Author Topic: Flat top piston and valve install - is this right? (Read 2667 times))

WobblyHand · « **on:** July 23, 2019, 03:33:43 PM »

A couple days ago, I installed a flat top piston and valve from Airgunsmith into my 1377. (Brass adjustable piston and aluminum valve) Just got a chance to shoot it to day. (Haven't used a chrony yet, but at least I've re-zeroed the red dot.) Occasionally, the pump seems to "slip", meaning that there isn't much resistance. This is different than the general trend of getting harder and harder to pump. Is this behavior normal? Or not. If not, does anyone have and idea where to look for issues? This is the first flat top I've done, so I don't know what to watch out for, nor the correct general behavior. Any insight would be gratefully accepted.

CraigH · « **Reply #1 on:** July 23, 2019, 05:23:10 PM »

It's certainly not normal. The only thing I can think of is a bad o-ring on the piston or valve.

Or are you talking about the feel as the pump arm assembly goes over center and snaps closed?

WobblyHand · « **Reply #2 on:** July 23, 2019, 07:33:22 PM »

I kind of thought so.

I thought the valve went in the pump tube a little hard.

The slip is during the pump, not the cam over part at the end.

Before I take it apart I'll chrony 10 shots. I'd guess the velocities will be erratic.

Back_Roads · « **Reply #3 on:** July 23, 2019, 08:42:48 PM »

When the pump arm is fully extended , or as it is getting there do you hear air being sucked into the pump tube ?

Tom Tucker · « **Reply #4 on:** July 24, 2019, 09:45:09 AM »

A flat-top setup can have less perceived pumping effort than a stock setup, especially for the first several pumps, but it should always feel about the same every time you pump. I have 2 13XX guns with adjustable flat-tops. One does 10 FPE at 12 pumps with a 10-inch barrel and +18% valve volume, and the other does 14 FPE at 15 pumps with an 18-inch barrel and +67% valve volume. Both guns are easier to pump than an unmodified gun. So I would say, chrony it first and then decide if there's a problem. The 2 biggest concerns with a flat top are proper adjustment and protecting the o-rings by deburring all the edges in the pump tube and careful installation.

WobblyHand · « **Reply #5 on:** August 08, 2019, 06:09:14 PM »

Quote from: Back_Roads on July 23, 2019, 08:42:48 PM

When the pump arm is fully extended , or as it is getting there do you hear air being sucked into the pump tube ?

Not sure I understand.

It's more that some pumps (past the first 20 degrees or so) just don't feel like they are compressing as much air as a previous stroke. Sometimes this is really noticeable, it feels obviously like it is leaking, or rather that the compression is way down for that pump. Other times the pumping is normal. In 10 pumps, the bad/low compression pump can be the 2nd stroke, or the 9th. There doesn't seem to be a pattern. And it doesn't alway happen. Sometimes all 10 pumps are 'normal', meaning that they get successively harder.

WobblyHand · « **Reply #6 on:** August 08, 2019, 06:51:34 PM »

When I was looking at the 1377, I noticed that I had accidentally squashed the compression spring (#15). I ordered a new one. Just got the spring, but haven't installed it. I wanted to chrony a few shots of the 1377. I have the AliExpress $30 chrony, (thanks to the BargainGate!) but I thought I'd try something different. I've been slowly plodding away at making my own chrony. (If you want to know, it's taken over 4 months.) Today, I had the first successful run of my home built radar chrony. It's still a work in progress, but it mostly works. I'll have to make a separate thread on it.

I did 10 shots, and had 8 detects. 1377, 7.48gr Excite ECON pellets, 10 pumps, homebrew radar chrony

446.45 fps - pumping felt really strange most pumps
534.94 fps - pumping felt off
548.65 fps - pumping felt better but still not normal
-- - no detect
585.72 fps - pumping felt normal
575.03 fps - don't remember anything notable
564.34 fps - one stroke felt weird
582.12 fps - felt normal
580.97 fps - pump 9 didn't have much resistance, 8 & 10 seems normal
-- - no detect. pumps felt good

Sorry about the hand done table - couldn't figure out how to do it using the forum table tool.

Not much data, and from a sketchy homebrew instrument. But there is some correlation between the radar chrony and "feel". Worth opening up the 1377 again. I've got some spare o-rings, and a new spring. Down the rabbit hole I go...

WobblyHand · « **Reply #7 on:** August 08, 2019, 06:59:56 PM »

Picture of homebrew (unpackaged) radar chrony.
Calibrated it with a tuning fork. 440 Hz (A) = 6.27 m/s = 20.57 fps

I'm seriously stoked that it works!

Robert 5mm · « **Reply #8 on:** August 08, 2019, 07:09:37 PM »

Man you should be on call by the Avengers

Back_Roads · « **Reply #9 on:** August 08, 2019, 07:17:54 PM »

The reason I asked is that it may not be always allowing air into the pump , the other thing would be the pump cup is not always forming a full seal on compression.
Nice work on the radar, are there some microwave oven parts in the mix ?

Tom Tucker · « **Reply #10 on:** August 08, 2019, 07:31:17 PM »

Quote from: WobblyHand on August 08, 2019, 06:59:56 PM

Picture of homebrew (unpackaged) radar chrony.
Calibrated it with a tuning fork. 440 Hz (A) = 6.27 m/s = 20.57 fps

I'm seriously stoked that it works!

Good lord! That's impressive.

Check those o-rings. They probably got damaged somehow. It's a good thing that a 13XX can be totally stripped in about 5 minutes.

