FWB 124 Pro Mac Kit issue

[jimk1963]

1978 FWB 124, original owner. Gun had a kit from 2009, installed by Air Venture Airguns in Bellflower. No idea what kit was used, the piston seal is black synthetic and the spring appears to be an OEM full-length FWB spring. Gun was shooting great, around 870fps w/ 7g, 785fps with 8.2g. However, still a bit of twang and I'm fresh off rebuilding my Anschutz 380 so was feeling my oats. Ordered the Maccari Pro Mac Kit...

No issues at all disassembling the gun, installing the Pro Mac Kit, and reassembling. Gun is actually a joy to work on, very simple mechanicals and thanks to all you guys who post such great videos/photos/explanations. Prior lube job looked great... a light coating of heavy tar on the spring, and moly around the piston seal and piston-rear. Also a slight amount of moly on the spring guide. The receiver tube was very clean - guessing Air Venture did the hard work of cleaning up the old piston crumbs.

All I did was (per ARH instructions) clean up the old spring guide with a rag and lightly apply new moly, also on both sides of the ARH spring guide spacer, the top hat, and the piston front (around periphery of seal but not in front of the seal of course) and rear.

First shot - 400fps
Second shot - 380 fps
Third shot - 250 fps
Fourth shot - pellet didn't eject. Popped barrel open and a "pffff" sound happened as the piston was able to drive the last little bit to its end.
Fifth shot - same result

Disassembled the gun, I don't see anything obvious. The piston is definitely tight, it's one of those green "anti-vibration technology" seals ARH sells. Comparing it to the old seal (which I accidentally damaged so can't re-use), the green seal is slightly oversized as expected. I had no problem driving the piston home on reassembly, although I don't know how many lbs of force I'm using. It is snugger than the old seal but not by much.

This is baffling. Anyone ever had an experience like this where the piston doesn't even make it to the end?

 

[jimk1963]

Put the piston on my drill press (poor man’s large) and sanded down the seal very slightly, a few thousandths. On reassembly the gun shot a 10-shot avg of 687fps, spread 650-707fps. Still very crappy, but obviously the oversized piston seal was causing major problems. Will run more pellets through it to see if it picks up any, but I’m doubtful it will be much.

 

[Jonah]

If it ain't broke.......

 

[Feinwerk]

@jimk1963 ,
From your original description, it sounds like you did everything right, very similar to how I rebuilt my own 124. The piston seal should not be so tight that it causes a lot of drag in the receiver. I think you are on the right track, fitting it by sanding it down. I did a similar method, chucking the piston latch rod into my hand drill and spinning the seal against a section of sandpaper laid on a flat surface and checking the O.D. very carefully. Because of the elastomeric nature of the seal, it's difficult to get a consistent measurement with calipers. You have to go by the feel of the drag of the piston in the cylinder bore. I think if you hold the cylinder vertically, the drag should just barely be enough to keep the Piston from falling out by gravity or something close to that. Different tuners use different criteria I believe. It sounds kind of like yours was way way too tight.

If it's the parachute seal type (most are) that has a lip on its periphery that gets expanded as the pressure increases, it does not have to fit terribly tightly but you want to take care and not to remove too much material from the outer edge of the lip to be sure that it performs its function. I would not call myself an airgun tuner, but I used my own intuition as to how much to take off the seal in order to create just the right amount of light friction.

One thing that you very much have to watch out for is cutting the piston seal during assembly as the lip gets tucked into the receiver and the cocking slot cut outs. These are places where burrs can form during honing. You have to use a tiny probe to tuck the leading edge of the seal in as you advance the piston forward. Check to make sure you haven't accidentally cut your seal. You probably would have seen it when you took it out the first time for resizing.

Also, just double check that you have not lost or damaged your breech seal during maintenance. If it gets overly flattened, it begins to fail. You can do the tissue test, placing a tissue over the breech during firing and watching for it to jump from blow by air.

Lastly, run a nylon bristle brush through your bore a few times if you haven't already, to make sure that years of lubricant debris haven't built up in the rifling grooves causing excess drag. Based on the numbers from your original kit, this probably is not a problem.

Of course you can always put your old kit back in, assuming you didn't damage anything during the disassembly. You can also mix and match piston seals. You can try measuring the original piston seal and comparing it with the new one.

Experiment with different pellets and differing head sizes as well.

