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More HW77 Notes: Made my first o-ring piston cap...

April 9 2012 at 2:14 PM

Mike Aber  (Login Boomer_Mikey)

I made my first o-ring piston cap for my old 25mm piston Santa Rosa vintage HW77 yesterday. I've been using a Vortek VacSeal-25 in this rifle for the last year or so because it gave me the best consistency compared to the factory parachute seal and picked up an extra 45 fps with this gun's favorite 8.4 grain 4.53mm pellet at 853 fps. I like Brownell's Action Lube however, this stuff is so slippery it migrates past both seals causing varying degrees of dieseling that destroy consistency so I thought I would try an o-ring cap. I read in one of the earlier o-ring cap posts that o-rings did a good job with this lubricant.

I ordered a bag of Viton 75 milspec 2X21mm ID o-rings from Mc Master-Carr and went to work.

I used 6061 aluminum and the piston had a threaded 10-32 hole in front so I used it with a flat head ss machine screw to hold the cap on with blue lock-tite to seal the threads and the cap.

I ran a flap wheel through the compression tube with 1000 grit silicon carbide paper to soften the edges of the crosshatch surface finish in the compression tube before taking measurements so the surface was smooth and clean for the new o-ring piston cap.

I measured everything several times:
Compression tube ID 0.983" - 0.984"
Piston OD 0.973"
Piston Head Diameter 0.775"
Piston Head Length 0.230"

Factory parachute seal:
Depth 0.415"
Body diameter 0.975"
Lip diameter 0.990"

Vortek VacSeal 25:
Depth 0.370"
Body diameter 0.975"
Lip diameter 0.993"

Piston Cap Dimensions:
Depth 0.380"
Diameter 0.975"
Piston Head Pocket Diameter 0.775"
Piston Head Pocket Depth 0.235"
Front Lip 0.060"
o-ring Groove Width 0.080"
o-ring Groove Depth Diameter:
0.030" Crush 0.857"
0.025" Crush 0.852"
0.020" Crush 0.847"
0.015" Crush 0.842"
0.010" Crush 0.837"

No problems making the cap and I started with an o-ring groove depth for 0.030" of o-ring "crush" or interference however, I couldn't get the compression tube over a 0.030" interference fit and reduced the o-ring groove depth to 0.852"; still too tight, then I reduced it further to 0.847" for a 0.020" interference fit and the compression tube went on with a snug fit.

I lubed the piston with Action Lube and a slight wipe of lube inside the compression chamber, just enough to lube the o-ring until the shot cycle distributed the Action Lube on the coated piston.

The first 50 shots displayed a 150 fps loss in velocity with no dieseling or smoking and settled to a consistent 700 fps and around shot 90 the rifle started smoking as lube started migrating again and consistency went away.

I decided to go back to the VacSeal with only JM Moly Paste. I cleaned all of the compression chamber, piston and spring components and burnished a slight film of Moly Paste inside the compression chamber, a thin film of paste on the spring components and a thin film of paste on the piston body and VacSeal. I like to coat the entire surface of the piston for corrosion protection.

With JM Moly Paste as the only lube present the rifle settled down in the first 25 shots to a consistent (4fps) 830 fps with it's favorite pellet. This doesn't mean I'm done... Now I get to start over without Action Lube.

Airguns are a gas


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does not sound like you are getting the potential

April 9 2012, 2:47 PM 

of the O ring piston cap. I have no idea where you need to go with it but judging by what Ed is getting (and Me with his cap) something is not right.

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(Login Knobs)

Some ideas.

April 9 2012, 2:51 PM 

Hi Mike

I'm the guy that uses the Brownell's Action grease. I also use Mobiltemp SHC 32 which has a considerably lighter base oil than Brownells. The only time I get any inconsistency is in the first three shots after the gun has been sitting for a long time, like in weeks.

I also don't set up for much more than .009 compression per side on a nominal .070 o-ring. I've did the first one ever at .10 thou per side, or about .020 overall and it was so tight on the R-9 that i had to grease the ring in order to get it in the tube without cutting the o-ring. You should be just fine at around 7 thou per side, but I hve found that when using a tube tha has not been precision honed that you can find an optimal compression for velocity if you want to take the time to play with it.

Brownell AG is slicker than Owl **** and it works well with factory finished tubes. It might be that your 1000 grit honing polished the surface to the point that it became quite easy for the grease to slide under the seal. Just a guess.

