I’ve hit meltdown
Bob Clare seeks advice after post-rebore problems with his 1974 Triumph T120. First time he opened it up, despite having regularly torqued down the bolts, the head started leaking and he had to bump start it down a hill. A strip revealed the head was slightly warped and there were unexplained scores on the new pistons. Reassembling with a skimmed head and new pistons, there was no compression. Suspecting the head bolts bottoming in their holes, Bob cleaned them out and this time got 100psi, which seemed adequate with new rings not yet bedded in, and the bike fired up and ran well. Once cool, Bob re-torqued the head and went for a ride; 32 miles later, riding uphill, he lost all compression again and this time found that one piston had partly melted with a blued gudgeon pin and scored bore.
“Any Ideas? I‘ve never seen piston failure this bad. The carb to manifold O-rings seem to be a bit thin. Maybe that caused a weak mixture?”
Well, maybe... although provided the flange is properly flat; in cases of carb icing I’ve tried running without an O-ring at all with no trouble and besides, you’d expect ‘pinking’ and spit-backs. I also doubt a leaking head gasket would be enough to demand a bump start unless a bit had actually blown out; even then it should still fire up on one cylinder unless it’s the middle bit. I find tight tappets cause the worst compression leaks and the stock two and four thou Bob’s been using leaves little margin for error –
I prefer four thou inlet and six exhaust. Always check after re-torquing the head, as they may close up.
Bob sent the photo of the piston below and although he says it wasn’t seized, it’s certainly tried it’s best to do that. Thing is, if only one cylinder of a twin seizes, the other may keep the engine turning. Lubrication is unlikely to be at fault if the other side is OK, and the piston crown looks dark enough to make weak mix unlikely, so I would check the ignition timing. If it’s on points, the two sides are independently adjustable –you need to check both are right, not just one – and with electronic ignition bear in mind that issues with charging/battery voltage can interfere with the advance curve.