Yes, I agree about the temperature difference and that was under active investigation when the piston holed. Whatever was causing it wasn't the obvious stuff - I'd checked that as mentioned above. I was quite specific about piston clearances when I had the cylinder work done. Yamaha say 1.5 thou and that's the figure I gave the engineering co. Not being one to take this on trust I checked it when I got everything back and my measurements came out at between 1.6 and 1.7 thou on both sides. I don't have pro grade measuring equipment so there may have been some slight errors in that but it seemed near enough to what it should be to put my mind at ease. The engine specialist checked it (once a new piston had gone in), came up with 1.8 thou and he was happy with that.

Crank seals were new on both sides. You can't get OE seals (or I couldn't find any) so it has to be aftermarket, but there is a YDS specialist supplier in Scotland and the seals (plus a lot of other bits) came from him. Because of the centre labyrinth seal you have to leak down test the whole engine - you can't do it one side at a time. The seals passed that test. The faulty cylinder is on the primary drive side so oil would be sucked in if the seal was faulty. Often that results in a one cylinder smokescreen but there was no evidence of that.

Exhausts are std - as you can see from the picture - so no half developed expansion boxes setting up obscure pressure harmonics. The inside of the silencers had been cleaned out both chemically and physically - and a messy job it was. I got about a kilo of carbon out using a kind of wire flail I welded up. Baffles are std, cleaned back to bare metal, and most importantly, in there.

Carb balance is a bit more of an issue. It's easy to balance the carbs themselves - the lolly stick method works well and I've used it for years (decades) on two strokes. You can't really use vacuum gauges on these old two strokes. However that's the idle balance. Keeping that balance when you twist the throttle has (and is) proving more tricky. The cable is a one into three via a junction box type and is inconsistent. I have three cables at present - the OE Yamaha one that was on the bike, a specialist co (Venhill) custom replacement and a DIY one that I soldered together myself. All three of them behave similarly - the carb balance varies depending on cable routing and handlebar position. I'm still messing around with this as it's unacceptable but I have a feeling I'm missing the obvious. While in that area I'm currently looking closely at fuel flow. The carb jets may be std but if there was some restriction before that point the cylinder could still run weak. So everything from the tank filter to the float chamber is being tested. Similarly the oil flow from the tank to the pump. No point having a good pump if the oil feed isn't fast enough.

I'm going to resolve the cable issue one way or another as carb balance does need to be maintained, and then it's going to be gently gently to get a few more miles on it before it goes on the dyno. That hopefully should identify any fueling weaknesses and optimise the ignition settings. In the meantime I've been trying to get some luggage capacity sorted out and spent a happy few days (!) welding up a custom fit luggage rack. I just hope I get a chance to use it.