HW77 .177 transfer port size

[magnummike]

Hi chap's in 1993 I bought a 77k in .177 with a 25mm piston . Back then most people were fitting OX kits including me Anyway a year later I sold it to a mate and he has had it since then . The other week I managed to buy it back after 19 years and I stripped it . The ox was still in there so degreased etc and plan to fit a standard spring when I get 1 . But all them years ago I cant remember if i opened up the transfer port when I fitted the OX kit , just wondering if anyone knows the standard size so I can check it ATB Mike

[brucegill]

I think it's 2.8mm or something like that. If yours is bigger than 3mm you opened it up

[magnummike]

I think it's 2.8mm or something like that. If yours is bigger than 3mm you opened it up

Cheer's Bruce I'll get the caliper out , fingers crossed E'h

[MattyBoy]

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

[Phil Russell]

Could optimal tp size depend on the spring as well?
The mention of the 'Maximiser' reminded me of my own, new in about 1984, HW77k.
The Maximiser tuning kit was all the rage then and I bought one as all the adverts promised me it would transform the feel of the standard rifle. It was my first attempt at any form of tuning so I leapt at the chance. I sweated all morning to get it fitted. I remember the square section spring (Ox??) took a lot of preload and I did not have a compressor of any sort. But I did it ..... all fitted with the minimum of spilt blood.
What a transformation. From a pretty docile but bit twangy rifle I now had a rifle that bucked and jerked around in my grasp. I had no chrono so did not know what it was doing; but assumed it was still within spec. But I hated the firing cycle. Late afternoon I took it apart again and refitted the original spring. All became nice again ... the piston liner and 'debur' carried out as per Maximiser instructions had helped. No more twang and a good sweet firing cycle.
The rifle stayed like that for about 20 years when a chrono check showed power was declining so I put a new standard UK power spring in. A chrono check revealed power ott so spring reduction was needed. No problem.
I have never touched the transfer port.

Cheers, Phil

[Bulsaye]

or .118" if you want the decimal size.

[TonyL]

Could optimal tp size depend on the spring as well?
The mention of the 'Maximiser' reminded me of my own, new in about 1984, HW77k.
The Maximiser tuning kit was all the rage then and I bought one as all the adverts promised me it would transform the feel of the standard rifle. It was my first attempt at any form of tuning so I leapt at the chance. I sweated all morning to get it fitted. I remember the square section spring (Ox??) took a lot of preload and I did not have a compressor of any sort. But I did it ..... all fitted with the minimum of spilt blood.
What a transformation. From a pretty docile but bit twangy rifle I now had a rifle that bucked and jerked around in my grasp. I had no chrono so did not know what it was doing; but assumed it was still within spec. But I hated the firing cycle. Late afternoon I took it apart again and refitted the original spring. All became nice again ... the piston liner and 'debur' carried out as per Maximiser instructions had helped. No more twang and a good sweet firing cycle.
The rifle stayed like that for about 20 years when a chrono check showed power was declining so I put a new standard UK power spring in. A chrono check revealed power ott so spring reduction was needed. No problem.
I have never touched the transfer port.

Cheers, Phil

Hi Phil,
Nice little write up. I'm sure many fell for these kits/springs back then, resulting in harsh firing cycles, probably OTT power and maybe the square edges of the square section springs nibbling away at the precious internals. I just thank my lucky stars that I never fell for them....I stripped and serviced a few rifles belonging to friends, all containing Ox springs. At least two HW35s and a Mercury to two, and all became much nicer once fitted with nice, sensible, round springs.

I know some have posted on here that some of the square section springs, such as the ones HW fitted to the 97 as standard for a while, were actually fine, as they didn't exhibit those nasty, sharp edges.

And, yes, big time. Any tune, on any gun, will be down to balance. So a certain TP size will suit a certain set-up better than another. There will be cases, however, where the standard size is too small or large for any ideal set-up, I bet. And, as Matty Boy says, yes, by the sounds of it, with the 77 you can go right out to 4mm, depending on personal preference. A TP this wide would be a bad move on many guns with longer TPs, due to more lost volume, certainly at our power level.

[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

[bigtoe01]

remember however, if the gun you have is nice already, why alter it?

If you must experiment drill the TP to 4.2mm and tap to m5, get some 6mm long m5 grubscrews and have them drilled from 3 to 4mm in .1mm steps, use a little blue loctite to hold them in and seal the threads, you can now play with TP dia and see what works for you

A little heat and an allen key gets them out, run the tap back in to clean the threads and screw a new one in to resume testing.