Hi All, I've decided to buy a new set of rings for my M21,I measured across the piston half way up and it seems to be under 82mm so I thought I'd check the part number and do it that way but the number doesn't tally with my parts book or the dealers websites i've tried. Its clearly a BSA piston with the piled arms logo and the number 66-1039 on the top. Could someone help me identify it please. Thanks, Mike.

email (option): michaelggilbert@btinternet.com

hi Mike,just checked my parts book 1949-1958,piston complete 66-1215
just looked at Russell motors and they list four
std 66-1198
+020 66-1199
+040 66-1200
+060 66-1201
these are complete with pin rings and clips so you will have to pick
hope this helps
cheers Rick

email (option): richardholt@rocketmail.com

Hi Mike, 66-1039 is listed in the 1947 parts list as a standard size piston but without rings, clips and pin. The number for the complete piston in that size is 66-1190...Ian

email (option): ian@wright52.plus.com

Rick, I went on Russells & Draganflys website but no luck with my number, thanks anyway.Ian JACKPOT so its just a STD piston, just one thats pre-dates the present part numbers.I will order a STD set.Many thanks.Mike.

email (option): michaelggilbert@btinternet.com

Hi Mike...The BSA method of using part numbers to identify piston sizes can be problematic if you don't happen to have the right parts list, particularly as the numbers changed over the years and there are also numbers for 'bare' pistons in each size, as in your case...I guess it was a lot easier back then when you just rang one of the 8,000 BSA dealers... ...Ian

email (option): ian@wright52.plus.com