Anyway i hope you stick with it. I'm sure you'll find your own soluitons that work for you.


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Mike Aber
(Login Boomer_Mikey)

Much more to explore.

April 9 2012, 2:54 PM 

1.53mm (0.060") from the face of the piston to the front edge of the o-ring groove, chosen for mechanical strength.

I was attempting to solve the lube migration issue as my issue with consistency. So many ideas and methods have been used that it's hard to find a corilation in so many posts and articles on this subject.

I didn't polish the compression chamber, it still has a crosshatch finish inside, I ran the flap-wheel just enough to clean the surface. I like the Action lube, been using it for over 10 years and use it in many applications but it looks like it's too good for this one...

After I check the velocity/temperature relationship with the current setup for a baseline I'll try the o-ring cap with moly paste and less crush... maybe 0.25mm (0.010").

This rifle is my research testbed, there's much more to explore.

Thanks for the ideas Knobs, I'm sure they are leading me in the right direction.

Airguns are a gas


This message has been edited by Boomer_Mikey on Apr 9, 2012 3:14 PM

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(Login Knobs)

I've got an early comp tube

April 9 2012, 6:27 PM 

set up at 12 fpe with a Nylatron cap and Brownells Action Grease. Works like a champ.

One thing about the AG though, it can cause high breakout forces to get the piston moving. It seems to lock the O-ring in place once it sit's in position for a bit. As soon as it breaks out it's off to the races though.

I don't get big velocity changes when i switch lube methods either.

I like the consistency I get withO-ring tunes but the shot cycle can lead to ringing in the 77/97 and R-9's


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Ed Canoles
(Login nced1)

I have a couple ideas for you.

April 9 2012, 4:17 PM 

For a dynamic oring seal the recommended "squish" is 15-20%. Here is the oring chart I've used............
[linked image]

If too much "squish" is used, piston drag goes up and velocity goes down, however, if too little "squish" is used then sealing becomes problematic, especially with receiver tubes like those of the R9. With the R9 receiver the geometry varies a lot along the tube due to the cocking slot, knockouts & dowel holes. In the beginning I used only about 5% oring compression and this worked well for some tubes giving real good velocities, however more recently I've increased my oring "squish" a few thou more to better accommodate a varying receiver.

Now with my HW77 and the TX compression tubes I've sealed using orings are less problematic since my particular HW77 compression tube and the TXs I've worked have had real consistent geometry.

Please consider that a real cylinder is expected to function for 100s of thousands of full pressure/length cycles, which is many times more severe than the shot cycle of a springer. Ya see, the springer piston only sees peak pressure for a split second during the last small fraction of an inch of stroke. With a springer I consider that even a 25,000 shot count longevity would probably out last most springs, and I've found that the oring in a well fitted oring sealed piston cap will outlast the spring so I see no reason to design for a 50,000 shot cycle when the spring might only last for 20,000 shots.

By the way, I recently learned that the size consistency of even military spec orings can vary a lot. I did a conversion for a fellow and the conversion sealed well, however I put the oring I used for testing in the package (I usually supply a couple extra orings for "playin' around") and fitted a brand new oring on the cap. I later learned that the oring conversion didn't work very well. After inspecting the "returned under warranty" piston and compression tube I found that the seal indeed wasn't very good. I substituted a new oring and all worked well, however I did go ahead an cut a new cap with a bit more "squish" and it's being used.

Anywhoo......good luck on your oring sealing attempts!

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Mike Aber
(Login Boomer_Mikey)

Re: I have a couple ideas for you.

April 10 2012, 4:14 AM 

I see in the Parker O-ring Handbook that the 0.020" I used was maximum squeeze in the recommended range; however, it is recommended that the minimum squeeze regardless of cross section size be no smaller than 0.007" as some lesser compounds can't maintain a seal (take a set) and will not recover 100% with less than 0.005" of squeeze.

All of this supports your experience and comments that a few additional thousandths is good insurance against variations in compounds and bore variation. I completely agree with you about receiver conditions. I own 2 HW98's; one with excellent concentricity with the piston and airtube, and the other one out of round with 0.010" variation. All three of my 77/97's have 0.001"-0.002" variation in concentricity and/or taper.

Anyway, thanks for giving me the motivation to look at design recommendations from an engineering standpoint. I went through years of articles and posts on this subject.

Now I'm confident I can get an O-ring cap to work.

