Hey Mark- Perhaps in an effort to re-track this thread away from clutch options, you could elaborate your preference for modifying a cast 750 piston versus purchasing a new forged unit? I know from other posts, you appear to be gun shy over forged units, but with all the guys running them, it seems these issues are no longer prevalent. So is there a performance reason, a reliability reason, or a romantic reason that drives your preference?
Yeah, I am kind of gun-shy from not having the best results (long-term) with forged pistons in these engines. At least a part of that is the type of engines these are: they are made for cast pistons, with the alloys of the cylinders being very close to the alloys of the pistons - for good reason: this matches their thermal characteristics closely. The rings, too, are designed with the steel liners in mind, which is one reason why the 1-piece oil rings outlast the 3-piece types: their thermal masses are similar. The 1-piece oil rings are very light and lose or gain heat more rapidly than the thicker liners, so during the startup cycle these rings wear a bit more than the 1-piece type: this is the primary point where increased wear occurs.
But, back to the pistons: the forged pistons grow and shrink at rates different from the cylinders. For reasons that are not clear to me, the manufacturers of these forged pistons require more than twice as much piston-to-bore clearance than the cast pistons. And, if not clearanced this much, they will stick if the pistons are hotter than the cylinders, seen mostly during the initial startup-to-run-temperature cycle. This seems to infer that the forged pistons do not cool as well as the cast pistons, or else the cast pistons heat up more slowly and do not grow in diameter as much. All this presumes, though, that at running temperatures both types of pistons are running at the same clearance, which is probably near 0.0002". So, this raises the appearance that the forged pistons must grow much more than the cast pistons, because advocates of the forged pistons say they seal up the bores "better than cast pistons", whatever that means?
In practice, this is what I have experienced that makes me shy away from forged pistons in these engines: I have NEVER seen forged pistons from 811/812cc, 825cc, or 836cc versions of these engines that were NOT deeply scarred on their skirts on the front and rear sides where they are largest diameter. On the other hand, I have seen cast pistons with as much as 80k miles come out of these engines with even the original dull luster of their finish still intact, and at worst a barely-perceptible mark on the same skirt surfaces. I have also not seen very long ring life on these forged pistons, which has often been the reason I was inside the engine: 25k miles and the rings were toast, burning oil and low compression. This was usually caused by the debris from the scratched-up skirts getting caught up in the rings, chipping away their edges so they no longer sealed. Twice (that I remember) I was asked to simply re-ring them (honing it first, making even MORE clearance as the result), and the bikes I did this on did not fare well: one did not stop burning oil until the owner simply sold it, the other one would overheat on hot summer days in heavy traffic and stop (with stuck piston(s)) until it cooled down. I don't know the rest of that bike's history, as I moved away from that area soon after.
So...I don't like making engines that don't last for a long time. To that end, I prefer to stick with those parts that will help me do this? If someone is making a race bike, that's sort of a whole different thing and I may (and have) participate if asked. But racing engines' lives are measured in minutes, so lots of things are acceptable in that game that simply aren't when you're 100 miles from Nowhere, in the dark, and hoping to make it to the next town to put it down for the night. And, especially here in the American West, that's barely a trip to the next town.
