I was riding my 1970 Thunderbolt last August when it started to run poorly, then had a major loss of power. I coasted it onto a side road where we stopped and called for assistance. My son was following me on his Commando and said there was a spectacular plume of smoke coming from the left exhaust. While waiting for the roadside assistance, we pulled the left plug to find a nickel sized hole in the top of the piston.
We disassembled it about two weeks ago and other than the piston (and some cleanup) the head and cylinder look fine. The piston is a Hepolite marked with 7892-1a and 71 H underneath. The top of the piston has 010 in fairly large text, and STD in smaller letters in the same spot as the 010. It seems like if I could find another of these pistons the repair could be fairly cheap with new rings and gaskets. I haven't found any of the 7892-1A pistons, only the 7892-1B. Anyone have a source for these?
We haven't found a definite cause for this yet but think that maybe the timing advanced too much. It was running the original points ignition. My son bought me a Boyer
ignition for Christmas that is still in the box.
Any help would be appreciated. Thanks for looking.