Honing HW77 25mm cylinder

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Lpgc

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I recently rebuilt my old HW77 that has 25mm internals. During the rebuild I noticed that when completely cleaned up the piston seal was extremely tight but even more tight, as though it had reached a sticking point, at around half way along the piston stroke inside the cylinder.

I am no stranger to honing cylinders, not too long ago I honed the cylinders on my boat's V8 engine (during a full rebuild with new pistons etc) to the extent of removing a couple of thou" of metal lol. But what are people's opinions on honing springer airgun cylinders / main blocks?

If it wasn't for the sticking point I wouldn't be considering it but the sticking point half way along the stroke length points to a tight area or deformity in that area, so I'm thinking a hone would help with that. Also, it seems to me that if the breech seals are tight we could either remove some material from the seal or remove some material from the cylinder, much the same difference, but the later HW77's use a 26mm piston anyway... and if seals were generally too tight then removing some material from the cylinder might be the preferred way to go.

Then there is the problem of the piston rod in the HW77 preventing getting into the cylinder with a honing tool. But I have read that the rod can be removed from the 25mm piston more easily than from the 26mm piston because the retaining pin for the rod in the 25mm piston is accessible and can be tapped out then the rod removed... Is this correct and if so how easy is it to disassemble the 25mm piston? When disassembled is the cylinder just a hollow tube or is it still capped at the 'cylinder head' end?
 
The latch rod does not need to come out unless there is a valid reason for it and I have never heard of any one honing inside the piston.

The piston comes out of the compression tube ( Cylinder). .
1778679566775.webp


The compression tube is sealed at the end .

1778679729890.webp



sizing a piston seal.


A

Thread 'Heat Sizing A Hw Piston SealBy Tyler Durdon'

Original post by @Tyler durdon

C0588E97-B9DC-4E4C-A535-2FD9A848A3E0.webp

994AA74A-BD91-418E-9866-4B44FDC736D5.webp

5E7F1B11-05C1-4C12-813C-1C231CDC376F.webp

I’ve got a project on the go .swapping my hw77 to the older 25 mm internals,
I felt the piston seal was very tight compared to the one in the rifle . Didn’t fancy sizing with paper and a piston in the drill .
So seal fitted to piston .
Light tube with silicone grease
Insert into comp tube
In the pan for 2 mins 30
Let totally cool
Piston now falls slowly under own weight .block the tp and stops dead .
This might have worked out quite well . Will post results when building up on Saturday



Honing out a cylinder needs a proper hone and 3 legged one would be the last one I would use . Saw plenty of engine blocks ruined due to to and they required a rebore . Fine for glaze busting though but others may disagree.
 
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Bighit said:
The latch rod does not need to come out unless there is a valid reason for it and I have never heard of any one honing inside the piston.

The piston comes out of the compression tube ( Cylinder). .View attachment 976569

The compression tube is sealed at the end .

View attachment 976575

Honing out a cylinder needs a proper hone and 3 legged one would be the last one i would use . Saw plenty of engine blocks ruined due to ti and they required a rebore . Fine for glaze busting though.
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Oh dear what was I thinking hehe! It was the cylinder I wanted to hone inside the cylinder not the piston and of course the piston rod doesn't need to come out to hone inside the cylinder.

If the cylinder has a tight spot at the moment I think it would be better to remove the tight spot than to use it as is. I doubt doing a DIY job using a 3 legged hone would make it any worse than having the tight spot but I think you'd disagree? Do any other DIY methods come to mind?

When I was honing my engine cylinders I did regular bore diameter checks at many points inside the cylinders and I'm confident I did a good job, even though I removed a lot more metal than would be considered normal for a hone. I DIY honed it for 2 reasons, it was cheaper than having it bored by a machine shop and time to completion was a lot quicker, I would've missed most of a summer's boating waiting for a machine shop to turn the job around.
 
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Lpgc said:
Oh dear what was I thinking hehe! It was the cylinder I wanted to hone inside not the piston and of course the piston rod doesn't need to come out to hone inside the cylinder.

If the cylinder has a tight spot at the moment I think it would be better to remove the tight spot than to use it as is. I doubt doing a DIY job using a 3 legged hone would make it any worse than having the tight spot but I think you'd disagree? Do any other DIY methods come to mind?

