Canadian Airgun Forum

Today's testing was bitter-sweet

Things started out quite nicely as I picked up on my further testing of the MPP LGR.

Curiosity got the best of me, so I added a bit of weight to the LGR hammer, and got it to reliably exhaust 5 strokes. With the newfound ability to exhaust 5 strokes with no problem, I began to push the envelope, and gave it a whirl with a few shots at 6 strokes. WOW - using JSB 8.44 grain pellets, I managed 766 FPS, for 10.99 FPE !!!!!
I did however begin to notice a few things at these power levels that were not as noticeable at 3 & 4 strokes. The area of the compression chamber where the check valve is located became very slightly warm to the touch at 4 strokes, but became significantly warmer at 5 strokes, and warmer again at 6 strokes. After only a very few shots at 6 strokes, I also began to notice a curl of smoke coming from the compression chamber as the piston was drawn back for the next stroke, and also the smell of burning plastic. Not good. Sure enough, after a few more shots, the check valve seems to lock up solid - I couldn't get any air into the valve. The gun had to be torn down to see what failed.

It didn't take long to determine what had happened - the burned out craters in the face of the piston seal clearly indicate that detonation was taking place during the higher compression strokes. Note how the burnt sections extend from the slot of the seal retaining screw. Having seen the results of such detonation in the past, I sort of knew what to expect when I examined the check valve, and sure enough, it had become completely plugged by the burnt residue from the piston seal. After removing the retaining screw, the ball had to be tapped out with a pin punch - that is how solidly it had been jammed in by the molten urethane. After a thorough cleaning of the entire action, and all the exhaust valve and check valve parts, the gun was reassembled with all new o-rings, and a brand new ball for the check valve. The piston seal had no damage to the sealing edge, so it was re-used, but I knew that the extra headspace created by the burnt areas would reduce the effectiveness a bit, so a new piston seal will eventually be required for maximum efficiency. The pic below shows the damage to the piston seal, and on the right, the discoloured ball bearing from the check valve with some urethane particles still stuck to it.



A completely different lubricant was tried for the compression chamber and piston this time, in hopes of eliminating any possibility of detonation. As expected, the overall power level per stroke of the lever is down by about 20 or so FPS.

With the gun back shooting well again, I tried shooting a few test targets with various pellets, and using only 3 strokes per shot. The tests went smoothly, and as expected, pretty much all the pellets tested produced small, ragged, one hole groups. No signs of smoke or nasty odours regardless of the number of shots taken.

I may once again test the limits of the gun with the new lubricant, to see if it will resist detonation, even if it means another teardown.
All part of the fun of experimentation.

interesting project an results , id never seen this gun before but seems perfect for you mod.

I was just wondering if it is possible that the piston face is simply melting from the heat of compression?.... I had a Delrin piston face melt in my Uber-Pumper using a 1400 pump tube and linkage with an F-T-P and had to change to metal.... The info I looked up on the internet gives the melting point of Polyurethane as 240*C and Delrin as 347*C.... Let us know if it doesn't do it with the lube change....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!

Life is too short to be wasted worrying about the opinions of others.

That is a possibility I suppose.
I based my assumption on the way the cratering seems to extend from the screw slots, and as well, the way that burnt particulates had worked their way into the check valve, and the exhaust valve. This would seem to indicate detonation to me, whereas melting from the heat of compression might not result in the same residue, but don't know for sure.

Did you find that burnt residue from your Delrin piston face found it's way into the valve body, and the exhaust porting as well?

Nope, I didn't really see any residue.... the end of the piston was simply melted, blackened, and cup shaped (instead of flat), reducing the compression ratio by the addition of dead space.... However, since the the piston started as flat (not having a screw slot) that may account for the difference, plus the higher melting point of the Delrin....

I was just wondering if the air in the slot was superheated from compression and "jetting" out to the sides, carving away the Polyurethane like a cutting torch.... just a thought....

I would imagine that the answer will be clear, once you change the lube to something non-combustible.... I'll be following the results with interest / curiosity.... Always something new to learn....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal; Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!

Life is too short to be wasted worrying about the opinions of others.

Greetings. First, I introduce myself. My name is Antonio and I live in the Canary Islands. I have a Walther LGR and I have been a joy to discover this link. Would be very, very grateful if you send me email photos and details to enhance my walther power. It is a weapon with excellent precision and FT, I have used but little power problems ...

I hope your help and I thank you in advance ...

tonyolympus@gmail.com
www.tonyolympus.blogspot.com
Thank you very much.

Antonio - I will try to get the pics back up for you in the next few days.