HW/Weihrauch HW77 25mm piston vs. 26mm question

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baranjhn

baranjhn

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Jan 16, 2016
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Lafayette, Indiana, United States
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Gentlemen, a few questions to any of you uber knowledgable gurus out there regarding the 25mm piston vs. the 26mm piston

I just stumbled upon an older shooter grade (cosmetically rough) HW77 in .177. I put it over the chrony with 10.3 grain JSB pellets and I was averaging 638 fps for 9.3 ft. lbs, which I think is a bit a bit low. Now the rifle is marked Hull Cartridge, which is an airgun seller in the U.K., so it could be a sub-12 gun, but still a bit low. I took it apart and it was way over lubricated with a thick tar (Maccarri tar?). The seals look ok, but since its apart, I’ll replace them. The spring is pretty good too, a very slight bend at the end. I’m not sure I’ll change that out yet. After degreasing and cleaning, I measured the piston and it’s 25mm. Which leads me to my main question:

is there a power difference between a 25mm and 26mm piston?

Also, could a 25mm piston be a FAC gun?

Thanks!
 
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Which series is yours?

MK1 - no anti-beartrap, and a ball bearing underlever detent. 25mm

MK2 - anti-beartrap, sliding underlever catch. Most all were 25mm

MK3 - push button underlever catch, bigger loading port, 26mm

Hull Cartridge was indeed the importer and the gun was likely sold in the UK originally. Given that, it was probably a "legal limit" gun.

All the 25mm guns I've had that were originals were under 12 ft/lbs anyway. But they are old, and i don't have a solid spec sheet that I trust for when they were new.

A 25mm gun can make more than 12 ft/lbs fairly easy with tuning changes. I've had some of mine up near 14 ft/lb.

A 26mm gun can make more than 12 ft/lbs easily, because of the obvious increase in swept volume.

There really ain't a whole lot of difference, but the 25mm shoots better to me (and most other people) because it's a little more efficient, especially at lower power. Smaller piston takes less spring to push it, and it's usually snappier since the piston is lighter.
 
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Here's an old article from a British magazine. The gun they test the best I can distinguish is a MK1. *Click the link at the bottom for the whole thing*

Note that 855fps with a 7.9gr pellet is about 12.8 ft/bs.

Screenshot_20240907-203447.png


View attachment grapr85weihrauchhw77.pdf
 
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Apart from the change in piston/cylinder diameter, the thing that marks the later rifles is piston weight, at 270g against the 210g of the early rifle, so, for any given piston velocity, the 26mm piston has far greater momentum, and is better able to compress the air to a higher pressure/temperature.

The early 25mm rifles can achieve a little over 12 ft.lb., but the 26mm rifles have greater FAC potential.
 
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baranjhn said:
Gentlemen, a few questions to any of you uber knowledgable gurus out there regarding the 25mm piston vs. the 26mm piston

I just stumbled upon an older shooter grade (cosmetically rough) HW77 in .177. I put it over the chrony with 10.3 grain JSB pellets and I was averaging 638 fps for 9.3 ft. lbs, which I think is a bit a bit low. Now the rifle is marked Hull Cartridge, which is an airgun seller in the U.K., so it could be a sub-12 gun, but still a bit low. I took it apart and it was way over lubricated with a thick tar (Maccarri tar?). The seals look ok, but since its apart, I’ll replace them. The spring is pretty good too, a very slight bend at the end. I’m not sure I’ll change that out yet. After degreasing and cleaning, I measured the piston and it’s 25mm. Which leads me to my main question:

is there a power difference between a 25mm and 26mm piston?

Also, could a 25mm piston be a FAC gun?

Thanks!
Click to expand...

There is a definite difference in power levels between the two. Older 25mm .177 HW77 guns were rated @ 830fps (according to the Blue Book) where the modern 26mm guns are 930fps. The 25mm guns can easily make FAC levels, my 97 with a 25mm conversion makes 13+fpe with FTS pellets.
 
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Great info gentlemen, thanks!

From the info gathered above, it may be a MK2 rifle. There is no bear trap (removed or transitional?) and it has a sliding underlever catch. I’ve only had a few HW97’s in the past and they had the 26mm piston. I was pretty sure it would be a non FAC rifle, but your information confirms that.

I was just going to replace the seals, but went with a Vortek PG-4 kit instead.
 
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baranjhn said:
Great info gentlemen, thanks!

From the info gathered above, it may be a MK2 rifle. There is no bear trap (removed or transitional?) and it has a sliding underlever catch. I’ve only had a few HW97’s in the past and they had the 26mm piston. I was pretty sure it would be a non FAC rifle, but your information confirms that.

I was just going to replace the seals, but went with a Vortek PG-4 kit instead.
Click to expand...
I assume the kit is for the 25mm piston?
 
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baranjhn said:
Great info gentlemen, thanks!

From the info gathered above, it may be a MK2 rifle. There is no bear trap (removed or transitional?) and it has a sliding underlever catch. I’ve only had a few HW97’s in the past and they had the 26mm piston. I was pretty sure it would be a non FAC rifle, but your information confirms that.

I was just going to replace the seals, but went with a Vortek PG-4 kit instead.
Click to expand...
The early MK1s werent even inletted for the anti-beartrap. You can check the stock to see if you're curious. I bet it was just removed. If so, they should have left the little silver spacer under the threaded receiver post that the forward trigger guard screw threads into.

I always remove the thing on all of mine anyway so you can decock the gun.
 
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baranjhn said:
From what I gather on the Vortek site is that it's only the piston seal that is different.
Click to expand...
That's entirely possible the 26mm 77/97 kit uses a 0.785" OD spring. That's ~ 20mm. I just dont know the ID of the 25mm piston offhand. The bore is actually 25mm so the piston OD is likely 24-24.5mm to start. I guess that still leaves enough room for a 20mm spring and slightly larger tophat.

That should be a real nice shooting rifle when you're done. FYI that 25mm piston and sliding compression tube will fit any modern 77 or 97. I've been hoping to find a set to convert my green 97 to 25 mm.

Enjoy the rifle. Nice find.
 
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beerthief said:
any way to tell if you have a 25 or 26 MM without tearing it down ?
Click to expand...
Not that I'm aware of and, even if your rifle was originally 25mm, there's always a chance someone swapped the original cylinder and piston for 26mm versions - it's not unknown here in the UK for an unscrupulous tuner to offer to fit the 26mm innards from his own 'specially tuned' rifle to an unwary customer's rifle, and fit the cuztomer's 25mm piston and cylinder into his own.
 
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My Vortek parts came yesterday from the U.K., after only 3 days! I installed them tonight, which was a simple job and did not require my spring compressor. I did not add any of the spacers for preload, but I may use them in the future. I shot 10 pellets through her for a quick test and she averaged about 760 fps for 13.5 ft. lbs with FX 10.3 pellets. After a tin of pellets, it should wear in a bit and get down towards 12 ft. lbs.

My intial impression of this brief session had me impressed. I don’t have a HW-97 (or HW-77) with a 26mm piston right now, but this definitely feels different (in a good way) than my last Tinbum Tuned one (which was nice too).

I’ll mount a scope this weekend and start breaking it in, in earnest.
 
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baranjhn said:
I shot 10 pellets through her for a quick test and she averaged about 670 fps for 13.5 ft. lbs with FX 10.3 pellets. After a tin of pellets, it should wear in a bit and get down towards 12 ft. lbs.
Click to expand...
Not trying to be a smartass here, but a friendly heads-up—a 10.34g pellet at 670 fps gives you 10.3 ft-lbs of energy.
 
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