Cylinder & Piston Tolerance / Wossner Piston Kit

My cylinder ('82 250) needs a 2nd bore up to 70.5mm. In looking at piston kits, I noticed Wossner's next suitable kit has an apparent piston diameter of 70.44mm, that leaves a gap of 0.06mm, which seems too narrow?

The piston that I pulled stated 69.68 on a 70mm bore, which is a 0.32mm gap.

Is there a minimum gap requirement?

Am I missing something on the Wossner's, or should I just go with Wiesco and not worry about it?

.06mm= .0023" make it .0025 and you be fine. .32mm =.0126" I think you misread the top of your piston

I just put a Wossner in my 84 250 and finished honed it for a .0025 fit. The newer alloys dont expand as much as the older ones and the Wossner is coated also.

hopefully this helps,

For the 430 and 500 which start out with an 86.00mm bore and a 85.92 piston. Oversizes are in half millimeter and the .08mm difference would remain necessary to add to the number on the piston to reach a sensible number.

The Mahle original pistons are cast both the Woosner and the Wiseco are forged. Forged generally are assumed to expand with heat more. I have not measured and warmed any pistons. It might not be a bad idea to buy the piston from a place that bores the cylinder and understands how much space you need in order to avoid seizure. I suspect if you bore as if it was an original one you stand a good chance of quite a significant seizure. Or smearing of the piston material if it doesn't seize to a stop.

I think something is screwy with what you took out of your engine. The owners manual might not tell you how loose to bore it but has a picture sticking feeler gauges between the skirt and the wall of the cylinder and I suspect what you are posting is like a factor of 10 too big. At least with the 86mm ones the numbers on top are the same mahle, wiseco, woosner.

Original Piston??

Here's a picture of the top of the piston. I just assumed the stamping on the cylinder was the diameter as I've seen pistons with such stamping.

However, in measuring it at the skirt, although without quality calipers, it looks more like around 69.5mm.

Can you guys tell if this is the original piston? The guy that I purchased it from said it was orignal bore, but I was skeptical. Does the number stamping 69,68 (or maybe 89,69) mean anything? It also has a stamping N2.

Thanks,
Adam

Thanks for confirming the piston details Huskydogg!

So now I just need to insure that the bore is .0025'' over whatever the piston measures, Wiesco or Wossner.

Thanks

Adam

i found that over sizing just a bit with forged pistons keeps from light seizing.
had a 400CR at one time that was super fast, until it got hot, and everything stopped.
let it cool down...she would fire right back up.
had a Wiseco piston, we switched to a standard Mahle piston, and a new ring, worked awesome, wasnt as plistering fast,,,but still fast enough.