Just wondering whether, with small mods, lightweight 250 pistons, if available, could serve?
I say this because . . . about 15 years ago I acquired a Model 31 fitted with a G9 engine which I discovered had been over-bored and was running on a pair of ALCAM 15125 pistons at 70.5mm. The late Rik Edwards kindly identified these as +20 7.8:1 lightweight items. (The M31 motor came with it as a pile of bits, some badly broken, in a separate box.)
Rik's info came in the context of a long and convoluted thread back in 2008 (the complexity of which was all my fault as I was wallowing in ignorance while being educated by Rik, by Don Madden and by Ken Bryant as I tried to figure out what I had):
viewtopic.php?t=4421
What I can say though is that the G9 - complete with the annular grooves on the cylinder liner exteriors (not found on the G9B) - had been bored to that 70.5mm. Although it ran like an absolute rocket when I finally got the thing together, I 'd been so taken aback by what I'd seen re the thinness of those cyl liners that I rebuilt the motor back to a standard 66mm.
The key point though, is that Don M reckoned G9Bs could be rebored owing to the 'solid' liners and machined channels in the crankcase to get the oil up to the top end, reversing the stock G9 layout.
So, given there's enough metal, I would have thought it possible to resleeve and take the bores out to the lightweight's 69.8mm or more even, and then carry out any minor mods to the pistons.
The pic here shows the +20 pistons I was running on, which are in excellent order - if they would be any use (to anyone), they aren't ever going to get used round here because I subsequently rebuilt the bike in question one more time, using the correct 650 motor when the new Club crankshafts became available. The rebuilt-to-standard G9 is my ready-to-go spare, capable of serving temporarily in either my Mod 20 or Mod 31.
PIstons AMC 70.5mm.jpg