Dig out my old frame that I made to hold the engine, - whip off the cylinder head and there it is - a blown gasket. I did notice while removing cylinder head nuts that one didnt feel particularly tight, so wonder if that was the root cause of the blow.
Anyway, remove chaincase side as i have bought a set of nylon chain guides to replace the old original ones. ( noticed when running that you can hear grating sound that possibly is the chain rubbing against the old coated metal ones that were well worn) downside is that the chain has to come off before I can remove the old metal guides, which means removing front or rear drive, - opt for the clutch removal, can have a peek at clutch plate wear while out.
eventually, job done -
- would have preferred black or blue though!
Have bought in a 185 cylinder conversion kit, so thats the next job when I get round to it.