When I was honing my engine cylinders I did regular bore diameter checks at many points inside the cylinders and I'm confident I did a good job, even though I removed a lot more metal than would be considered normal for a hone. I DIY honed it for 2 reasons, it was cheaper than having it bored by a machine shop and time to completion was a lot quicker, I would've missed most of a summer's boating waiting for a machine shop to turn the job around.
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The problem with a 3 legged hone is that they are spring loaded and once they hit the high spot they can push the stone back and ride over it and take material out on the other bits .
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This type does not do it as they are fixed.

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Lpgc said:
But it seems that type cannot reach the end of a closed ended cylinder (like an airgun compression tube) because a part of it protrudes below the bottom end of the stones?
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There is hone's for that job. Blind hole Hone. The stones are level with the bottom


one here
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You can get extended stones that go beyond the bottom but they can be damaged easily if you go to deep and can make a mess of the bore when damaged . I remember seeing a brand new set of stones getting wreaked on a tractor block as my work mate did not set the depth stop and went too deep hit the casting inside and ripped the stones apart.


1778684461994.webp
 
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What grit to hone inside an airgun compression tube (if I were to do it)?

If the idea is to remove a tight spot I would imagine starting with a fairly coarse grit but finishing with a very fine grit, finer than we'd use to hone an engine cylinder to suit the (none metal) piston seals and the fact the seals run without lubrication or with only a very thin layer of moly grease as lubrication..?

Also thinking about the honing tool contacting the end of the cylinder - there is potential for the tool to take material from the outer edges off the cylinder end cap in which case the end cap would have a small outer groove which could maybe slightly affect compression if deep enough. We'd only be talking a negligible amount but I wonder if it'd be better if the stones didn't quite contact the end of the cylinder... just as long as we didn't end up with a 'step' that the piston seal would have to pass?

It does occur to me that it might be a good idea to ask an airgun piston seal maker's advice. But I recently bought an aftermarket piston seal from such manufacturer for the HW80, asked them a few questions but get the impression they either don't have the tech knowledge or couldn't be a***d to give decently thorough answers. I doubt they'd know and/or bother to explain what internal cylinder finish best suits airgun piston seals. A bit alien to me because I built my reputation in what I do for a living partly by knowing all the answers and being willing to discuss and advise, going into great detail for those interested in the tech like engineers - A positive review or recommendation from a techy / engineer type person is often worth many positive reviews or recommendations from average customers. I'm not into airguns in any professional sense but I am that kind of techy engineering person. I stripped and rebuilt my HW77 5 times last week trying to find out what was going on with it's tight piston seal and now I've got a chrono I will be doing some experiments, comparing results, maybe discuss results and theories on forums and probably go on to give advice if asked.
 
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Mine is the 25mm setup. I did consider switching to a 26mm setup and have seen people selling 26mm pistons on Ebay, I don't know how they've ended up with several 26mm pistons but no compression tubes for sale but that seems to be the case.

I don't think I'd have much bother honing out the sticking point in the cylinder. I've honed engine cylinders that would have been a far more expensive fail if I'd messed up and not nearly as easy to put right. If I messed up the simple 25mm compression tube I could attempt to buy another, probably not be able to source one at reasonable cost because 25mm HW77 tubes seem in demand.. but I would be able to cheaply buy a used 26mm piston and a replacement 26mm tube and not have to wait for them to become available or ship from the US.
 
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Lpgc said:
Mine is the 25mm setup. I did consider switching to a 26mm setup and have seen people selling 26mm pistons on Ebay, I don't know how they've ended up with several 26mm pistons but no compression tubes for sale but that seems to be the case.

I don't think I'd have much bother honing out the sticking point in the cylinder. I've honed engine cylinders that would have been a far more expensive fail if I'd messed up and not nearly as easy to put right. If I messed up the simple 25mm compression tube I could attempt to buy another, probably not be able to source one at reasonable cost because 25mm HW77 tubes seem in demand.. but I would be able to cheaply buy a used 26mm piston and a replacement 26mm tube and not have to wait for them to become available or ship from the US.
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Put up a wanted add and tag @Jasper7 and see if he has one. He did my a good deal on a HW80 piston .
 
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