Well, that was an interesting afternoon. I got the head and barrels off. right hand piston looks as though it has been overheating, or something fell down there sometime and melted. pulled the barrel, the bores look good, but have not measured yet. Left piston has too much play, can feel it with my hand. The pistons have a slightly concave crown which I have never seen before on later BSA models, is this normal? No BSA part # in the crowns either.
Biggest issue is the right hand big end has way too much play, I suspect it would have have been too long before it let go, so glad I am stripping it down. I should have done this at the start, but you live and learn. I will doubtless be asking more questions as I did deeper.