P17 Piston Experiment in Progress

[Tom Tucker]

I've always been able to get about 3.15-3.2 FPE out of my P17's.  I've tried for more, but they seem to hit a wall where more compression hurts instead of helps.

So, on this Sunday morning, I decided to try to something different....

[UCChris]

Excuse my ignorance, as I've only had my P17 piston out of the gun once, but what did you do to it? I can't tell just by looking.

[Tom Tucker]

Excuse my ignorance, as I've only had my P17 piston out of the gun once, but what did you do to it? I can't tell just by looking.

I epoxied a 1/32" thick aluminum disk to the face of it and cut out a section to clear the transfer port.  The idea is to max out compression without sequestering any high pressure air between the o-ring and the side of the piston / end of the compression tube.

[UCChris]

Excuse my ignorance, as I've only had my P17 piston out of the gun once, but what did you do to it? I can't tell just by looking.

I epoxied a 1/32" thick aluminum disk to the face of it and cut out a section to clear the transfer port.  The idea is to max out compression without sequestering any high pressure air between the o-ring and the side of the piston / end of the compression tube.

Ah, gotcha. Let us know how it goes! I'm thinking about doing a longer barrel on my P17 so I can attach an LDC. A longer barrel with your piston mod might gain us a foot pound or even two.

[Tom Tucker]

So the epoxy has been on for 5 hours, and I decided to install the piston.  Since you can't install the piston without setting up a shot, I fired that one shot over the chrony to see what it would do.  461fps with an H&N Econ 7.5 grain pellet.  My other P17 with the same 1/32" disk but no cut-out section shoots that pellet at 430fps.  Once the epoxy has had more time to cure, I'll shoot it some more to make sure that reading wasn't a fluke.  I'll also shoot some JSB RS, since they really fly out of these guns.

[UCChris]

So the epoxy has been on for 5 hours, and I decided to install the piston.  Since you can't install the piston without setting up a shot, I fired that one shot over the chrony to see what it would do.  461fps with an H&N Econ 7.5 grain pellet.  My other P17 with the same 1/32" disk but no cut-out section shoots that pellet at 430fps.  Once the epoxy has had more time to cure, I'll shoot it some more to make sure that reading wasn't a fluke.  I'll also shoot some JSB RS, since they really fly out of these guns.

Oh wow. I'm getting 394 with 8.44 JSB in my stock P17 for 2.91 fpe. You're getting 3.54 fpe by comparison.

[Techie]

Tom, what elevation are you at?  This affects velocity in these guns, so without knowing your elevation, I can't make a valid comparison with my guns.  I'm at 6300 feet elevation and so get about 20 percent less velocity than at sea level.

[Tom Tucker]

I'm at about 100 feet.

[UCChris]

Tom, what elevation are you at?  This affects velocity in these guns, so without knowing your elevation, I can't make a valid comparison with my guns.  I'm at 6300 feet elevation and so get about 20 percent less velocity than at sea level.

I'm at 4500 for reference. My p17 is doing just under 400 fps with 8.44 JSB.

[Tom Tucker]

OK, testing is done!

JSB Exact RS 7.33 grain: Average 464 fps for 3.51 FPE
H&N Econ 7.5 grain (Tin #1, tight fit): Average 450 fps for 3.38 FPE
H&N Econ 7.5 grain (Tin #2, loose fit): Average 456 fps for 3.47 FPE

I have to give this mod a thumbs up.  This is the fastest I've ever gotten a P17 to shoot.  The lesson here is if you're going to build up the piston on a P17, always leave some clearance for the transfer port.

[Techie]

Thanks for that info Tom. 

I'm at 4500 for reference. My p17 is doing just under 400 fps with 8.44 JSB.

Chris, that's really high velocity with that weight pellet at 4500 ft. elevation!  Is yours stock or did you do some mods?

[UCChris]

Thanks for that info Tom. 

I'm at 4500 for reference. My p17 is doing just under 400 fps with 8.44 JSB.

Chris, that's really high velocity with that weight pellet at 4500 ft. elevation!  Is yours stock or did you do some mods?

Stock out of the box

[USAFANG6799]

Has anyone tried the Predator GTO's which come in at 6.75 grain ?

[jackssmirkingrevenge]

Is it markedly louder than the stock?  What about the compression effort?

[Tom Tucker]

The difference is not noticeable to me, but there must be a little extra effort.  I wouldn't say it's louder at all.

[superchikn]

I have 3 layers of aluminum tape on the face of mine and I get 430 fps with Crosman WCs.  I will try the channel tonight.  Thanks for sharing your findings.

[Buldawg76]

I haven't shot mine in awhile so do not remember the fps numbers but I have a similar mod done to mine. I used a hardened washer that I had from my RC cars that is used on the differential where the hardened balls ride to allow for different amounts of slip. Its a perfect fit for the piston/chamber and is around 1/32" also. It has a 7/16' center hole and is held in place in the bottom of the chamber by the silicone lube used on the o ring. It stays there with no issues while pumping. If I remember it shooting right at or close to the same fps as the OPs after installing the mod.

The wiper on the rear of the piston is interesting as well and I can see where it would help prevent to excess lube from being pushed off the back of the piston during use.

BD 

[Tom Tucker]

The funny thing is that I have 2 P17's and one of them is weaker no matter I what I do to it.  The one I bought a few months ago is shooting JSB 7.33 at 460-465fps, while the one I bought a little over a year ago will not shoot them faster than 430-435.  That's with all the mods, new o-rings, and tight breech seal.  Completely stock, the faster one did about 420, and the slower one did about 390.  The only thing I haven't done is take them apart side by side and compare every single component.  Something has to be different in there, I just don't know what...

[Tom Tucker]

Just an update - I have been tinkering with slower P17 more than I care to admit...  and after trying everything I could think of, the only thing I have succeeded at is making it even slower.  Actually, I did learn one thing.

Do NOT make the transfer port at the breech larger than 7/64" inch.  Stock size is about 3/32".  Going up to 7/64 will get you 5fps.  Going from 7/64" to 1/8" will cause you to lose 10fps.  So don't ever do that.

In an effort to get some of that power back, I tried filling the top of the transfer block with some JB weld and carefully contouring it between the valve seat and the transfer port in order to take up some the lost volume I created with the oversized transfer port.  I did what I consider a good job of it, but it resulted in no change at all.  I might just epoxy the whole thing shut, and re-drill the transfer port to 7/64.

I still can't figure out why the other gun is so much faster, and I may never figure it out.

[jackssmirkingrevenge]

I still can't figure out why the other gun is so much faster, and I may never figure it out.

Could it be that one barrel is tighter than the other?  Maybe switching them out will answer that question.