Friday, March 28, 2008

Fixing the Predom Lucznik Rear Sight

I recently bought a Predom Lucznik air pistol from Military Gun Supply. There is a lot written about the pistol already on the web, so I won't rehash that all here. MGS doesn't have the pistols up on their site any more, but they may still have a few in stock. The price was $50.00 plus about $15.00 S&H. The gun as received was a tad rough, which is to be expected from military surplus.
As received, my main problem was the looseness in the rear sight. Generally you want your sight to be accurately adjustable, and once adjusted, to stay put! The rear sight on the Predom could easily be wobbled back and forth. So I set out to fix it. Let me state that I would not attempt such a repair on a valuable airgun, as it is possible that in trying to fix a minor problem, a problem of far greater magnitude could be created, in this case a broken rear sight...

The rear sight in place.

With a ruler placed approximately perpendicular to the pistol, the sight can be tweaked right...

...and left by maybe 10 or 15 degrees...not good.

The sight disassembled. Remove the elevation screw, then the screw which affixes the sight to the pistol. Snap the sight mount out from the rear sight unit (see the barbed spring fork?) and then unscrew the adjusting screw and knurled nut. Don't lose the spring washer(s)!

This is the angled notch that bears against the spring of the sight mount.

It's hard to see, but basically there is a gap, the spring does not press against the notch and thus the sight is loose. I took some measurements and the slot in the spring (I'm just going to call the
sight mount the "spring" for now, bear with me) measured .083" near the oval cutout to the left of the picture, and .076" up near the notch. So it had clearly compressed over time.

First off, let me state that I am wearing safety glasses while I do this.
I put the spring in a small machine vise (a very handy Palmgren vise ), stuck a stubby Craftsman screwdriver in the slot and put it under my arbor press. Then with only mild pressure from truning the handwheel, rather than the lever arm...

...I pushed the screwdriver down into the slot. This is the point at which things could go terribly wrong. Worst case would be a fracture of the spring, and slightly less worse would be a deforming of the spring so it was no longer flat. So again, don't do this if breaking the sight would break your heart as well. In any case keeping one hand on the screwdriver to minimize twisting, and going slowly with very little force seems to have worked. I pushed down a little, removed the screwdriver, measured the width, and repeated, pushing down a little further each time until the gap was spread far enough. Taking it slowly is important for things like this. The spring was spread beyond the point at which it would spring back to the original loose dimension, and took a set so that it was slightly wider at top than near the oval.

The sight adjustment screw snapped into the spring, and was held firmly.

Now we had to fix the other issue, which was the folded ears on the sight body which fold around the spring. They were loose as well. The spring thickness is .047", and the sight ears had a gap of .053"

I placed a stack of automotive leaf gage/shims measuring .045" into the slot and squished the ears (one side at a time) in the vise (I told you it was handy). Because the ears will spring back slightly when compresses you want the shims to be undersize by a few thousandths.

Sure you could eyeball it without the shims, but why? With the shims in place you are guaranteed to not squish the slot too far, and get even pressure over the entire ear. If it's still to big, reduce the shim stack another couple of thousandths and try again.

The sight is tight! Although there is a flaw in the design. Because the spring is designed to allow the insertion of the sight body, it has to spring inward under pressure. So you can still wiggle the sight if you grip it forcefully. But it is no longer loose.

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