Hi ,can anyone please identify this piston,it came whith a small fin barrel, looks to be unused, thanks20260702_144236.jpg20260702_144213.jpg
I don't know for sure, but it looks very similar to a b441 victor piston.
79mm approximately diameter.
Quote from: Steve 80 on 02 Jul, 2026, 19:10 Hi ,can anyone please identify this piston,it came whith a small fin barrel, looks to be unused, thanks
You don't say what bike it's from but "small fin" suggest to me a ZB or BB Gold Star, or maybe an early B31/B33 which had smaller fins than the later models. It looks very like a Hepolite 10.5:1 ZB32GS piston that I have.
BSA B44 441 victor 79mm
Looks like a High Compression Piston, suitable for Methanol Fuel. The road going versions had a slightly 1/4 inch domed crown and the low compression trials version were flat across the crown.
9.5 comp ratio
Quote from: Roy on 04 Jul, 2026, 20:35 9.5 comp ratio
This is a 9.5:1 piston for a B32. The piston in the OP's post has a much higher crown.
Roy is right. It's a B44 piston.
Perhaps when the original poster said " small finned barrel " they were referring to the round barrel used on early bikes as opposed to the later one with square barrel fins.
That's right Kev D and Roy the barrel is small fin apposed to large fin, I'll try and post a photo of barrel later,
Sorry for delay in replying not been on for couple of days.
Quote from: Kev D on 05 Jul, 2026, 18:12 Roy is right. It's a B44 piston.
Perhaps when the original poster said " small finned barrel " they were referring to the round barrel used on early bikes as opposed to the later one with square barrel fins.
If the original poster had said it was for a B44 in the first place he wouldn't have had us all guessing.
Hi, here are a few photos of piston and barrel sizes piston is std ,barrel I don't know maybe b31 b33?
Ring gap is approx 14 thou,
The barrel is either a B31 or a B33 but 78.86mm bore is a strange size. The B31 is 71mm x 88mm bore and stroke while the B33 is 85mm x 88mm. So it's either very bored out B31 barrel or a sleeved own B33. It's common to bore out a B31 to 75mm for Triumph T140 piston but to take it out to nearly 80mm, if that's possible, would leave it very weak I think.
Either way I think that piston will have far too high a compression ratio, especially for an iron engine.
350 b31 bored out ( have done it my self in the past ) you would be wise to measure to OD of casting see how much thickness you have , some casting had more meat than others. Head would ideally benefit machining to suit new bore size and yes need to check compression ratio ,or you be plague with detonation/ pinking and soon have hole in piston , get it right and get Big improvement, Roy
It may be a trick of the camera angles but that bore looks the same size as the OD of the cylinder at the base. As said above, measure carefully all round and look at the depth of any pits in the casting. You have the benefit that its through bolted so its always in compression but if its too thin I think it could warp.
Hi, thanks all for info, it's not that I want to use the barrel and piston just curious as to what the piston was ,sounds more like b44,
Someone must have bored barrel to suit but not used it "no surprise "
I will probably put them up for sale as separate items,
My ride is a 1955 b31,just rebuilt and sorting out a few gremlins as always things to keep you busy with these machines.