Ariel Owners Motorcycle Club

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?

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

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.

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

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.

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.

it also happened to me with my original piston. I couldn't find the cause. I sanded the damaged area, and the error did not happen again I think I adjusted the mixture in the carburetor a bit

A wise man once told me that original Black Ariel pistons had a habit of expanding, but not contracting. His advice if it nipped up was to clean up the high spots on the piston (file not emery) and try again.

What looks like high spots has become low spots due seizing and shrinking the metal. Check with bearing blue or a micrometer. Most just hone a bit out and try again. It makes the ring gap wider though.. Filing a piston skirt all over takes skill and time. It was more common with two strokes that have NO ovality or crankcase compression disappears . Nev

OK so I not had much experience with ariels but from my experience rebuild other engines that's got hot very hot even the grudgion pin kissing been the bore through expansion. Most likely you were running a bit lean.