BSA supersport Transfer port modifocations

[evert]

Many years ago, I shot a tuned .25 XL lightning that went "thokk" when fired.
It had a V-Mach kit and the transfer port restrictor removed (no power limits here in norway).

I've been happy with my own Lightnings/Supersports so far, using the Titan #1 on tight delrin guides, but they seem a bit bouncy and dont have the same "thokk" but more of a muted "thudd" when firing. As they have 29mm pistons I've not thought much about it as large diameter pistons probably bounce more than small diameter pistons.

Thinking about the transfer port, removal of the restrictor screw on the gun I shot meany years ago probably meant that gun had a 4mm port? The original ports on these (At least the Norwegian full power guns that came with the heavy piston weight) measure about 3,2x10mm, which means the may benefit from opening up?

[Waddy]

Many years ago, I shot a tuned .25 XL lightning that went "thokk" when fired.
It had a V-Mach kit and the transfer port restrictor removed (no power limits here in norway).

I've been happy with my own Lightnings/Supersports so far, using the Titan #1 on tight delrin guides, but they seem a bit bouncy and dont have the same "thokk" but more of a muted "thudd" when firing. As they have 29mm pistons I've not thought much about it as large diameter pistons probably bounce more than small diameter pistons.

Thinking about the transfer port, removal of the restrictor screw on the gun I shot meany years ago probably meant that gun had a 4mm port? The original ports on these (At least the Norwegian full power guns that came with the heavy piston weight) measure about 3,2x10mm, which means the may benefit from opening up?

I'm not an expert but I think 3.2 mm is for sub 12 UK air rifles. I have a Hatsan 55 in .25 that came with a 5mm TP, with the original TP & a Titan spring it was doing nearly 19 ft/lbs, now it's sub 12 with a few coils off the spring & the 3.2 mm TP insert I made for it.

It's all relative & has to be balanced. So yes I think a bit bigger TP would be the way to go.

[Shed tuner]

how heavy is the piston ?

for a large 29mm bore, 3.2x10 is OK, but 3.5mm is probably more optimal. 4.0 is too much IMHO, unless the piston is very light.

[evert]

how heavy is the piston ?

Heres the piston I have outside of a gun:
246 grams with piston seal and liner.
312 grams with added FAC piston weight.

Theres usually two 23mm steel washers in front of the piston liner, they were missing on this piston, so you can probably add a few grams with these in place.

[evert]

I'm not an expert but I think 3.2 mm is for sub 12 UK air rifles. I have a Hatsan 55 in .25 that came with a 5mm TP, with the original TP & a Titan spring it was doing nearly 19 ft/lbs, now it's sub 12 with a few coils off the spring & the 3.2 mm TP insert I made for it.

It's all relative & has to be balanced. So yes I think a bit bigger TP would be the way to go.

I'm thinking the smaller TP would be OK for FAC guns able to counter bounce with heavy pistons, and that 12fpe setups without extra piston weight might benefit from larger ports?

[evert]

for a large 29mm bore, 3.2x10 is OK, but 3.5mm is probably more optimal. 4.0 is too much IMHO, unless the piston is very light.

I have a Spanish cylinder (heavily polished with rounded corners and lightly engraved lettering) that can be subjected to experimentation.

[Shed tuner]

throw away the piston weight.
open the port to 3.5
try it out. I reckon it'll be better.

the piston is not that heavy without the weight.

Also see if has a marked preference for lighter or heavier (otherwise identical) pellets - e.g. JSB in 13.43 / 15.9. It's not a perfect test, but it does show up significant imbalances.

[evert]

throw away the piston weight.
open the port to 3.5
try it out. I reckon it'll be better.

the piston is not that heavy without the weight.

Also see if has a marked preference for lighter or heavier (otherwise identical) pellets - e.g. JSB in 13.43 / 15.9. It's not a perfect test, but it does show up significant imbalances.

Found a Supersport tube I believe to be Birmingham made that I forgot about.
It has a lot of scratches in the finish, so it can serve as a test subject.
A reamer in my box of small reamers took the port to about 3,6+mm.

Set the gun up with one of Maccaris HW50/99 springs, which fit a guide I made some time ago snugly, (this spring works extremely well in the Webley Vulcan when it is spaced up a bit) and found a delrin tophat that fit well. I'm not expecting full power with this spring, but just want to test how the shotcycle feels with it.

The barrel is an old Meteor Mk5 barrel block that came to me some time ago, which had been bored of to fit a .177 barrel I dont remember the origin of. Gamo or chinese barrel probably, and feels like a loose bore when entering pellets.
The gun recoils very linearly with this setup, and doen not feel choked like the Supersport can feel with some setups.
So I guess the port opening might be a success

When time allows I'll take the barrel and Titan #1 from another Supersport tube which shoots very well.
It will be interesting to see how the opened port performs with the full power spring and a tighter barrel.

[Shed tuner]

So I guess the port opening might be a success

Don't go being overly cautious now... I seem to be on a roll

[evert]

The action with the 3,6mm port now has a full length Titan XS spring on tight delrin guides, and a tighter original BSA barrel.
Not sure if I like it, but will shoot it for a while before I decide if is better than the 3,2mm port.

[Shed tuner]

The action with the 3,6mm port now has a full length Titan XS spring on tight delrin guides, and a tighter original BSA barrel.
Not sure if I like it, but will shoot it for a while before I decide if is better than the 3,2mm port.

I think that's a bit too much spring for that size port... chrono it, chop off (or collapse) a coil, retest...

[evert]

I think that's a bit too much spring for that size port... chrono it, chop off (or collapse) a coil, retest...

No power limits here in Norway ;-)

But compared to the Maccari HW50 spring, it is a bit harsh. I'd like to try something in between.

[Shed tuner]

No power limits here in Norway ;-)

But compared to the Maccari HW50 spring, it is a bit harsh. I'd like to try something in between.

I wasn't thinking of power limits, I was thinking of the amount of preload and the port size, together making it a bit slammy...

I shoot FAC springers too, and have a pro elite that shoots softer than plenty of 12FP springers I've fired.. whilst ticking over at 20 FP. Incredibly accurate too...

[evert]

I wasn't thinking of power limits, I was thinking of the amount of preload and the port size, together making it a bit slammy...

I shoot FAC springers too, and have a pro elite that shoots softer than plenty of 12FP springers I've fired.. whilst ticking over at 20 FP. Incredibly accurate too...

Yeah, it feels a bit too much now. But it is still recoils quite flat, and seems to have less tendency to vibrate than the 3,2mm port.
The .177 XL Lightning with the same spring/guide setup is softer, but I'll have to set up another Supersport with the same spring to do a side by side comparison.

[Shed tuner]

Yeah, it feels a bit too much now. But it is still recoils quite flat, and seems to have less tendency to vibrate than the 3,2mm port.
The .177 XL Lightning with the same spring/guide setup is softer, but I'll have to set up another Supersport with the same spring to do a side by side comparison.

but the .177 generates more back pressure / softer piston landing than the .22