TAU 7 valve & spring issues / trouble-shooting help

[quillhyde]

I acquired a TAU 7 - it's from the 80s - in an auction and have been struggling with it to get it to run correctly. Thanks to this forum, and gwhite, I've been able to get pretty far.

Initially, the valve and seal leaked really badly. I took it apart, and found crud on the valve seal - bits of loose rust and oil. I cleaned it thoroughly but was unable to get it to stop leaking. It came with a spare seal, thankfully, so I swapped in the spare, and it stopped the leak. So far so good.

The issue I'm having now is that the valve will bind open, randomly (as far as I can tell) on a shot and drain the powerlet in a woosh. It's shot a couple powerlet bulbs completely, but sometimes I only get a few shots before it fails. It's great when it works. I did get 108 shots out of a single bulb, so I don't think the power is set too high.

Here's what I've observed: After the fail, the valve / spring assembly is bound open. I can see that it's not seated by looking in behind the hammer, and down the breech hole. Usually, when I start to loosen the access screw there's a click as the valve extends and reseats. Once, the spring was actually partially slipped over the valve, and stuck onto it. In any event, once I take the plug out, the spring and valve fall out freely and don't appear bound up at all. I can then reinstall and reassemble and carry on for a while until it does it again.

I'm wondering if I have the wrong spring, or if this spring is just tired, or maybe someone knows something. I can't find any reference to a similar problem. I don't think I messed anything up with the seal, I was pretty careful not to bung it up during the install, and everything else appears in working order.

I measure the spring I have as having a .279" OD, 1.120" free length, wire approx .017" diameter.

thanks for any help, I really appreciate it.

Quill

[Gwhite]

Unfortunately, that's a new one on me. Does the seal you replaced have the steel sleeve in it (old style), or was it all plastic (new style)? The hole is pretty big on the plastic ones, so I doubt it's hanging up on the new seal unless there is a burr someplace.

The inside of the hole behind the seal can be a little rough, and the flange on the valve may be catching on something. This can be aggravated if the spring isn't pushing the valve straight, or slightly up to fight gravity. I would take it apart carefully, and keep track of which way is up. This may be complicated by the fact that the spring & valve may rotate with the unscrewing of the rear plug. You can try to mark the end of the valve through the hammer slot on one side with a extra-fine tip marker before you take it apart. You can then try to rotate it 180 degrees and see it that helps, but also examine the spring and see if it has any sort of bend or curve to it. The spring should be smaller in front, and it should hold onto the back of the valve. Also, look at the end of the valve at the little step in front, and on the flange to see if you see any wear marks from where it has been catching. The front portion should be guided by the hole in the frame, so my prime suspect is the flange. The valve may also be getting pushed sideways by any lack of squareness on the end where the hammer hits it. Again, rotating the valve may fix that.

You can also look inside and see if there is any sort of prominent burr that it could be catching on. Stoning that down might help, but you have to be SUPER careful not to get any metal particles on the face of the plastic seal. There may be a burr on the bottom around the hole where the CO2 comes up from inside the grip, or even a bit of extra brazing material from when they assembled it. That may be far enough to the rear that it won't be a problem.

I have three springs, all different lengths, number of turns & wire size, so it's not very critical. Mine vary from ~ 30 mm long to 37 mm. I suspect the heavier wire ones may help center the valve better than the thinner ones.

Good luck & let us know what you find out.

[quillhyde]

OK, great, thanks for the feedback.

The replacement seal was the old-style, with the steel insert. I think the clue I was looking for is that your springs are smaller at the valve end.

My spring is the same at both ends, and a pretty sloppy fit on the back of the valve flange. It's not centered very well at all ... so maybe it's slipping off sideways and getting caught on the lip of the brazed in tube that supports the seal. If that makes sense. You can hopefully see in the pictures what I mean.

I can either try and tighten up the spring at the valve end - or maybe make a bushing that presses on the valve projection and centers the spring... or maybe I better think about it for a minute.

[Gwhite]

I'm pretty sure that is your problem. Every spring I've seen tapers down in the front to hold onto & center the back of the valve. A sleeve that centers the back of the vale in the spring you have might do the trick. I suspect when yours first started leaking, the original owner substituted a stronger spring. Also, getting 108 shots from a cartridge is unusually high, so the stronger spring is probably closing quicker and reducing your velocity.

As I said, the spring isn't super critical. I'd check a hardware store & see if you can find something close, buy a couple and see if you can reshape the end of one to hang onto the valve & keep it centered.

[quillhyde]

Awesome! Glad to have the corroboration, I'll proceed. Thanks.

[quillhyde]

Success!

Made a little aluminum spacer, it does the trick! It's a snug fit in the spring, a slip fit on the valve. Lathe for the win.

Shot off a bulbs worth - cranked up the power by a couple of turns - but I'm just guessing at the settings - next step is to find a chronograph. It's such a relief to have the gun working.

Thanks for having this forum, and being there to answer questions!

[Gwhite]

Nicely done! Having a lathe handy is a wonderful thing...

[Star7]

Please forgive an off-topic but possibly relevant question. Is this valve and spring remotely similar to the mechanism of the Tau 8? My Tau 8 just now started to dump it's (air) cylinders after a shot.....seems like the valve is sticking open. thanks!

[Gwhite]

I think the internals are a bit different, but almost all air/CO2 guns use a similar valve system. You cock a "hammer" against a spring, and when the trigger is pulled, the hammer taps the firing valve open. A combination of gas pressure & (usually) a spring close the valve after it releases a short blast of gas.

I couldn't find an exploded diagram of the Tau-8 on-line, but the external geometry looks very different from the Tau-7's. In the 7's, the hammer is in front of the trigger, and taps the valve from the front. The Tau-8 looks more like a conventional PCP pistol, where the valve is in front, and gets tapped from a rear mounted hammer.

It certainly sounds like your firing valve is sticking in a similar fashion, but fixing it could be quite different.

[Star7]

THANK YOU
I greatly appreciate your posts.