AJS & Matchless Owners Club

Hi, I'm looking at replacing the piston rings on my AJS 600 twin, but can't identify the piston type. There is a size stamped on top of the piston (72.8), but nothing else. The club spares stock rings and states that they are for GPM pistons only, just checking if they are suitable. Also 0.8mm = 31.4961 thou., so would I need +.030 rings? See photo of piston attached, any help would be appreciated.
Cheers Mark.

if its marked as metric then its a pattern +030" ( +0.75mm)
best thing is to measure old rings
if was a genuine it would have BHB on crown

but tha'ts my guess ! , alternatively talk to Cox and turner and they can advise 01935 826816. Email: ian@coxandturner.co.uk.

Mark are there any markings inside the piston?

Thanks for the Cox and Turner contact, i'll contact them tomorrow... What measurement info. would they need from the rings?, never measured any before, only the gap clearance when inside the cylinder.
No markings at all Duncan...

thickness of old rings , and width , that should tell them if its metric and obviously the cylinder size

Cheers