I am the defacto primary gunsmith for the MIT collegiate pistol team. They have a large number of Czech CO2 pistols that were made about 20 or more years ago. They shoot very accurately, and are generally pretty reliable. The primary firing seal is dying on a few of them, and the replacements we have don't fit due to a design change at some point. seal is a short hard white plastic tube, press fit into a hole in the pistol frame. The actual firing seal is formed with a conical pin that presses into the hole through the center of the plastic seal. On the theory that the most likely seal material is Nylon 6/6, I bought some rod stock, and I'm going to try making new seals from scratch.The recommended approach for removing the old seals it to run a brass wood screw into the seal & then pull it out. The pulling part takes quite a bit of force, and the process means the seal can't be re-used once removed. However, the one I extracted is sufficiently intact to take some measurements from. Give or take a few tenths, the seal measures 0.3157" OD. The bore in the pistol measures 0.3138" ID.I've done some reading on both Nylon & press fitting plastics, and there are several issues that complicate matters:1) Nylon is hygroscopic, and will swell depending on the moisture content. That means if I machine one now when it is humid, it will shrink in the winter. On the otherhand, machining it will heat it a bit & dry it out slightly. 2) Nylon has a pretty high thermal coefficient of expansion. That means I probably need to let it cool off to measure it when I'm machining it. They recommend chilling the part to shrink it for the press fitting, but with high humidity, if I get it at all cool, it will be covered with moisture (if not frost). I'd have to drop the temperature ~ 60 degrees F to shrink the part by 1 mil, which definitely gets it into the frost category.3) If I make it sufficiently oversized to make sure that moisture & thermal variations don't allow it to...

Try pulling the seal with a brass screw fitted in the head of a small slide hammer. Available at Harbor Freight for less than $15.

Press Fit Nylon Seal?

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azjohn 14 years ago

Maybe a small wood stud? Wood screw on one end an machine threads on the other.

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Doug White 14 years ago

Doug White wrote in news:XnsA088BC87555B6gwhitealummitedu@69.16.186.7:

I never had time to chase down a good oil for annealing the nylon, so I just tried it as is. With sufficiently sharp tools & the right speeds, it machined OK. I could get within a tenth or two on the critical dimension. The only minor annoyance was cleaning off the burr inside the hole left from parting off. That's not a critial location, and a scalpel worked fine. The stuff is pretty chewy.

I greased the seal bore in the pistol and the seal, and then chilled the seal by inverting a can of freon "duster" spray & hosing it down. I had made a brass installation tool to guide the seal, and it pressed into place without too much force.

The matching valve stem that seats against the nylon had gotten lightly etched by corrosion, so I polished that up, greased that as well, and reassembled the pistol. I put a little CO2 in from a bulk-fill cyclinder, and Voila! No hissing, and I cycled it about 20 times without a hint of problems. Not sure if it will last another 20 years like the original, but I suspect we'll replace these pistols before then.

Now that I have the tools, material, dimensions & procedures, I can make more any time I need to. I made up one spare for now. We have a couple more pistols with the same problem, but none of the same exact style & vintage, so there's no telling what size seal they will need.

The whole adventure was quite a learning experience, which (up to a point) I always enjoy . Especially when one of the things I learned is that I _can_ actually fix stuff like this.

Thanks for all the inputs.

Doug White

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Larry Jaques 14 years ago

Congrats, and a hearty Well Done, Doug.

-- [Television is] the triumph of machine over people. -- Fred Allen

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Press Fit Nylon Seal?

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