R9 is relubed with Ultimox 226, and added a couple piston & oring seal "twists".

[nced]

The Ultimox 226 is some pretty awesome stuff (so far)! Compared to the Krytox GPL205 used previously the new stuff is noticiable thicker, however still buttery and really slick. I bought a 40 gram (about 1oz) tub of the Ultimox 226 (contains corrosion and wear inhibitors) and after stripping my r9 with aerosol brake cleaner and completely relubing all internals (piston, spring, spring guide, and oring sealed piston cap) plus the cocking shoe sliding surface, barrel pivot bolt, pivot block and shims.......I only used this much grease.....
Before lubing......

And after the R9 lube job.........


After reassembly and a few shots to check things out I had NO "combustion smell". After the previous lubing using the thinner GPL205 I initially had occasionally smelled a trace of "combustion". After disassembly I noticed that the white Krytox GPL205 contained traces of "black" so I thought that perhaps the Krytox was leaching molly paste that was embedded in the "nooks and crannies", and the leached molly paste was combusting.

Well....the Krytox GPL205 lubed R9 has been in use for a while and all "combustion smell" was long gone so I was wondering if I would get any combustion smell with the Ultimox 226. I pretty liberally smeared the Ultimox over the sides of the oring sealed piston cap and piston, pushed the piston into the receiver, cycled the piston back and forth to distribute the grease on the receiver wall, pulled the piston and repeated the "grease, push, pull" cycle, then buttoned up the R9 for a few test shots. The result was NO combustion smell even though the Ultimox was applied considerably more liberally than when I used molly paste.

I also revised my R9 piston and piston cap design. My factory R9 piston weighed 250 grams so i machined a few thou from the piston OD to bring the piston weight down to 220 grams including a 10mm flat washer. When machining the piston on my lathe the first half of the "turned length" machined really nice with a tool steel bit, but as soon as the bit started cutting the second half of the "piston turn length", the piston shell was too hard to cut with the tool steel bit so the machining was finished with a carbide bit. I also made the 6061 T6 aluminum oring sealed piston cap thicker than usual at .500" and I added a Delrin bearing behind the oring groove. Here are a few pics showing the lengthened piston cap with the bearing groove and bearing. Matter of fact, you can see where a portion of the piston shell was machined to reduce the piston weight.........


Another change for this setup was the use of a 90 durometer military spec Viton oring rather than the 75 durometer ring I previously used.

A surprise was in store when I did my initial chrony and shot testing..........
Even though I reduced the swept volume by using a thicker oring sealed piston cap and the piston was lightened 30 grams, the chrony showed CPLs travelling 880fps with the minimally preloaded 34 turn .120 wire main spring, plus the shot cycle was really nice. The previous testing with the unaltered R9 piston and my usual .400" thick gave a CPL velocity of 870fps with the same spring! I actually thought that I would have gotten less velocity with the reduced swept volume and lighter piston of the new setup.

well.....I'm interested in seeing how the new setup holds up over the long run, and especially how stable the poi is with shooting session temperature changes using the thicker Ultimox 226.

[breakfastchef]

Great read as always, Mr. C.!

What is the purpose of the delrin bearing? I assume that is the white ring behing the piston head.

[nced]

Great read as always, Mr. C.!

What is the purpose of the delrin bearing? I assume that is the white ring behing the piston head.

LOL.......just to see IF it made any difference since I know folks have buttoned both the heads and skirts of their pistons. Also, Air Arms goes to the trouble of providing front and rear bearings for their TX200 style springers so at least they think the bearings are useful. In all actuality though, I see little benefit for adding "buttons or bearings" the piston of MY newer .177 R9 with proper lubrication. The cocking shoe puts little pressure on the head of the piston but a lot of pressure is on the skirt which is why HW and RWS take the trouble to harden the piston skirts to provide "dissimilar metals" sliding together,  and dissimilar metals minimize galling. Also, the new style HW cocking shoe that rides the receiver cocking shoe slot instead of a piston liner puts VERY little pressure on the piston skirt!

