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.
This sound like the 7.5 piston in my B33.
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
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!
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.