Just realised I've not posted owt about the KH's pistons for a while.
After a lot of help and advice from the KH register triples board (thanks all), I still had no real idea what happened. So I bought another set of cruzin image pistons, and this time made sure every single clearance was right-not just ring end gap, but side gap and piston/bore clearance. Put a drop of oil in the petrol too for protection while the oil pump primed on start up.
So far, it has done 30 miles very happily. I'm still not entirely sure what happened, my best guess is I over revved it maybe? Anyway, it would appear cruzin image pistons are OK, which is a relief for the economy minded.
Next up, assembling a BMW boxer...
