BSA B33 Piston

[Cardiboy1]

Opening up my 1959 B33 for a top-end overhaul (-see my other post) I was surprised to find a very sophisticated looking piston in there. Fortunately it is in very good condition, I say fortunately because I see that new ones cost upwards of £150 !
It is a cut-off dome crown, with a split skirt. The band of slope around the dome is about 1/2in wide. There are pronounced valve cutaways.
The crown is marked:-
" 65 1661   BSA(with piled arms logo)   3/55
ID16
C
FRONT "
I can't find '1661' listed in any suppliers websites. There are suggestions that late B33 s had a higher compression than the previous 6.8:1, and old photos I've seen show flat-top pistons only. Does anyone know what my bike's engine is running on? Some of my research suggests that it's a 7.5:1 piston, but that's not definitive. I'm not complaining! It looks like this might be a beneficial change to spec.

[idie]

This sound like the 7.5 piston in my B33.

[Derek996]

65-1660 is a 7.5:1 piston assembly c/w rings, gudgeon pin and circles. The number 65-1661 is the bare piston only

[Cardiboy1]

Thanks Derek.
That explains why 1660 is listed in the parts book. I have heard it said that the BSA parts numbering system could get a bit arcane at times, but at least in this case it made sense!

[Derek996]

The casting number is usually one digit different from the part number. e.g. The ZB32 alloy head is part number 65-1300 but the number cast into the head is 65-1301.