new piston seal

[max115]

Does anyone know how long it takes for a new piston seal to break in before the pellet velocity will improve?  I have shot about 80 shots after replacing the seal on my Trail NP AW.  It is still dieseling and the pellet velocities were lower than before, but it seems to be increasing in my last round.

Thanks for any input.

[ShakySarge]

After 80 shots there should be no more dieseling. Something is not quite right there.  An initial decrease in velocity is normal with a gradual increase as the seal sets with the compression chamber. How much dieseling are we talking about?

Matt

[mtbdrew]

x2 shouldn't be dieseling after 80 shots. As for the velocity increasing, that will depend on the size of the replacement seal? If it was actually smaller than the stock then it will likely never have the same FPS as before. If it is a larger seal then it will depend on how much larger/tighter the new fit is?

Firstly you need to find out why the gun is still dieseling. Either you have too much lube in the chamber or there is something else wrong.

[max115]

thanks for your input.

The dieseling has been reduced now that I removed some of the moly paste inside the chamber. I did a few more test shots in groups of 10 and my average velocity still not as high as before.

I have a Trail NP AW .22 495fps version.  Using H&N Sport, my average velocity was 518 fps.
After the new seal, it was 533 fps (first grout shots with lots of dieseling), now after 16 group shots, it was 498 fps almost 35 fps difference! I check the seal and the piston it was still quite tight and not sliding smoothly inside the chamber.  I am hoping that the seal will get smaller thru wear and the piston will glide easier to increase my velocity.

I found the readings of my F1 Chrony varied under different lighting conditions.  I used a lamp placed in the middle of the chrony to do most of my indoor tests.  I also used natural sunlight by a window.  The readings from the window was lower.  Does the intensity of the lamp also affect the velocity reading?

Thanks for any input.

[mtbdrew]

If it is tight then then the numbers should pick up as the seal is broken in and honed down to size. Could take several hundred rounds before that happens depending on the amount of seal that needs to be removed.

[thekid]

Well now, good to see more canadian boys on the forum.

You should see a drop in velocity, up to around 500 shots, as this is the time the seal will be sized, by the action of cocking and shooting.
You however will see a greater drop in velocity due to the weeping hole in the piston. The longer it takes for the piston to reach the end of its stroke, the longer the weeping hole will effect velocity.

Even if the seal was sized correctly, you would not notice a great deal of difference in velocity...as the weeping hole will continuely keep the output of the rifle around the 500 fps mark give or take a few.
With that weeping hole, you would be best to keep to lighter pellets around the 14.3gr mark. Crosmans are cheap and fairly accurate, but they are harder lead and require more pressure to get moving. A softer lead will yeild slightly better numbers and performance.

I re-read your post, those h&n pellets are good pellets, they are generally a tight fit being they are 5.53mm.
If you are so inclined, JSB or Daisy make a nice soft lead that will help pick up some speed.

Did you treat the receiver with ptfe...using "Permatex" multi purpose synthetic grease, formulated with ptfe.
Put some grease on a rag wrapped around a dowel, then insert it into the receiver, spin the dowel in the receiver to apply it to the walls. Then put the piston back in and move the piston in and out to fully coat the inside.
After this is done, remove the piston, wipe all lubes out of the receiver with the same dowel and a clean rag. Clean the piston seal off and re-apply a small amount of Molly and reinstall.
Should see an increase in velocity as this will make the seal slide easier.
All this info is in the GTA library on the GTA menu bar...
Good luck.

[max115]

nice to be on board!

I did not use the "Permatex" grease on my seal.  I only used small amount of molly paste 60 inside my chamber and on metal to metal parts.  Where would I get this grease?

I am hoping that over time the seal will form its shape and thus slide easier.

Thanks for the advice.

[thekid]

nice to be on board!

I did not use the "Permatex" grease on my seal.  I only used small amount of molly paste 60 inside my chamber and on metal to metal parts.  Where would I get this grease?

I am hoping that over time the seal will form its shape and thus slide easier.

Thanks for the advice.

Not on the seal.
I picked up my grease at Canadian tire in the auto section, walmart most likely has it.

The whole Idea behind this, is ptfe (Teflon) is the slickest stuff there is basically. You treat the receiver walls, then put the piston in the receiver and move it back and fourth to help coat it evenly. You then wipe out the grease and clean the piston seal off.
After you re-apply moly, then re-assemble.
There should be no visible grease in the receiver before final assembly, this will help the seal slide easier against the walls.

[mtbdrew]

For future reference you don't put any molly in the chamber.

[max115]

Thanks "drew" and "kid" for the advice.  I will strip it down and clean the inside thoroughly and no molly inside the chamber except on the piston itself. 

[mtbdrew]

No problem. BTW, here is a good video guide for tuning/lubing Air gas ram airguns:

[QVTom]

This is why you don't use a single light source with a chrony.

Tom

[max115]

thanks "drew", I saw that video too and I was following his steps also.  Now my rifle does not diesel anymore but the velocity seems to be constant around 500fps average.

As with the light source, I have tried using the LED puck lights taped directly over the sensors, but kept on getting error readings.  I guess I will have to try using two lamps one over each sensor to check the chrony readings.  Weather has been crappy lately in Surrey, BC so I haven't got a chance to try the chrony outside since I got it two weeks ago.

Tom, are you suggesting that one light source will not give accurate readings of the pellets?

thank you all for sharing.

Alex

[QVTom]

yep, unless it can be adequately diffused so it doesn't cast any shadows that are not perpendicular to the sensors.  Two light solution is much easier.

Tom

[mtbdrew]

Should have asked earlier but where did you get the replacement seal? 

[max115]

I ordered the seal from D&L airgun and they ordered it from Crosman.