So I have an aperature sight that’s on a piece of threaded rod. It has a special threaded lock washer on the threaded section to lock it into place once you set it to the height you need.

I have a tapered tap I bought for some project long ago, it is marked 8-40, NS, 28, 16.

The threaded lock washer threads perfectly on the tap mentioned above.

Can you explain to me what the numbers mean?

I know the 8-40 is the size screw/post and 40 per inch.

I think the NS means special size threads(like for gunsmithing)

Don’t know what the 28 is.

Don’t know what the 16 is.

The sight

This message has been edited. Last edited by: MikeinNC,

quote:

8-40, NS, 28, 16

To tap for a 8-40 NS screw, use a #28 drill bit.

The "16" escapes me at the moment.

NS is National Standard as opposed to National Fine, or National Course in relation to threads per inch.

16 is the size if you use a "clearance bit" and are not going to cut threads, but will allow the #8 screw head to pass through the bore.

quote:

8-40

#8 is the thread diameter (5/32", decimal .16"), 40 is threads per inch. Similarly, ¼-20 is, respectively, ¼" diameter, 20 tpi, and so on. In metric, e.g., 8x1.25 is 8mm diameter, 1.25mm the distance between the crests of the threads. In both inch and metric this is known as the thread pitch.

Got it done last night.

The portion the aperture goes was hardened, and I had to anneal it. I don’t have a numbered drill set but are fractions. I was diggin thru the dremel set and found a #29 bit. Worked perfectly.

Tapped the hole and got it installed. Then I lost the tiny pin that acts like a keeper for the whole thing. Where the arrow is in the pic. Spent 15 minutes looking for it and when I moved Mrs. mikes fancy new ride, there it was.

The company that makes them for $65, includes a pre threaded metal plug for the hole making the install much easier, but hey I just saved $65 !! Now I gotta go to the range and test fire it. And find a dovetail blank for the buckhorn sight I’m going to remove.and grub screws for the hidden holes in the barrel.

This is a Rossi model 92 a copy of the Winchester. The only difference is this lawyer safety that’s right where the aperture is now installed-the gun has a half cock notch.

For trivia (and perhaps trivial interest), the numbering of screw sizes does have a formula.

Number x 0.013 + 0.060 = major diameter (inches)

https://en.wikipedia.org/wiki/Unified_Thread_Standard