Tx200 velocity loss mystery

[Jonah]

Tx200 mystery losing 150+ fps velocity. My TX was averaging 648 fps with H&N FTT 14.66 grain 5.53 head pellets. Decided to do a Tune-in-a-tube. After the tune, velocity was 500 fps.

This was unexpected and a shock.  However while running shots across chronograph, I detected a blast of air from the loading port.  A tissue test confirmed a blast of air coming from that area.

This indicated that the breech seal was bad.  Replaced breech seals. No change, still about 500 fps. Then replaced piston seal and mainspring although I knew that neither of these parts should be causing the problem.  I was stumped and would have been pulling out my hair except my hair is too short to grab hold of.

Disassembled the gun again looking for a damaged piston or compression chamber. The nut on the end of the compression chamber containing the transfer port looked suspicious. It looked like a small gap between the nut and the compression chamber itself. Carefully clamped the nut in the vise and was able to turn the chamber about 1/16 th turn tighter. Took the chance and unscrewed the nut from the compression chamber. There is a thin o-ring on rear of the nut that was damaged in two places. I did not remove the o-ring waiting on finding a replacement. Put one wrap of Teflon plumbing tape around the threads and carefully screwed the nut back into the compression chamber being very careful not to cross thread the nut.

Reassembled the gun and shot it across the chronograph. Velocity was 614 fps. This may increase after all the new parts break in.

I know this is a stopgap repair.  However, no schematic I can find shows the nut dissembled from the compression chamber.

This is the first TX problem I have ever seen that no one has addressed. Anybody know the o-ring size for the transfer port nut?  Comments anyone?

Do I just have to go ahead and purchase a new compression chamber. I have a feeling that the factory never intended for this repair needing to be done.

[Jim-in-UK]

The same issue with the cylinder end staring to unscrew happened to a friend's TX, but that's the only case I'm aware of.

PTFE tape will serve as a stop-gap measure but, PTFE being slippery, the end cap will probably unscrew in the fullness of time.

I'll try and measure up an o ring in the morning, and would suggest fitting a new one, and using thread lock compound.

If you email Air Arms, describe the fault, and supply the rifle serial number, they will log it against your rifle's production record.

[Jim-in-UK]

As near as I can measure, the o ring is 1.7mm x 25mm, though this was a used example, and I don't have a new one to measure.

I would suggest visiting the air arms site and using the 'Contact us' facility to report the issue, and ask what size the o ring is, just to make certain.

[Jonah]

Got an answer from Air Arms. O-ring size is 1.6mm CS x 22.1mm ID. Thread sealer is Loctite 638.

[mustangtc65]

I applied Loctite 638 the the end nut and it solved my issue with fps loss.