WobblyHand · « **Reply #11 on:** August 08, 2019, 07:34:02 PM »

Ahh, I understand. The stock pump cup has been replaced by the flat top. It's probably just the o-ring which has been damaged. I'll go at it again. Hopefully I'll get it right the second time... or third.

No microwave oven parts. I used an X-band radar door opener module - low power (which should be good enough) and $3, if you shop around on ebay or ali express. Microwave oven parts could be used, technically, but there is a significant shock hazard, not to mention a regulatory issue. Low power stuff is much easier and safer to play with - if you screw up, you're not dead.

Robert 5mm · « **Reply #12 on:** August 08, 2019, 07:39:02 PM »

did Nick Fury tell you to say that...

WobblyHand · « **Reply #13 on:** August 08, 2019, 09:25:40 PM »

Quote from: Robert 5mm on August 08, 2019, 07:39:02 PM

did Nick Fury tell you to say that...

No. Nick is an imaginary character, created by Jack Kirby and Stan Lee in 1963, he doesn't exist.

On the other hand, my experience with radar is over 35 years. High voltage in radars can kill you. That's why I chose a low power module. If you have something of value to add to the thread, please feel free to contribute.

Robert 5mm · « **Reply #14 on:** August 08, 2019, 09:39:12 PM »

Sorry for making a joke...
I was actually making a compliment about your knowledge.

WobblyHand · « **Reply #15 on:** August 08, 2019, 11:33:10 PM »

It seems that, somehow, I missed the joke. Guess it went right over my head. My apologies.

Thanks for the compliment. Doing this chrony has been a long, long, frustrating exercise. It was good today to see it work, for the first time with live fire. Lots to work on still. Like somehow putting all that "stuff" into a box and still having it operate.

But next, I've got to fix the o-rings... Have to stay on topic

WobblyHand · « **Reply #16 on:** October 22, 2019, 07:15:08 PM »

And several rebuilds later, I still have erratic chrony values. (Bargain gate aliexpress chrony) It's getting a little frustrating. I want to shoot it, not futz with it.

I used Beeman WC pellets (I weighed them to be around 8.2 grains) and got some dieseling for the first 3 shots, and then things calmed down. But the velocity variation is still pretty large.

I then ran a second group of 10 shots the next day (today) and the values are all over the place. The first 2 shots were at 342 fps, then the following 8 shots had significantly higher velocities. (344.82, 341.86, 490.49, 404.53, 383.20, 402.89, 483.60, 443.57, 549.54, 474.74) ES 207.68! The first 2 shots seemed not to pump very well, or rather offer no resistance at all.

Thought it might be the pellets (nah, but it was worth a try) and reran with Crosman Premier 7.9 grain, 8/14/15, Lot B pellets. I then got (474.74, 459.32, 494.42, 538.06, 509.19, 538.71, 486.22, 467.85, 519.69, 478.35) ES 79.4. I should be able to do better than this right?

New seals everywhere, including the transfer port. Using a 70D #111 o'ring for the flat top piston and 90D #111 o'ring for the valve. When I opened it up, the pump tube seemed to have a small rust spot in it. I don't remember seeing it before. I attempted to polish it out, using 400 grit sandpaper on a 1/2" dowel, but there seems to be a small pit. I have no idea how the rust spot got there. The piston o-ring traverses the pit every pump. I put a tissue paper over the breech and nothing moved when shot.

Any suggestions where to look? What to do? Am I looking at a new pump tube? I'm guessing a new pump tube isn't that expensive, but it kind of stinks to replace one that is less than 18 months old.

maraudinglizard · « **Reply #17 on:** October 22, 2019, 08:23:23 PM »

You have to cock the gun before pumping, the poppet may be opened enough to let air pass through. I have a rear cocker 1377 that I rebuilt a few years back and ran into a cup problem with the flat top piston. I average about 620fps with a cphp 7.9 pellet. Here is my post, it might help.

https://www.gatewaytoairguns.org/GTA/index.php?topic=83670.msg802677#msg802677

WobblyHand · « **Reply #18 on:** October 22, 2019, 08:54:52 PM »

maraudinglizard, I have been cocking the gun prior to pumping for every shot. This is a new flat top valve - it could have a problem, or junk in the poppet. I never disassembled the valve. Looks like that is something to pursue. I will read your post.

WyoMan · « **Reply #19 on:** October 22, 2019, 10:06:36 PM »

Quote

Occasionally, the pump seems to "slip", meaning that there isn't much resistance.

Quote

The first 2 shots seemed not to pump very well, or rather offer no resistance at all.

From that I gather the piston o-ring seal to the pump tube is intermittent. I've had that problem with dry or damaged o-rings, or a rough surface on the inside of the pump tube. I had one with rust on the inside that did that. Lots of oil and pumping smoothed it out. I wouldn't take 400 grit to the inside of the pump tube though.

Are you keeping the oil-wick wet with the supplied (airgunsmith) oil? You are using the right piston o-ring and I assume that you're certain it is not damaged. But the real test for a good piston / pump tube seal is that it should pull a vacuum on the intake with an audible hiss with the incoming air... as Back_Roads mentioned earlier. You may need a new pump tube.

I would not suspect the valve. If the exhaust valve doesn't seal, you would hear flatulence noises out of the barrel. If the check valve doesn't seal, the pump arm will fly back. If the valve body or tube o-ring doesn't seal you will feel air coming out of the tube (under the breech and above the grip frame).

You completely lost me with this...

Quote

got some dieseling for the first 3 shots, and then things calmed down

But I hope you get it sorted...

Wyo

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Flat top piston and valve install - is this right?

Author Topic: Flat top piston and valve install - is this right? (Read 2667 times))