I am now onto my fourth piston with this bike, any ideas what the problem could be? Started off with 70cc of oil to 5 gals of petrol (what I run in my gas gas) and am now up to 120cc of oil per 5 gals. Luckily piston and rings are only

Is 70 cc the same as 70ml if so you should be using approx 70ml yo 5 litres not gallons

Sorry, I did mean litres not gallons.

Have you been checking the piston ring gaps down the bore before fitting?

Is the piston being fitted correctly? (Arrow to the front)

Have you recently had a rebore and haven't had the ports gone over with a riffler file to take off the sharp edges?

Have the ports got sharp edges anyway?

What is the piston/bore clearance measured at the piston inlet skirt mid way down the bore. Look for approx 1.5 thou here.

Thats about all I can think of for now.

Bye, PeterB.

Is it air cooles cast iron bore?

Have you been checking the piston ring gaps down the bore before fitting?

Is the piston being fitted correctly? (Arrow to the front)

Have you recently had a rebore and haven't had the ports gone over with a riffler file to take off the sharp edges?

Have the ports got sharp edges anyway?

What is the piston/bore clearance measured at the piston inlet skirt mid way down the bore. Look for approx 1.5 thou here.

Thats about all I can think of for now.

Bye, PeterB.

If the rings are actually breaking, which is unusual, I'd look at the champhering on the ports, especially the exhaust, which is generally the widest one (and the area that heats the rings the most). You'll want to make the angle of the cut at about 15 degrees and widest at the centers (top and bottom) of the port (maybe about 1.5mm), tapering down as you reach the sides. A lot of mechanics will just "break the edge" of the port sides which will usually end up with a 30-45 degree cut, but the idea is to ease the ring back into the land and avoid dangerous harmonics in the process as they can contribute to ring breakage.

Jon