I've got my head of the A50.
At TDC there is quite a bit of movement when I move the tops of the pistons but at the bottom of the barrel the pistons are quite firm with little movement. The bore looks good and with my calipers measures 65.5 to 65.65 across the bore so am assuming it is still standard bore.
One valve guide ( left exhaust ) is quite worn and will need replacing but otherwise all looks OK.
Further to the original post, I pulled the barrel and measured the pistons and bore lower down. I think they are still within spec so may take some rings off and check them in the bore.
However taking the barrel off probably revealed the source of my engine noise that no one has yet explained. Big ends fine, little ends fine, camshaft and followers fine, but play when I really try to lift the crank so am assuming the dreaded timing side bush is bo**oxed.
So now the engine has to come out!
in that case ,, false economy to leave the bores. good luck, I picked up a pair of oem pistons +.020" on the bay for less than £100.
My A50 was standard not much play but one ring in halves, so as the motor and complete bike is apart Ive gone the whole hog , bearings, bores ends, etc
the unfortunate thing is the 650's are well catered for for over size pistons, the 500's are harder to find.
I will get someone with better measuring equipment to check pistons and bore sizes.
Just a shame the amount of work needed to get the engine out on my own. Over the next few days will be removing as much as possible before trying to get the crankcases etc out. No mad rush, got a lot of tidying up in the garden when weather ok,!