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[Blown265]

G'day Wheaters
For a standard bike I'd agree, but for any performance engine, maximising its potential and keeping it alive, having those details is the only way I hand over a customers build.
Gday SW
The piston to bore clearance, if sitting on the low end of spec (0.0053 thou) is getting dangerously tight when excess heat is present. And, given an unknown fuel curve, thats the first and most likely culprit.
Hope this helps.
Regards
Paul

[Adrian]

Maybe a 5 speed gearbox would give you a bit more flexibility for getting up those hills if the jump between 3rd and 4th in the 4 speed is a bit steep (unless you already have the close-ratio conversion, of course).

Is that a cast piston or one of the Accralite/Omega forged ones?

A.

[stinkwheel]

It's got the close ratio gearbox. It was sitting at just over 70mph in top when it seized (on a 21t front sprocket, standard engine sprocket), so not lugging, not screaming. They are a comparatively revvy motor though, I try not to ride about in an over-high gear.

It really does look like the seizure was a result of the crack in the piston, not the cause of it and the tapping noise I was assuming was detonation, may not have been, it could have been play between the piston and pin or side-to-side piston slap.

It was an accralite forged piston. Interestingly, I just took delivery of an 8/10 used piston from our hosts this morning, presumably left over from development of their 612 kit (I can't see that they'd have a supply of used forged big-bore pistons from elsewhere). Guess what? It's already been mostly radiused. Just needs a light clean-up and a re-polish, may get a bit of dowel and some polish into a couple of the corners to open them out a bit more.

be interesting to see how it sounds with the new piston fitted. I knocked the timing back a degree just in case.

[Adrian]

It was an accralite forged piston.

Now that IS disappointing, for one of those to have cracked.

A.