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JimH, we never used an overbore cylinder on any H2R factory engine, that would have put us over the AMA size limit for the engine.
That said, we also had numerous issues with pistons and cylinders when we were forced to go to street cylinders, from the special, altered stud first H2R cylinders, but, the worst issue was the forged pistons. We had the factory forged H2R pistons literally collapse a few thou on the first warm up in the pits, and more on the first couple of laps on new pistons. The skirts literally had not enough support, and bent inwards, making for excessive clearance, which allowed more rock over in the bore, beating the piston up even more.
This would then allow flexing of the wrist pin boss area of the piston, eventually cracking the pistons, and pulling the pin bosses out of them, and, whammo, crash, bang, disaster.
We'd have to set the piston clearance to .003 to begin, and it got worse real fast. To make things worse in early 1973, the factory determined that it was wrist pin flex that was causing the pistons to frag themselves, and literally went form 16mm to 18mm in diameters, on the same 16mm forged pistons. Things got seriously worse, in one fast heart beat.
Finally, the factory got its act together, and asked Ford for assistance, came up with a centrifugal spin cast piston, auto-thermic and austinitic, with serious ribs on the skirts and more strength in the pin bosses, which we installed with .001 clearance. Those pistons worked well, didn't collapse, didn't have rock over problems, and lived long,and prospered, didn't tear up rings like the forged pistons did.
This was basically what racers of all sorts had with Yamaha pistons form the TD series engines, YDS derived, to the RD derived TZ, forged pistons in the TD, cast in the TZ's. We finally paid ART pistons (Azumi Rendering and Turning) to build a few sets of cast pistons, and, the factory finally saw the light.
I have never seen, nor worked with a Wossner forged piston, but, I know I wouldn't want it in one of my engines if I couldn't set it up as tight as possible, like right down to .00125/.0015. Big piston clearances in two strokes are just begging for ring and/or piston problems, with adverse cylinder issues as well.
On my almost Laguna Seca Yvon H2R spec street H2A, I ran the same set of pistons and rings for 20K, before the bike got stolen, and had no ring/piston/cylinder issues. I pulled it down once in all those miles, to de-carbon the ports, didn't need anything else, and, it used stock cast H2 street pistons and rings, two per cylinder.
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The Wossners are a work of art......
