BSA A65 PISTONS

[bikerbob]

I am restoring a1963 BSA A65 Star Twin and having removed the head and barrels  the pistons are stamped BSA 68-251 now according to the books they should be  68-250 standard 68-252 first rebore 68-254 second rebore. I cannot find anywhere a record of 68-251 can anyone tell me what these pistons are. Also on the crown is stamped V30  7/62 M I understand that the 7/62M is the month and year that they were made and the M has something to do with boring out of the cylinders. 68-251 is also cast onto the flats on side of the pistons next to  the gudgeon pins.

[L.A.B.]

the pistons are stamped BSA 68-251 now according to the books they should be  68-250 standard 68-252 first rebore 68-254 second rebore. I cannot find anywhere a record of 68-251 can anyone tell me what these pistons are.

68-0250/252/254 are the part numbers for the piston assemblies, (piston, gudgeon pin, rings)  as the pistons were not generally available separately, so the '251' marking could be the number for the bare piston. Or it could be that the 251 pistons are a different compression ratio to the early 7.25:1 pistons?

the M has something to do with boring out of the cylinders.

It was standard procedure to accurately measure the bore sizes after (boring and honing) and then grade them (as L, M or H) according to whether they were found to be very slightly slightly under, slightly over, or dead-on the nominal bore size.

L M or H pistons would then be selected to give the ideal running clearance with a particular set of barrels.
 
So your barrels should also have an 'M' stamp to match the pistons?  




  

[bikerbob]

Thanks for the reply. There is some confusion about compression ratios for the A65 Star Twin I have a BSA instruction book which lists the compresson at 7.25 but a  BSA Workshop manual for the same year lists it as 8.00 also Roy Bacon's book lists the part numbers for the different compression ratios from7.25 upto 11.00 but no mention of 68 0251.

[L.A.B.]

You could try asking at the BSA 'Piled Arms' Pub (BSA forum)?

http://piled-arms.com/pub.html

[MikeN]

You could try asking at the BSA 'Piled Arms' Pub (BSA forum)?

http://piled-arms.com/pub.html

If the A65 is a star twin the compression ratio was 7.25:1 from Jan 62 and in Jun62 raised to 7.5:1 which remained until 1965 when it the star was replaced by the thunderbolt. 0250 0252 and 0254 are the 7.25:1 pistons but what are the 7.5 part numbers

[MikeN]

You could try asking at the BSA 'Piled Arms' Pub (BSA forum)?

http://piled-arms.com/pub.html

If the A65 is a star twin the compression ratio was 7.25:1 from Jan 62 and in Jun62 raised to 7.5:1 which remained until 1965 when it the star was replaced by the thunderbolt. 0250 0252 and 0254 are the 7.25:1 pistons but what are the 7.5 part numbers

Just found the 7.5 part numbers which are0819 0820 and 0821