Could be a lot of things. it never saw any detonation or lean conditions or wrong timing.However the other possibility is that with a 90mm bore, that is FAR too much bore size, and left you with a thin cylinder wall which probably severely distorted when it got hot, and no piston can run in a distorted bore.
The short stroke engine has much less pin movement and much less side thrust angle, which involve things in the piston design requirement.
I'm just guessing but the short piston shown in the photos looks like it is made for a short stroke engine.Doing a bit more guessing, if I use the 90mm size as the diameter piston and the 398cc capacity I find listed for the Suzuki DRZ that bikes engine would have a 62.6mm stroke (2.46 inches).If this number crunching is right it may be that the 90mm (3.54") stroke the Royal Enfield has is too much for that short piston and is causing the Suzuki piston to rock about the wrist pin (gudgeon) which could create very heavy loads on the front and rear of the piston.These heavy loads could wipe off the bores lubricating oil resulting in the galling shown in the photos.As I say, I'm just guessing but I would expect to see a much longer skirted piston in a engine with the long stroke our RE's have.
As I say, I'm just guessing but I would expect to see a much longer skirted piston in a engine with the long stroke our RE's have.
It's a pretty big stretch to expect a piston designed for a 62mm stroke to perform in an engine with a 90mm stroke.
You guys do have a point there. But my engine is destroked to 84mm. And I had measured the 2 pistons side by side: the JE piston skirt was 53 mm and the DRZ piston skirt was 46 mm. A difference of 6 mm which doesn't seem like much.