I do have to say though that buttons on the piston skirt and bearing behind the oring seal does break metal to metal sliding contact. The bearing also centers the oring in the receiver for even compression, but in all reality I don't know if there is a PRACTICAL benefit over the plain oring sealed piston caps I've used in the past. LOL....I figured the bearing couldn't hurt so why not try it since the R9 was broken down anyway for testing of the "new to me" grease.

Ya know......sometimes I wonder if a lot of the "latest and greatest" springer mods are simply an excuse to legitimize "messing with" a perfectly good springer as it was sold by the manufacturer. For ME, unless modding actually IMPROVES the function, I'm of the opinion that it's simply unnecessary at best and damaging at worst. I've proven to my personal satisfaction that a properly fitted oring sealed piston cap gives a more consistent poi when there is a large temperature shift during a shooting session. I've experienced this when early spring field target matches are started with snow flurries but the temp rises to mid 50s by the end of the match.

[eeler1]

Hey Ed, I just bought an R7 and an HW50 with the oring pistons, maybe your conversions?  So, I figure I ought to learn a bit about them.  I'm wondering, since the older o-ring piston guns often had a bumper in the front of the piston to prevent it from slamming metal-to-metal into the cylinder wall, have you seen any evidence that such slamming happens?

[nced]

Hey Ed, I just bought an R7 and an HW50 with the oring pistons, maybe your conversions?  So, I figure I ought to learn a bit about them.  I'm wondering, since the older o-ring piston guns often had a bumper in the front of the piston to prevent it from slamming metal-to-metal into the cylinder wall, have you seen any evidence that such slamming happens?

When I first started oring sealing I was concerned about "piston slamming" so I used a urethane bumper between the piston cap skirt and the piston with a bit of space between the piston button face and the cap c-bore for "cap travel" and it looked like this (the capped piston)........

After using this arrangement for a while I nixed the bumper all together and have been running "bumperless" for over a decade since. I found that when the tune is properly balanced (spring strength, pellet and piston weight) the piston decelerates on the "air cushion" and doesn't slam. Here is a pic of a piston cap I pulled after thousands of 14.5fpe shots to inspect for "impact marks" and there were none.......only discoloring from molly paste combustion products.........


Keep in mind that the high power Theoben Eliminator "rammer" had a steel oring sealed piston so I figured that if the Eliminator would hold up the oring sealed piston cap would certainly hold up on my 14 fpe HW springers.   

[chiro972b]

I've shot Ed's R9 and it is as smooth as buttah! I don't think there is any piston slamming going on. Seems like there would be but I don't think the shot cycle could feel that soft and smooth if there was. My stock r9 is a twangy beast by comparison (although it is still accurate!)

[David.Soliman]

Is there a thread on how you made that piston?

[eeler1]

I'm getting my head around the o-ring piston.  Only experience up to now is the FWB300 and an old Sterling (Benjamin), both had bumpers at the face of the piston, but both are relatively low powered compared to an R9.  I haven't gotten inside the R7 or HW50 yet to check them out, but I can say that the R7 shoots very smooth. 

[nced]

Is there a thread on how you made that piston?

No, but the fabrication is pretty simple, however tedious the way I do it for oring sealing a HW springer.
1. Hacksaw a piece of 1 1/8" diameter 3031 T6 aluminum rod about 1" long
2. Chuck the aluminum puck in the lathe and face one end flat.
3. C-bore the faced end so the C-bore is the same depth as the piston seal retaining button on the piston, however make the diameter so it's a light press fit on the button.
4. Reverse the puck in the lathe chuck and face the hacksawed end flat.
5. Take the faced and C-bored puck to the milling machine and mount it in the chuck of a rotary table mounted so the puck is horizontal.
6. Using the mill I drill 3 equally spaced holes in the side of the cap base for the cap retaining screw holes. I used to do this by eye, however now I have a rotary table to ease this operation.
7. Tap the 3 holes in the base for #6-32 nylon screws.
8. Put a bit of super glue on the face of the piston button and press the c-bore onto the piston seal retaining button.
9. Install the 3 nylon screws till they bottom out on the bottom of the piston seal retaining button groove and trim the ends flush with the sides of the puck.

