OK, got moving on it again, while I'm awaiting parts on the 750K6 resto project...
We've switched to the CI 605cc pistons. They are a little over 3mm shorter, and have flat tops, so the valve clearance won't be an issue anymore. But, like most of the CI pistons I have used, a little hand-finishing is in order.
The bottom edges of the piston skirts on these pistons are frequently as-cast rough. This will allow crud to attach well to it over time, increasing piston weight and possibly disturbing the wall-to-piston-skirt oil transfer during the downstrokes. Also, all edges on the CI pistons are usually very sharp (i.e., not "broken"), which cause stress risers and heat concentrations at those sites. With mild tuning this isn't an issue, but I clean them up anyway. To do this: hand-sand the bottom edges of the skirts, in between the slippers, and touch off the sharp edges everywhere else. DON'T sand the skirts or piston surfaces that contact the bores. I also break the sharp edges of the oil-feed holes for the wristpins, and the edges of the piston-pin holes on the outside surfaces of the piston.
The wristpin clips for the CI pistons are the fancy ones with the upright ends that allow easy removal (someday) of the clips. While handy, they are also more easily dislodged should the piston encounter a shock one day (i.e., momentary valve float, burnout-style clutch drop, big road bump...), so I don't use them on engines that will be running high RPM a lot. The original clips will stay in their grooves better, so those type will replace these, here.
Pix of the pistons, along with the original GEX piston attempt(s), and a set of CI 0.25mm 750 pistons, are included below. The picture with 4 rows of pistons includes the original CB500 pistons form this engine as well.
