Piston ID photos

[Superflash]

Let's try this...

[Superflash]

And this

[fffcycles]

Ah yes, the old casting number game.  BSA were very much enamoured of having a number marked or cast into an item then it would get machined and become another part number. It looks like it is closely related to 68-906(?) perhaps?? I amongst many others have never quite figured out how this ended up working for anybody.
Frank
ps the real part number is possibly inside the skirt of the piston or inside surface of the crown sometimes.

[Swarfcut]

 This link may help

 https://www.britishbikebits.com/hepolite-branded-piston-ass-std-9-1-bsa-a65

 and another....

  https://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=623795
 

 Frank's almost got the measure of the BSA Parts system. Casting numbers are sometimes the same as the part number, but vary by one or two digits when that casting becomes part of a sub assembly, pistons with rings, circlips and pins being a typical example. Gearbox covers supplied with bushes, etc are another case.

 Numbers and codes inside the piston identify the parent casting in manufacture, which is then machined to the appropriate standard or oversize. Crown makings are the best guide.

 BSA  Model type can be established with some certainty from the part number prefix.  67 for early A's, 42 for S/A, 68 for Unit A's.  The other model ranges have their own prefixes. This is fine until confusion ensues where a part is common across the whole range and this part has a number completely different from all the others because it started out on a different model range. Think A65 still using BSW threaded studs from a '47 Longstroke....

 I reckon I've got BSA parts sorted......Pre Unit Triumphs are another law to themselves!!!

 Swarfy.

[Superflash]

Thanks for the reply. Have done some research and it looks like 907 is the cast number for std size. The M and H are also factory stamps.
This engine has never been apart. The original laquer jointing compound is right through it all.
For its age its in pretty good condition. Cam and followers are pristine. Bores look good.
Bit gungy in places, but have cleaned up well.
Only real concern is the sludge trap and timing bush wear.
But I'll take the crank and cases in to a machine shop here and see if they can get accurate readings.
Cheers