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

[MattyBoy]

Just started going up the weight range of pellets.

FAP @ 7.3 running 11.70 - feel nice

AA Field at 8.4 running 11.50 - feel fairly nice

H&N FTT at 8.6 (I put these in as 8.7 on the chrono as they are known to be heavier) running 11.20 - feel a bit bouncy

Barracuda Hunter Extreme @ 9.6 running 10.30 - very bouncy.


So still bouncy with heavier pellets but no possibility to add preload... or I just shot AA Field which feel OK at 11.50...

11.70 with FAP is still too high for my liking - 11.40 is my usual top limit... If I reduce spring length I will simply dial back in the bounce...

I think I need to short stroke this gun by 5mm - no other option. 3.5mm TP may be perfect already; just too much stroke for 11 ft lb...

Matty

[vobster]

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!

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

I should have said that my 25mm is standard except the spring, which is a standard short stiff vmach jobbie and the piston seal which is from TTI.
It shoots really nice with either the original long or new short one in I only pulled the original out as it took me over power.
So, it doesnt feal bouncy even with little preload ala vmach.

[MattyBoy]

I should have said that my 25mm is standard except the spring, which is a standard short stiff vmach jobbie and the piston seal which is from TTI.
It shoots really nice with either the original long or new short one in I only pulled the original out as it took me over power.
So, it doesnt feal bouncy even with little preload ala vmach.

I guess the Vmach springs are pretty stiff to counteract bounce even with low preload?

Sorry if I have rambled a bit above

TP of 3.5-3.9 all seem to have their place - it's just a tough balancing act.

I am sure after my testing with mine that a Short-Stroke is needed to get a sweet cycle with 26mm piston.

Matty

[bozzer]

I'm sure we looked at the drill out and tap with 5mm and then make grubs up with various holes in them before.

Did we have some concern that when a 5mm grub is drilled out to say 3.5mm ish then the Allen key fitting on the end of the grub would be taken out?

So I can't remember if we said to use grubs with flat driver heads and not Allen key.

I looked at doing that as I was concerned my 3.8TP was too large. I spent some time looking up about aerodynamics and air flow through a port. I was a little concerned about getting a smooth air flow entry on the inside of the piston.

I think I asked if anyone had actually done this or whether it was just theory.

Has anyone actually done this and it's been a success?

[vobster]

I'm sure we looked at the drill out and tap with 5mm and then make grubs up with various holes in them before.

Did we have some concern that when a 5mm grub is drilled out to say 3.5mm ish then the Allen key fitting on the end of the grub would be taken out?

So I can't remember if we said to use grubs with flat driver heads and not Allen key.

I looked at doing that as I was concerned my 3.8TP was too large. I spent some time looking up about aerodynamics and air flow through a port. I was a little concerned about getting a smooth air flow entry on the inside of the piston.

I think I asked if anyone had actually done this or whether it was just theory.

Has anyone actually done this and it's been a success?

I'll report back when my m5 tap arrives and I've tried it. I think that when I've settled on the size I want I'll red threadlock it in and drill a slight venturi on the piston side. Just to complicate things like

[MattyBoy]

With M5 grubs you lose the hex faces at 2.7mm

Matty

[vobster]

With M5 grubs you lose the hex faces at 2.7mm

Matty

Cool, should work then