Hi All,
I've been given a new but badly stored non-BSA 83.5mm M21 piston, the rings are scrap and the crown looks like the surface of the moon.
I'm trying to identify it so I can see if rings are available. There are no part numbers stamped on it but the number 2336 is cast under the crown.
The ring thicknesses are top to bottom
2.47mm
2.49 mm (undercut)
3.97mm (oil ring)
I think the oil ring land is the biggest clue to the origin of the piston. Most pistons I've seen have had a few small holes drilled to allow the oil to pass, this has a massive slot cut in both the front and back.
So anyone know who made this piston, hepolite? or some other after market supplier.
Finally, the skirt is in unused condition but the crown, is it junk or useable even with the pitting which reminds me of holing pistons on an RD400 many years ago.
The crown, although it looks bad, is entirely serviceable...A picture of the gudgeon pin taken looking at the end would help to identify the piston...Did you get a pin and circlips with it?...If not you will have to examine the circlip grooves closely to ensure you use the correct type (flat form or wire) and ideally, pick up a BSA gudgeon pin...
I'm going away tomorrow for approx. 3 weeks and don't have sufficient time to make ring thickness comparisons with original BSA rings...However, it is quite possible I have an original set of BSA +.060" rings...Good luck with your hunt in the meantime..I'll catch up with the posts when I return home...Ian
The crown, although it looks bad, is entirely serviceable...A picture of the gudgeon pin taken looking at the end would help to identify the piston...Did you get a pin and circlips with it?...If not you will have to examine the circlip grooves closely to ensure you use the correct type (flat form or wire) and ideally, pick up a BSA gudgeon pin...
I'm going away tomorrow for approx. 3 weeks and don't have sufficient time to make ring thickness comparisons with original BSA rings...However, it is quite possible I have an original set of BSA +.060" rings...Good luck with your hunt in the meantime..I'll catch up with the posts when I return home...Ian
Hi Ian,
thanks for the reply. No pin or circlips but the pin from the old oem piston is a perfect fit. Here's a pic of the piston from the side.
https://ibb.co/kkPPZc0
Thanks again!.
T
It's probably been due a rebore for 50 years so I don't think another 3 weeks will make much difference!
