor advance springs become weak and allow the dizzy to go to full advance at idle. The detonation caused the top rings to break into many pieces. Fortunatly, he pulled his engine before it caused further damage. Unfortunatly, I didn't (his happened after mine). A bit of ring managed to "tunnel out" and made a mess of the cylinder head (repairable). It seems to me that to break the piston would require more than just a little detonation.

Don