Piston problems Model C

[M.Bagshaw]

Rebuilt bike a couple of years ago, set of this morning on a run 18 miles in piston nipped up! Been running great, plenty of oil?

[JohnnyBeckett]

is it Runing to rich and washing the oil off the boor or is the timeing out

[brenton.roy]

The pitting on the crown suggests it was burning really hot. Lean?
It's not easy to jet the mains as we tend not to ride at big throttle openings.

[nevhunter]

Insufficient clearance It looks like. Perhaps the day was a bit warmer or your riding a bit more enthusiastic than normal. It takes thousands of miles to wear a bit of extra clearance on the piston and sometimes a piston will "grow " slightly. Don't file off the "heavily"dragged areas, they will already be undersize most likely. It's also a good idea to relieve under the lower ring and drill a few holes at an angle through the skirt. (NOT in the ring groove). A bit of two stroke in the fuel, doesn't hurt either. An egg cup to a tank of fuel is enough. Nev

[john.whiting]

Insufficient,or deficient cam relief,if you ask me .....in fact it looks like the piston is just plain round ,with no taper,and no cam.....there was similar one pictured a few weeks ago,and I thought the same thing then........if you are going to use a round piston,give it plenty of clearance...like .006-.008........are both sides the same?.....the rod may be bossing in the piston and forcing it over.....ie crank not centred.

[M.Bagshaw]

Thanks for your suggestions, but its none of the above, with the exception of perhaps being a little too enthusiastic!
Barrel being lightly honed then its back together for another go.