Machinist (?) advice needed

[eaglemike]

If it's truly a 4-40, Brownells has some if you want to order. McMaster has a lot of them as well. They might not be exactly the same amount of unthreaded length, but will work.
How did that 13/64 number above show up? That's about .203.....

[baih777]

Try this website.

https://www.gunpartscorp.com

[baih777]

You need a thread pitch guage.
A lot of guns use a fine thread pitch. Not a standard fine pitch.

[heidad01]

The guy who made that new screw as a replacement (for pictured old one) must have been a blind goober.
Any half competant machinist should be able to chuck a piece of 4140 or drill rod and cut an exact thread and shape the bolt head. But, one of s are very expensive.

What is wrong with ordering the $5 part from the posted link above?
A lot of gun screws are of different threads from the very common industrial use screws.

Had you posted your location/vicinity, by now, some one with a lathe may have offered to make one for you.
Good luck.

[Half Cocked]

You need a thread pitch guage.
A lot of guns use a fine thread pitch. Not a standard fine pitch.

Gun makers sometimes use an oddball thread pitch in their products. An example is #6-48 thread. My local ACE hardware has a some bins with gun screws in them or you can try Brownells.

The first thing that you need to do is identify the thread diameter and pitch of your screw. Once you know that it will be much easier to find a replacement screw.

[ar15barrels]

Here is the formula for telling the screw size number:
(.013 X (screw number)) +.060 = nominal screw outside diameter

hah!
I know common screw diameters off the top of my head but I never knew the formula to determine them was so simple.

It would seem to be that determining the unknown screw number FROM a diameter would be something like:
(Unknown-screw-diameter - 0.06)/0.013

Allowing for 0.007 in thread top radiusing adjusts the formula to:
(Unknown-screw-diameter - 0.053)/0.013

So his 0.104 mystery screw is (0.104-0.053)/0.013=3.92
We round 3.92 to 4 because we are looking for a "standard" screw diameter number.

[Half Cocked]

It could be a 4-80 thread.

4-80 -- Die - (creates 4-80 ultra-precision thread)
Stock# DIE480 Price: $19.95

H.S.S. 4-80 dies are very rare. They are designed to thread 4-80 micro precision threads on rod stock. These threads are used by knife and gun makers to create a maximum adjust threading, or to create a thread that is less prone to "back-out", a common issue with standard coarse threads.

https://www.knifekits.com/vcom/die-creates-ultra-precision-thread-p-1502.html

[ar15barrels]

If it's a weird #4 screw that you can't get anymore, I would probably be tempted to upsize BOTH screws and tap the action bar to 6-48 and replace both screws as 6-48 are MUCH more available in different configurations of head style.

[ar15barrels]

It could be a 4-80 thread.

I think you could be right.

There is around 8 threads showing in what is about 1/10 of an inch which would correlate pretty well to 80 TPI on a #4 screw.

Brownells has screw blanks that could be threaded with that a 4-80 die.

https://www.brownells.com/gunsmith-tools-supplies/small-hardware/metal-screws/no-4-blank-screw-refill-pak-sku080011004-376-1718.aspx?sku=080011004

[eaglemike]

hah!
I know common screw diameters off the top of my head but I never knew the formula to determine them was so simple.

It would seem to be that determining the unknown screw number FROM a diameter would be something like:
(Unknown-screw-diameter - 0.06)/0.013

Allowing for 0.007 in thread top radiusing adjusts the formula to:
(Unknown-screw-diameter - 0.053)/0.013

So his 0.104 mystery screw is (0.104-0.053)/0.013=3.92
We round 3.92 to 4 because we are looking for a "standard" screw diameter number.

That's it. I'd hoped others might be able to keep this in the background for the next time they need to figure out a fastener.

Another useful formula I'm sure you know, but some won't:
minor diameter for a nut, using a cutting tap, is the nominal major diameter, minus the pitch of the thread.
For 1/4-20, drill size would be .250-(1/20) = .200, closest drill is .201
for 3/8-16, drill size would be .375-(1/16) = .3125 closes drill is 5/16. or .3125 exactly.
This is for general stuff, there will be differences for mil-spec stuff, etc, but this is close enough, using cut taps. Form taps are different, of course. Most guys here won't have them. I use form taps almost exclusively in aluminum.
Hope this helps somebody.

[sonofeugene]

It could be a 4-80 thread.



https://www.knifekits.com/vcom/die-c...ad-p-1502.html

That's what I said based on my measurements off the put. I've never heard of a 4-80, but you've just confirmed it.

The correct screws are out there. Unless you're in a hurry for some reason, get the correct screw.

[drkphibr]

Not quite sure how folks are coming up with a tread pitch of 80 as when I used my calipers I measured 5 threads in .125". So 5 threads in 1/8" tells me there are 40 threads per inch. Am I incorrect here?

[ar15barrels]

my attempt at general screw measurements...

Not quite sure how folks are coming up with a tread pitch of 80 as when I used my calipers I measured 5 threads in .125". So 5 threads in 1/8" tells me there are 40 threads per inch. Am I incorrect here?

Your very picture shows total screw length of 0.216".
The threads on the old screw are less than 1/2 of the total screw length.
8 threads can be counted in less than half of 0.216"

The NEW screw appears to be 4-40 or maybe even 4-48.
The OLD screw appears to be 4-80 by the fact that 7.5-8 threads are visible in right about 0.100" of screw length.

[eaglemike]

OP, if you have a complete set of drill, or a decent set of calipers, you could measure the minor (inside) diameter of the threaded hole.
By a complete set of drills - it would be like the big assortment sold that has fraction, letter, and number drills. You'd want to use #38 or #39. The shanks is a little smaller than the widest part of the cutting lips. A #38 or #39 would indicate it's pretty likely an 80 pitch thread.
There's a chance the hole is worn, and it's something else, but getting the minor diameter is a place to start if you have the drill set.
Good luck!

[drkphibr]

So these two views of the original screw are with my caliper set to 1/10" and there are 4 or potentially 5 threads at most showing so this thread pitch is either 40 or 48...definitely not 8 thread per 1/10" so I don't see how this screw's pitch could possible be 80.