and the price of new condrods & pistons for the 440? Factory 400cc pistons are about 30quid i think

Jon - I`ve been running a mildly tuned 444 engine in my NC for the last 10 years - 70bhp, lots of midrange, very nice to ride. No reliability problems but I was lucky that the liners were dead concentric in my engine so I didn`t get any breakthrough at the top of the bores which is what often happens. Also those JE pistons are pretty heavy so you need to avoid being rev-happy if you want the bottom end to stay together. The 433 kit has fewer potential issues.

Gringo - You use the stock rods but the pistons run out about £500 a set (plus boring work).

These are the the readily available (from TTS) parts I`m talking about - if you want a properly designed and built big bore engine go look at Mike Norman`s G-force site... (and take out a mortgage!)

This is an old post but going back to original problem

Scored bottom end bearing shells - usually caused by contaminant in oil. hard particles. If your head gasket was leaking and the bike was run on coolant with silica(sand) content ie car coolant, this could caused this problem if is carried over in piston blowby.