Thursday, May 7, 2009

Derrick's Crosman 180 Repair Part 2

First, a quick detour from the valve problem.
The end cap serrations were chewed up from pliers.
Note #1: Hey, don't use pliers.

I think every old CO2 gun I've seen has a chewed up cap. See note #1 above.

Long-time friend, Chuck, owner of Precision Airgun in Maple Heights, Ohio, mentioned that he cleans up cap serrations with a small diamond shaped needle file. Others (Mac1) use thread restoring files with success.

I'll go with Chuck's technique. I used the straight section of a riffler file and I tried a couple needle files. They worked just fine.

Cut and rotate. Cut and rotate. Tedious. Not the most exciting half hour.

The wire wheel finished the clean up.

The majority of the gouges are gone and the serrations look substantially better. Great. Now I've got to do all my old caps. Thanks for that, Chuck!

An out of sequence image. Reblued with Formula 44/40. All that work and now I'll probably use a bulk-fill cap.

OK, back to the valve problem that gave me fits.

Reassembly with a spanking new valve body o-ring was a complete failure. The CO2 was very rapidly leaking past the o-ring. (Remember, when the 2 brass valve halves are tightened together, the body o-ring expands outward against the ID of the gas tube forming the seal.) At first, I tried several different sizes of o-rings, thinking I was using the wrong size. No dice.

Then, I started paying more attention to exactly where the leak was coming from. Under pressurization, the valve was being pushed backwards in the gas tube very slightly. (--And yes, I did remember to tighten the locating lug. The valve was moving back a small amount, but it was enough that the o-ring didn't have full wall support of the gas tube at the front of the transfer port hole. I tried making a quick and dirty new locating lug from an allen bolt that was a tighter fit in the gas tube hole. Nope. The valve still moved back and leaked under pressure.

Depressurized the tube, disassembled the valve and took a hard look at the valve threads.

The threaded hole is stretched backward allowing the valve to push backward under gas pressure causing the leak at the transfer port.
To fix the gun, there appear to be but 2 choices. I can buy a new valve from Archer Airguns or I can attempt a repair on the existing valve. As it stands, this valve is useless, so I'll attempt to fix it.

I gutted the valve body of all internal parts, turned the threaded lug hole / exhaust vent hole 90 degrees from vertical and centered it up on the drill press. I don't want a repeat of the stretched hole scenario, so I located the new lug hole 0.0885" further back. I spotted the hole then drilled with a #21. Changed bits and flattened the bottom of the hole. Didn't drill completely through the valve wall and into the stem hole through the center of the valve. Though, no real problem if I had; the stud would have sealed the hole once installed.

Another view.

Here's a good shot showing how much further back I located the lug hole. New hole is up top on the right.
Yeah, I know that moving the valve forward in the gas tube will cause a drop in velocity. The hammer won't hit the valve stem as hard or move it as far forward. Either way, the valve will be open for less time and the gun will lose some power. I'll deal with that if I can first make the gun go "bang".

Tapped the hole #10-32. Finished the thread with a bottoming tap. (Actually I reground a bottoming tap and really took the thread to the bottom of the hole.)

Reassembled and ready to test.

This is where a bulk-fill cap can earn it's keep. Once the valve is inside the gas tube, the stud can be threaded into the valve to anchor it in place. Use the valve wrench, tighten the front of the valve to expand the body o-ring and thread down the bulk-fill cap. Now a very tiny amount of CO2 can be bled from the paintball tank to check for leaks.

By the way, failing to adequately tighten the locating stud could/will cause the valve to exit the end of the gas tube like a missile and lodge in your skull. --Uh, there's your warning. Remember, that stud is the ONLY thing holding the valve in place. There's no hammer assembly and end-cap to slow it down or stop it as in an assembled gun.

The pressure held. I used a piece of 1/2" aluminum rod about a foot long to tap the exhaust valve pin and the valve functioned. I checked it again at 24, then 48, hours later. Still holding gas. I initially added about 1 or 2 grams of CO2 using the bulk-fill cap. If I only had a standard cap and 12 gram cartridges, well, that's a lot more CO2 to pressure test with.

More to come...

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