Airguns are a gas


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Ed Canoles
(Login nced1)

Contrary to a static seal, a dynamic seal doesn't seal MAINLY..........

April 10 2012, 9:24 AM 

by compression, but by differential pressure during compression. As the pressure rises on the
"pressure side" the oring gets pressed tighter and tighter against the "non pressure side" till the pressure gets so high that it causes the oring to extrude between the piston and compression tube gap. This is one reason I like to keep a close fit between the piston/compression tube.

There are a couple ways to combat this extrusion, and one is to use a harder durometer oring, however I've personally had best service from an oring around 75-80 durometer given the geometric variations in the compression tube. Matter of fact, a while back I received a HW55 to oring seal and it couldn't be successfully sealed because the receiver ID got larger the closer the piston got to the transfer port. Since the oring had to be sized so it would fit past the smaller end plug end, it had insufficient compression at the transfer port end where the peak pressure was generated.

Anywhoo.....oring set for the dynamic seal applications in the springer hasn't been an issue for me.

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(Login Knobs)

I've had no extrusion problems.........

April 10 2012, 11:48 AM 

..for diametral clearances of 5-7 thou and 75A Buna N rings.


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brent saunders
(Login b1nine7o)

Why do you guys like the O ring seals?

April 14 2012, 10:41 AM 

Do you like it because you can a tighter velocity spread? Or that its easier to get lower power? Or that you have to work on the rifle more. It seems that you guys that are posting are always tearing into your rifles every thousand shots or so. Or is it a tinker kind of thing like how can we improve the wheel?

Brent s

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Ed Canoles
(Login nced1)

Well.........for me it's a matter of more CONSISTENT performance, however........

April 15 2012, 7:54 AM 

if I actually had a "regular style" piston seal that worked as well as a properly fitted oring seal I would be using it rather than messing with cutting and fitting an oring sealed piston cap. I'll give some personal opinions to your questions.............

"Do you like it because you can a tighter velocity spread?"
While the velocity spread of a properly fitted oring seal is pretty close, I've also had good aftermarket piston seals give VERY close velocity spreads.......the "fly in the ointment" for me has been that the "POI spread" can be "temperature dependent" which created havoc with early season FT matches where the temps can be "snow flurries" in the morning, yet mid 50s by the end of the match. Years ago both my brother experienced a 1" poi shift at only 20 yards during such a match where we did our zeroing in snow flurries and finished the match with temps in the mid 50s. It was at that time over a decade ago when I experimented with oring sealing in an attempt to minimize the contact of "rubber and lube" between the piston and receiver, thinking that changes in "rubber" durometer and lube viscosity was aggravating the poi consistency. Anywhoo......IMHO, form follows function, so while I personally would like to simply use a good piston seal, I've had a more consistent poi IF there is a large temperature shift due to ambient temps, sun heat load on the gun, etc. during a match.

"Or that its easier to get lower power?"
NO! Its actually HARDER to get lower power since "sliding friction" is less relative to a parachute seal. Matter of fact, for my HW77 I actually use a light wire after market spring to keep my velocity from exceeding about 900fps with CPLs. Normally I can count of at least a 50fps velocity boost from a properly fitted oring vs the parachute seal if I used the same spring. LOL.....at the beginning I used the factory spring in my R9 with the oring sealed cap and actually shot a bit with CPL velocities at 960fps. The R9 was much too jumpy at this velocity so I made efforts to minimize oring sealed velocities via aftermarket springs.

"Or that you have to work on the rifle more"
LOL....nope, it's been my experience that a well fitted oring seal will outlast a couple good aftermarket springs, so if you're replacing a spring you might as well also roll in a new 25 cent oring.

"seems that you guys that are posting are always tearing into your rifles every thousand shots or so"
LOL.....this happens regardless of the sealing method, however it not usually due to the oring going bad! Here are a couple pics of worn factory R9 parachute seals next to early oring sealed piston cap components......
[linked image]
[linked image]
Notice the "factory induced" slices on this R9 piston seal.

"Or is it a tinker kind of thing like how can we improve the wheel?"
Well.....I think that you may have nailed it here......"how we can improve the wheel". Hummm....perhaps it should be how we ATTEMPT to improve the wheel.

This message has been edited by nced1 on Apr 15, 2012 7:57 AM

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H├ęctor J Medina G
(Login HectorMedina)

Because we like shooting accurately n.t.

April 16 2012, 1:29 PM 


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