Weisco Pistond for Rebuild

[1976RD400C]

Probably the only reason to use forged is if you have a modified motor with porting for high rpm, big reeds and carbs, and a light rotor. If you miss a shift or it doesn't go into gear all the way and jumps out at full throttle, the rpms can go so high a cast piston can break.

[m in sc]

I'll be honest, never much of an issue in the triples i had. Just don't care for them in the RDs

[Alain2]

My two 1979 RD400 have Wiseco forged pistons. Both bikes had the cylinders bored to the recommended Wiseco specs. I have limited experience with 2 stroke engines so consider that. My experience is that the pistons will slap until you heat them up, I found that if your tune is rich, or if you don't ride hard enough, the pistons never get hot enough to fill the larger clearance and this could lead to premature wear.

One bike was initially tuned rich and after approximately 3000km was showing clearance of 0.0045" to 0.005", mostly from cylinder wear (front to back and in the lower portion of the cylinders). I honed the cylinders, put new rings, leaned the tune and put another 2000km on them. At this point the bike could only be ridden hot or it would slap. It was smooth as butter on highway when showing +320f CHT. The last 2000km added a little bit of wear to the bottom of the cylinders, front to back. The pistons also added a little bit of wear, anyway at that time I was measuring a total clearance of 0.005-0.006" and decided to stop there for now. Ring gaps were 0.017-0.018" for the top and 0.020-0.021" for the bottom, absolutely no sign of blow by.

Believe it or not, I decided to hone it again (no rebore) and I put new pistons (Wiseco) in these worn out cylinders, I have 2400km on them at the moment, I will open the engine at the end of this summer, unless the engine open itself! It shows 115-120psi compression, no sign of blow by.

The other bike was tuned leaner from the beginning and has 10000km on the pistons, but I seriously think it needs rebore. (Rings were replaced once) This bike is parked at the moment until I decide what to do.