I don't remember which ARH/ Maccari kit I put in my 124, but with just a little bit of heavy tar on the spring, it has a vibration-free shot cycle. If you're getting anywhere near 800 FPS from it with no vibration, you're doing quite well. Mine is shooting H&N Field Target Trophy 8.65gr 4.51mm head size at 800 FPS.

Good luck,
Feinwerk

 

[jimk1963]

@jimk1963 ,
From your original description, it sounds like you did everything right, very similar to how I rebuilt my own 124. The piston seal should not be so tight that it causes a lot of drag in the receiver. I think you are on the right track, fitting it by sanding it down. I did a similar method, chucking the piston latch rod into my hand drill and spinning the seal against a section of sandpaper laid on a flat surface and checking the O.D. very carefully. Because of the elastomeric nature of the seal, it's difficult to get a consistent measurement with calipers. You have to go by the feel of the drag of the piston in the cylinder bore. I think if you hold the cylinder vertically, the drag should just barely be enough to keep the Piston from falling out by gravity or something close to that. Different tuners use different criteria I believe. It sounds kind of like yours was way way too tight.

If it's the parachute seal type (most are) that has a lip on its periphery that gets expanded as the pressure increases, it does not have to fit terribly tightly but you want to take care and not to remove too much material from the outer edge of the lip to be sure that it performs its function. I would not call myself an airgun tuner, but I used my own intuition as to how much to take off the seal in order to create just the right amount of light friction.

One thing that you very much have to watch out for is cutting the piston seal during assembly as the lip gets tucked into the receiver and the cocking slot cut outs. These are places where burrs can form during honing. You have to use a tiny probe to tuck the leading edge of the seal in as you advance the piston forward. Check to make sure you haven't accidentally cut your seal. You probably would have seen it when you took it out the first time for resizing.

Also, just double check that you have not lost or damaged your breech seal during maintenance. If it gets overly flattened, it begins to fail. You can do the tissue test, placing a tissue over the breech during firing and watching for it to jump from blow by air.

Lastly, run a nylon bristle brush through your bore a few times if you haven't already, to make sure that years of lubricant debris haven't built up in the rifling grooves causing excess drag. Based on the numbers from your original kit, this probably is not a problem.

Of course you can always put your old kit back in, assuming you didn't damage anything during the disassembly. You can also mix and match piston seals. You can try measuring the original piston seal and comparing it with the new one.

Experiment with different pellets and differing head sizes as well.

I don't remember which ARH/ Maccari kit I put in my 124, but with just a little bit of heavy tar on the spring, it has a vibration-free shot cycle. If you're getting anywhere near 800 FPS from it with no vibration, you're doing quite well. Mine is shooting H&N Field Target Trophy 8.65gr 4.51mm head size at 800 FPS.

Good luck,
Feinwerk

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Thanks very much Feinwerk, I’m in lock-step with everything you said here. Unfortunately I damaged the original seal when removing it from the piston, but I still have it and am using it as a rough guide for sizing the new seal. To your point, it’s a real PITA to micrometer these parts due to their flex, so for this first round I removed only enough to get the gun shooting again. Will sand a little more and try again. I did pay attention to how the original seal felt in the gun, and you’re right… the piston slid fairly easily. So I’ll try to mimic that. Also found other posts where shooters had this exact same issue until they sanded down that seal. One shooter even had his stick like mine did. I ordered two more seals as well in case I bugger this one up (I have two 124’s). Thanks again for all the great input, this forum is just awesome.

 

[jimk1963]

OK Round 3... sanded down the piston seal another few thousandths. More improvement, Velocity up to 740fps avg (10 shots) with spread of 723-754fps on 7g pellets.

Also tested with 8.2g, 20 pellets: 663fps avg, spread 650-675fps, st dev 6.8fps. Shots were pretty consistent at 665fps, only a few flyers low and high.

Will move to Round 4 of fine-tuning the piston seal diameter, see what I can get. Eventually, will also try putting back in the original spring to see if the Pro Mac kit is contributing to the slow-down.

As far as shot cycle, the twang is definitely gone, but even at these lower velocities there's clearly a pronounced jolt (sharp recoil), that wasn't there before. On the plus side, the shorter Maccari spring is simpler to reassembe (minor point though). Cocking effort is noticeably harder as well, by a few pounds. Still nothing like the magnums out there of course.

 

[Feinwerk]

Good progress!