Now we're ready to do the actual fitting of the cap to the receiver and the oring groove.

1. Chuck the piston/cap assembly in the lathe chuck and center it so there is minimum piston runout using a dial indicator. This can be a bit of an issue because HE pistons aren't especially round or straight however I strive for .002 max runout.
2. Lathe cut the cap till it just slides past the smallest restriction in the compression area. With the R9 this can be a real challenge because it's quite common for the "end plug end" of the receiver tube to be constricted from fabricating the various knockouts and holes. Occasionally I need to hone the constrictions with a simple homemade hone to straighten the tube ID.
To check the fit of the piston cap to the receiver it's necessary to remove the piston/cap assembly from the lathe, check the fit, then remount the piston in the lathe and re-zero for another .003 cut. When the piston/cap assembly slides into the receiver after a .002 deep cut the cap diameter is good. Piston cap to receiver wall clearance should be .002 to .004 MAX (.005 if harder 90 durometer orings are used)
3. Re-center the piston/cap assembly in the lathe and face the cap to length. For the R9 I've been using .400 for the thickness, however for the arrangement in this thread I used .500 thick to make room for the Delrin bearing. If a bearing is to be added I lathe cut the pocket for the bearing, then I bore a delrin rod so the bore is .001 smaller than the pocket in the cap. I then parted off the bearing to length, diagonally split the bearing with a knife, then mounted the bearing in the pocket and tacked it in place with super glue. Then I sized the bearing on the lathe till it just slid past the smallest restriction in the compression area.
4. Now I lathe cut the oring groove till the oring slides into the receiver with the proper compression. Since HW receivers aren't very precise I judge the oring compression by "feel" based on over a decade of messing with this stuff. Fitting the oring is the same remove from lathe, test fit, re-chuck and re-center, cut about .001 deeper, remove bla bla procedure.
5. Test the function of the seal by lubing the piston/seal and receiver tube, install the piston/seal, put finger over the transfer port, with a hooked tool pull the piston to draw a vacuum......if the vacuum holds for 30 seconds the seal is good.

As a side note:
I don't use the standard oring groove size or compression rate recommended for dynamic sealing. I make the oring groove the nominal width of the oring and I don't use anywhere near the recommended 15-25% oring compression.

Pics of various oring sealed piston caps.........
R9 piston.......


My HW77k piston and compression tube.........



TX200 piston and compression tube.......



Bottom line......perhaps it's better to simply use a Vortek VAC seal! 

[SpringerForever]

Deleted.

[nced]

I've since nixed the longer piston cap with the Delrin bearing because I simply didn't like the feel of cocking the gun. The "extra slack" before the shoe engages the piston with a long piston cap gave a kind of a "double tap" cocking sensation that I didn't like.

I removed the extended piston cap recut a new cap at my usual .390 length and all is "back to normal". Matter of fact....after playing with the easy cocking/softer shot cycle of a 12fpe R9 tune I'm reverting back to the way I used to tune years ago, with a bit of a change. I used to tune to shoot CPLs at about 14fpe, however I'm now tuning the to shoot at a 13fpe level. I've found that the trajectory out to 55 yards is very similar between the two power levels and the 13fpe shot cycle isn't as sharp. When the R9 is zero'd at 30 yards the pellet drop difference is about 1/2" 55 yards.

I guess that another reason for nixing the 12fpe level is that it's a bit of a hastle for my to get the CPL velocity DOWN to that level with an oring sealed piston cap so I stopped fighting it! LOL.....last night shooting at 18 yards upstairs in the house I was still "touching pellet holes" shooting hft style (sitting on a bucket resting the gun on cross sticks) so there really wasn't any advantage to the 12fpe tune other than an easier cocking/softer shot cycle.

[eeler1]

After reading this, I got some of that dupont ultimox 226, the smallest tube I could find ($$$!!!!), and have started using it when I rebuild pcp guns.  Mainly on valve stems and o-rings that seal moving parts.  We'll see how it holds up over time under high pressure, but so far I like the stuff.  Still use the cheaper molykote 55 on stationary orings.