Tau CO2 rear sight elevation adjustment not working

[motorcycle_dan]

Due to TMB syndrome (too many birthdays)
I'm shooting left handed and left eye dominant. (and not terribly well by the way)
I'm limited on what air pistols I have that are left handed. I run a air pistol club and we have several loaner pistols but only one Tau is left handed.
I've been using it the last couple weeks. It has the standard Tau sights. Adjusting the elevation up isn't working.
I can turn the knob and cause the rear sight to go down. But adjusting it up only takes the sights to a CM hold at 10m. I have a couple more clicks of elevation but then the knob turns and keeps clicking but the sights do not continue up. At max elevation the shots land 6:00 in the 8-ring with a 6:00 hold. Even lifting the rear sight has no effect. It is as if there is something not allowing the sight to pivot up completely.

Any thoughts? what to take apart. what to look for?

I'd like to shoot it sub-6 hold but can't with the rear sight blade that I have. I have a couple of other TAU pistols and their rear sight lifts higher. a link to an exploded view or take apart video is very appreciated.

[Gwhite]

The team I help coach has about 20 Tau's, and we've never seen this problem. Here's a nice color diagram of all the fiddly bits:



Here's a somewhat smaller black & white diagram that shows how they go together:



I believe the Tau's use a sort of wedge arrangement to raise & lower the blade. As you turn the knob, a block (#16?) moves back & forth on an angled surface cut in the back of the frame that changes the height. You can just see a little diagonal line on the rear of the frame in the smaller diagram that I believe represents the ramp.

Is it possible the front sight is off?

[saran]

Speed is ok? So heap the front post by 2 mm.

[motorcycle_dan]

I can see the little wedge and the inclined plane plate that causes the sight to raise/lower. The wedge is moving full width across the angled inclined plane but does not come up. From about half way, turning the knob CCW it will lower all the way to the bottom. Which pushes the wedge all the way to the left side of the pistol. (shooters view) as I turn the knob CW it raises the sight but just shy of half way across it stops moving up. I can continue to crank the knob but the wedge just gets loose and no longer contacts the inclined plane. Pulling it up or wiggling it has yet to make a difference. There has to be something stuck inside preventing full elevation.

Was hoping someone would have a clue how to remove the rear sight then disassemble. I don't want to just go knocking pins out.

[Gwhite]

I've never had to tear one apart., so this is only speculation. I'm guessing if you take out the main pivot pin (#41), you will be able to remove the sight and diagnose the problem visually. I think the color photo has an error in it. The main elevation spring is #60. The photo shows three sets of detent spring/ball pairs (#61 & 50), which makes no sense. The ones next to the main cross pin (#41) are spurious. There is no indication of their existence in the smaller diagram. The only part that will try to escape is the #60 elevation spring, and it shouldn't be too hard to keep that under control. It you want to be extra cautious, work inside a large clear plastic bag. Dry cleaner bags work well...

I have had to reposition a #41 cross pin once. It had drifted to once side to the point that the sight had popped loose & up at an angle. It was pretty easy to align things & drive the pin back in, so I don't believe they are particularly tight. (At least this one wasn't, or it wouldn't have shifted...)

Once you figure out what is going on, it should be easy to fix. Parts are available from Tau BRNO, although the last time I looked, their web site was a bit hard to navigate (at least in English). The worse that might have happened is that the elevation screw got stripped. From the diagram, the knobs are attached with small cross pins. It should be straightforward to swap in a new screw unless the crosspin hole is drilled in place, in which case you may need a knob as well.

Good luck & keep us posted.

[motorcycle_dan]

Well it is fixed.
If it is already broke, you can't broke it any further.
The cross pin was knocked out despite my best effort, spring went flying.
(no I wasn't smart enough to use a bag)
I found, a big spring, small spring and a detent ball.
Assume the small spring went inside the big one and the ball sat on top.
The problem was the front of the sight forward of the pivot point was contacting the top of the frame, not allowing the sight to come to full elevation. I used a File to remove metal on the underside which allowed the sight to come all way up.. I thought about just shortening it closer to the pivot pin but was afraid it would become weak in the area of the hole. Removed enough metal to effectively sharpen the underside and it seems to pivot up near max elevation, (springs removed and just lifting)
Putting it back together was a bit tricky. I used grease to keep the ball on the top of the spring. You have to have the sight rearward, press the springs/ball, then slide forward to engage the wedge into the inclined plane. Put a temporary pin in place to hold it.
Verify you get full travel and haven't knocked the little ball off. Once you verify everything is moving correct, use the regular pivot pin to push out the temporary pin. Center it up and done. Now it goes up all the way.

[Gwhite]

So there really was a third set of spring & detent ball? I wonder what it is for? It's certainly not obvious from the drawing/picture. May have been a design change between the two diagrams I have, but I can't fathom its function...

I have seen some Tau's where the bluing was worn where the front of the sight leaf had contacted the frame (hard). However, with students shooting them, they often make totally unwarranted adjustments (and in the wrong direction...).

In any event, glad you got it all fixed.