sunrisemac wrote:Having had a look at your photos I think there may be a couple of points worth considering.
Although the burn colour on the head and piston crown look good, there is no squish band area on the head which seems to align with the position on the piston on the inlet side that appears to have had the seize. At the point of ignition, and just after, when the flame front is at its peak there is nothing to shield and slow the hot gases from flowing down the side of the piston towards the rings, (see the brown colouring at the top of the bore), and overheating that area of the piston which may possibly be causing problems with lubrication and clearances.
When running in IMHO it can be more detrimental to over oil the fuel mix, run at low revs and make the engine labour without much cooling being produced by the fan. Periodically give it a good few revs for a short burst then let it cool down so to get the new surfaces through a series of heat cycles to bed it all in.
Thanks, I do pretty much very revs and throttle position. I never let them labour whether run in or not, just because it's an unpleasant way to ride.
I did however definitely go too fast too soon on this one.
Not too sure what I can do about the head. I could buy a Ron moss head and machine from scratch but this is supposed to be a budget motor, so I'm not keen. You'd think that machining a 150 head for 175 would increase the squish band, but I seem to recall that I bought this as a 150 head...who knows? Just a lump of aluminium off Ebay. Could have been on on NVT for all I know!