HW77 .177 transfer port size

[brucegill]

I run all my 77s and 97s with a 3.2mm TP

I have a special "Gold" drill bit just for that job.

3.6mm is still perfectly fine but if it's over 4.0mm then it's sleeve-down time.

JB swore by 4mm so it seems the TP diameter is variable but different sizes will obviously affect the shot cycle considerably.

I like 3.2mm myself

Matty

Hi Matty

Do you just drill yours out, or do you ream them as well?

Cheers for the info
Bruce

[greenwayjames]

When asked JB said he used to use a 5/32" drill on FWBs, HW77s etc but advised against doing it unless the rest of the conversion that he did was carried out. He used a 5mm drill for HW80s

[bigtoe01]

4mm is perfectly safe on a 77, work out the static compression ratio and you will see why. A Diana 52 has a 6mm long TP with seal length and in unrestricted mode has 4mm dia TP...no different.

peak power comes under 4mm but not by much, i often see gains over over 1fpe going from 3 to the sweet spot with the same spring and preload, this then allows the spring to be backed off etc.

If using an O ring head the TP needs to be a tad smaller than on a synthetic seal, this is to raise the compression, O ring heads are more efficient and make power easier, so a smaller TP can be used and you get the same results.

[brucegill]

If you open up the TP on a 77 from standard to 3.5 (for augments sake), would it require less preload or more to get the same power levels?

Thanks
B

[bigtoe01]

Bruce, peak power is not produced with a TP of 3mm, that means a TP of 3mm is not efficient. As you increase the TP dia the power grows till eventually it peaks and for a small further increase in TP dia you can tune the distance from the breech face the piston stops before slamming into the breech face...so you can tune the bounce point.

Peak power is around 3.9mm, 4mm gets the piston a little closer to the breech face (and lets not forget piston weight plays a part also)

So if you increase efficiency you can use a weaker spring...however this may not be ideal but often does result in less felt recoil.

On an O ring piston you need to go smaller with TP dia as the pistons fly faster and generally the O ring is more efficient with less lost volume, so peak efficiency may be 3.5mm...i fell in the trap of expecting an o ring head to work like a synthetic seal ...they don't.

So quick answer is it should require less preload.

I tested 3 to 4.2mm in .1mm steps and settled on 3.9, i know Wonky likes 3.9 also, JB likes 5/32 which is just a nats over 3.9, T20 uses 4mm, BTDT runs 4mm on his 77k....the list is long for rifles that all run very close to the same transfer port dia...now ask yourself why

[MattyBoy]

In answer to Bruce, I just drill them then polish them with Autosol on a boot lace .

In answer to Tony, in that case I'll be taking my ports out from 3.2mm to 3.8mm immediately

Matty

[brucegill]

Bruce, peak power is not produced with a TP of 3mm, that means a TP of 3mm is not efficient. As you increase the TP dia the power grows till eventually it peaks and for a small further increase in TP dia you can tune the distance from the breech face the piston stops before slamming into the breech face...so you can tune the bounce point.

Peak power is around 3.9mm, 4mm gets the piston a little closer to the breech face (and lets not forget piston weight plays a part also)

So if you increase efficiency you can use a weaker spring...however this may not be ideal but often does result in less felt recoil.

On an O ring piston you need to go smaller with TP dia as the pistons fly faster and generally the O ring is more efficient with less lost volume, so peak efficiency may be 3.5mm...i fell in the trap of expecting an o ring head to work like a synthetic seal ...they don't.

So quick answer is it should require less preload.

I tested 3 to 4.2mm in .1mm steps and settled on 3.9, i know Wonky likes 3.9 also, JB likes 5/32 which is just a nats over 3.9, T20 uses 4mm, BTDT runs 4mm on his 77k....the list is long for rifles that all run very close to the same transfer port dia...now ask yourself why

Many thanks Tony! Better find some nice drill bits

In answer to Bruce, I just drill them then polish them with Autosol on a boot lace .

In answer to Tony, in that case I'll be taking my ports out from 3.2mm to 3.8mm immediately

Matty

Thanks Matty!

Appreciate the info guys