Transfer port size suggestions please

[vobster]

For a hw97 mk3 in .177

26mm setup
Piston weight is 210 grams (which I realise is on the light weight side for a 26mm piston, but when it works recoil is low)
Stroke with O ring piston head 77mm

I'm going to tap the present port to take M5 grubs screws and re drill. Can anyone give me a ballpark where I should be at for this configuration?

I expect I'll be using a fairly long soft spring rather than short/stiff, but I have options.

I've had the above working quite well with a 3.9mm port, but I'm not convinced its correct as it was feeling quite slammy which is why I'm going to re-drill smaller.

Thanks!

[Snooper601]

Do 3 new ones 3.5, 3.6, 3.7 and test...

Cheers

John

[vobster]

So, near the upper end of what people set them at. Thanks!

[greenwayjames]

Bowkett used to drill them out to 5/32" and had a lot of satisfied .177" FT customers. I got the impression from what he has said that there are a lot of other factors involved to make it work how you want so a system where port size can be changed is good. All rifles have to be treated differently - like women

[vobster]

Bowkett used to drill them out to 5/32" and had a lot of satisfied .177" FT customers. I got the impression from what he has said that there are a lot of other factors involved to make it work how you want so a system where port size can be changed is good. All rifles have to be treated differently - like women

Yes, but he tended to make up an extra weight to increase the piston weight from standard. My pistons gone the other way.

My guess is that a smaller port and a load of preload should work okay.

[MattyBoy]

I'm running 235g wth new styke HW seal, 3.5mm TP in a Full-length MK3 77

I previously ran a spring that was a bit longer and softer - 25mm longer with 27 coils of 3mm wire with a free length of 245mm

Piston bounce was very bad with anything other than the super light 7.3gr FAPs. These ran at 12.05 after putting 50 or so through the gun so I had to do something. Heavier pellets of course ran significantly lower power due to the bounce effect. mid 10s to low 11s but horrible.

On the basis that reducing pre-load by crushing or lopping off a coil to get below 12 would only enhance piston bounce I ditched this spring.

I am now trialing a shorter stiffish spring - Ox-stylee (I had it in a box and it fitted nicely ) which finishes just level with the back of the cylinder so just backblock thread preload. Free length around 220mm and 30 coils at 3.2mm square section.

Piston bounce is still very noticeable with heavier pellets like 9.6gr so I may go to 3.6mm and test.

I have yet to chrono the new set up with all pellets but FAPs are running 11.70-11.75 at the moment

It's a fun game

Matty

[vobster]

Its a darned confusing game! What seems to work for one person, doesnt for another. In theory my 25mm hw97 with standard internals should have too high a scr at about 900 to 1 (ish) and feel bouncey and kill scopes, but its lovely!

[MattyBoy]

The 2 un-messed-with older 77s I bought had 25mm piston at 220g, 3.2mm TP and a spring with about 4 INCHES of preload!

I have taken the TPs out to 3.5mm

Without the high preload I am sure these 25mm guns would be very bouncy?

I have not done much testing with my 3 x 25mm guns as they do run very nicely... particularly the MK1 97K in .22 4
I'll do more testing with a range of pellets and report back.

I will also (when I have time) drop the current spring out to double check the measurements.

Matty

[Shed tuner]

For a hw97 mk3 in .177

26mm setup
Piston weight is 210 grams (which I realise is on the light weight side for a 26mm piston, but when it works recoil is low)
Stroke with O ring piston head 77mm

I'm going to tap the present port to take M5 grubs screws and re drill. Can anyone give me a ballpark where I should be at for this configuration?

I expect I'll be using a fairly long soft spring rather than short/stiff, but I have options.

I've had the above working quite well with a 3.9mm port, but I'm not convinced its correct as it was feeling quite slammy which is why I'm going to re-drill smaller.

Thanks!

You'll need on a port on the larger side with 210g and 26mm. However o-ring will edge it a bit slammier, so I'd suggest ~3.7. Also if you have a longer, higher preload spring, that will tend to slam more. So maybe even 3.6, no smaller.

Safest is to start with 3.5, do a light/med/heavy JSB test (7.3 / 7.9 / 8.3), and see what it's actually doing. then open out as required.

HTH - JB

[vobster]

Good advice, thanks very much all!

Jon, I also have a short and extra short pair of springs from vmach, if you think the long and soft will pronounce the slam I'll start with the shorter, stiffer ones and 3.5 port