Tuesday, March 1, 2011

Modifying a T/C Rear Sight for a 22XX--Part 2

Ugh, got stuck for a couple days on this one. The adapter needs a simple through hole threaded for the windage adjustment bolt. Except the windage adjustment bolt is a #6-72. 72 threads per inch. Seventy-two. Most fine threads are around 32 to 40 threads per inch. I seriously thought about using a different size bolt--like a common #6-32. Would've saved an evening at the least. Except the base of that windage screw has a very fine serration to provide precise indexing detents that I'm quite sure I'm unable to duplicate. Hit my standby machine tool supply company, but Kromhard was out (or maybe never had) of 6-72 taps. Unfortunately, they had a threading die. "Unfortunately", because if they'd had neither one, I'd have either just ordered up a tap like a normal person, or switched out that bolt to something common and figured out a reasonable solution to the detents. (Like gone without them). Instead, I bought the die and figured I'd make a tap. Sure. For 72 tpi. Kromhard counter guy, Terry, thought I was borderline delusional. I didn't disagree.

Chucked and faced a piece of 0.250" drill rod. Centerdrilled in case it actually works and I ever need a piloted tap...

Then hand filed a square shank.

This part really didn't take long--maybe 10 or 15 minutes.

Then I turned about 1" down to 0.136", tapered the end and threaded 6-72. The low helix angle of the thread makes fine threading extremely easy.

Using the 13/16" end of the Taig die holder.

Heated the threads until a magnet was no longer attracted to the steel then quenched to harden the piece.

Then slowly reheated to approx 450 deg F in my Easy Bake oven for about 15 minutes to temper.

Ground a single flute.

Spotted the windage hole in the adapter and mounted it in a small vise.

Through drilled with a #33.

Then counterbored the right side approx 3/16" deep with a #7 for the coil spring.

Flattened the bottom of the counterbore with a modified bit.

And tapped 6-72.

Started in the drill press for alignment, but finished by hand. It actually worked.

Test fitting the adapter. I could just notch the top of the adapter and be done with it, but no, too easy.

Here's that spare leaf from the IZH-46M. It's double-sided.

Did some quick layout on the adapter. Scribed a line for the center of the mounting screws then did some math so the leaf would be centered.

The holes in the leaf are, in fact, already threaded by IZH. They're threaded M4 x 0.7mm. I'm not going to use those threads.

Center punched the marks.

As the rear of the adapter is angled, I wanted the threads 90 degrees to the face so the leaf would be drawn up flush. Drilled both holes with a #43.

Followed by a #4-40 tap.

Test fitting.

The threads need to be flush with the rear of the adapter for the windage adjustment to function. Cut the screws off with an Excel jeweler's saw.

Ran a piece of emery cloth across the piece to remove any remaining burrs.

Not shown. Blued the adapter with Birchwood Casey Perma Blue paste and assembled.

The original Thompson/Center sight leaf is there for size comparison.

Rear sight is done. At some point, I scratched two lines on the adapter and filled them with white paint. The middle of which is the center to act as a reference for the windage scale on the base.

Still need to attach it to the pistol and figure out an appropriate front sight. More to come.


Orin said...

Wow - 6-72. Who even has a thread gauge to measure 72 TPI? Crazy.

Derrick - Can you give me more info on starting the tap in the drill press? I've never done anything but hand tap, and you can imagine the results aren't always centered. Do you start on a low speed and use light pressure until the tap starts pulling itself in? Or are you just turning the chuck by hand? Thanks.

- Orin

derrick38 said...


I sure didn't have a gauge for 72 tpi. You need magnification to check the gauge.

Turn the chuck by hand. The drill press is used to simply align the tap vertically. On larger taps, sometimes all you need to do is get the thread started in the drill press, then finish with a tap handle.

Google "hand tapper" and you'll find some interesting tools to keep threads aligned. Tapping blocks are also very helpful if you're tapping by hand and need a vertical thread and/or are using a very small (fragile) tap.