BSA Superstar piston weight, stroke etc

[vobster]

The piston is surprisingly 265 grams, the same as the 26mm hw77/97's. Comp tube/seal is 28mm.
The port looks like 2.8mmx30m, the stroke looks like its around 81mm.

Can anyone confirm these measurements?

Has anyone played about with the specs?

[bigtoe01]

The piston is surprisingly 265 grams, the same as the 26mm hw77/97's. Comp tube/seal is 28mm.
The port looks like 2.8mmx30m, the stroke looks like its around 81mm.

Can anyone confirm these measurements?

Has anyone played about with the specs?

Just looked at the piston seal...why does it have that raised section in the middle?

If anything I would drop the TP down to 2.6mm, drop 25 to 30g off the piston and get some bearings on it... That said make the TP adjustable by threading it if you can for drilled out grub screws (if its like the RB2 it has a choking screw which needs doing away with)

Depending where the transfer port lies in regard to the piston seal I would profile the seal either like an LGV or an LGU...very efficient design.

[vobster]

If you are referring to the picture on evilbay, my one doesnt have it, its nice and flat.

I've compared the angle of attack of the lip on the lgv seal and the bsa, and while the bsa is shallower its not too bad. It looks like the whole front of the piston rides on the seal, and doesnt seem like the lip is very compressed. I'd say its not bad, if not ideal. If I get a second I'll try to replicate the lgv lip angle, they do make power very well

Rear of the piston is a nice fit in the comp tube, much better than what weihrauch are doing these days! I doubt buttons will do much.

I'll look into reducing the port, which would be doable with a brass rod insert and re drill, but 30mm is a long way to drill straight and true for me!

Do you think knocking 30 grams off the piston without touching the port would be a big no no?

[bigtoe01]

If you are referring to the picture on evilbay, my one doesnt have it, its nice and flat.

I've compared the angle of attack of the lip on the lgv seal and the bsa, and while the bsa is shallower its not too bad. It looks like the whole front of the piston rides on the seal, and doesnt seem like the lip is very compressed. I'd say its not bad, if not ideal. If I get a second I'll try to replicate the lgv lip angle, they do make power very well

Rear of the piston is a nice fit in the comp tube, much better than what weihrauch are doing these days! I doubt buttons will do much.

I'll look into reducing the port, which would be doable with a brass rod insert and re drill, but 30mm is a long way to drill straight and true for me!

Do you think knocking 30 grams off the piston without touching the port would be a big no no?

here is what I know...

Regardless of piston diameter upto 28mm around 80mm stroke is like a kind of sweet spot.
30mm long TP, 2.8mm will border on a low SCR, I would go smaller, if you want tap the last 10mm m4 and use 10mm long m4 grub screws, drill one at 2.5, 2.6, 2.7 and 2.8mm...add a little blue loctite to seal them..this will work near as good as a full sleeve.

If the tolerance is tight on the piston skirt, polish it, use a modern lube

I would have thought you could go lighter piston with the port at 2.8mm BUT you may need to fine tune..., I actually think its to big at 2.8mm with 260g piston, and bordering on to big with a 230g piston.

[vobster]

here is what I know...

Regardless of piston diameter upto 28mm around 80mm stroke is like a kind of sweet spot.
30mm long TP, 2.8mm will border on a low SCR, I would go smaller, if you want tap the last 10mm m4 and use 10mm long m4 grub screws, drill one at 2.5, 2.6, 2.7 and 2.8mm...add a little blue loctite to seal them..this will work near as good as a full sleeve.

If the tolerance is tight on the piston skirt, polish it, use a modern lube

I would have thought you could go lighter piston with the port at 2.8mm BUT you may need to fine tune..., I actually think its to big at 2.8mm with 260g piston, and bordering on to big with a 230g piston.

Thanks Tony, I'll knock the weight off the piston first, and if necessary go down the grub screw route. I'll try to find some soft ones this time, last were hard as nails.

[vobster]

Hmmm. How about a nylon grubscrew?

[evert]

Comp tube/seal is 28mm.

You sure about that? I was under the impression they use the same seal and piston as the supersport?

[bigtoe01]

Hmmm. How about a nylon grubscrew?

too flimsy

[vobster]

Just remeasured it, rear piston skirt is 28.8
Front piston 28.2
Piston seal 29.2 without crush.
With some crush more like 29, so I'd say its a 29mm piston seal.

I wont try the nylon grub them