Are stock tubes of AR15 threaded the same?

[LTC-J]

I've got a TM 80% lower I am building up.

I received a stock kit and the tube isn't threading onto the 80% I can get it just started and then it won't thread the rest of the way. I'm leery of putting too much force on it and stripping the 80%.

This is the second lower I've assembled using a TM 80%.

So the question is... are there different threads available for the tubes?

I was so darn close to having one complete rifle and I get this

[Chaos47]

All receivers should be threaded the same way.

Tubes have the same thread pitch but commercial and milspec threads are cut in a different process.

Maybe the anodizing is too thick?

[ar15barrels]

The threads should all be the same.
There are different specifications on buffer tube diameter, but they all thread into the same receiver threads.
If your lower is anodized, there is a chance the threads were made too tight for the type of anodizing that was performed on it.

The easiest solution is to get some 300-500 grit lapping compound and lap the tube into the receiver.

If you are in Los Angeles, I can do this for you in 10 minutes.

[LTC-J]

In the picture above, it appears that the thread is different between the mil spec and commercial.

I am trying to attach an A2 style stock to my TM 80% picked up from Vader ~Oct last year before the craze.

The vendor is claiming there is different thread. The stock is supposed to be a UTG which is mil spec thread. They are also claiming TM changed to commercial thread last year.

[ar15barrels]

In the picture above, it appears that the thread is different between the mil spec and commercial.

I am trying to attach an A2 style stock to my TM 80% picked up from Vader ~Oct last year before the craze.

The vendor is claiming there is different thread. The stock is supposed to be a UTG which is mil spec thread. They are also claiming TM changed to commercial thread last year.

You need to learn about the difference between pitch diameter and outside diameter.
Two different pieces can have the same pitch diameter, but different outside diameters.
The pitch diameter is the midpoint of the theoretical 100% thread on one side to the same point on the other side.
Pitch diameter is what determines the fit of two threaded parts.
The major diameter only determines the percentage of thread, or how big the flat top is on the crest of the thread.
75% threads are pretty normal and would have a flat too for 25% of the thread height.
The milspec threads are probably more like 60% threads.
That's why they have bigger flat tops on the threads.



There is no difference in the threads that are put inside the lower receivers.

[Baconator]

I had a TM that I finished that once anodized would not accept some tubes. Had to get a tap to clean out the threads.

[LTC-J]

ar15barrels, Thank you for the comments. I'm not a mechanical engineer by trade(software engineer by education/trade) but I dabble in lots of everything

I'm a little confused by this comment:

The easiest solution is to get some 300-500 grit lapping compound and lap the tube into the receiver.

Unfortunately, I'm in Sacramento area so I can't swing by for some expert help

In semi-layman terms, do I need to do a light sand job to the buffer tube threads to knock them down from perfect points?

[E-120]

Yes, only dufference is commercial is slightly larger in diameter

[ar15barrels]

In semi-layman terms, do I need to do a light sand job to the buffer tube threads to knock them down from perfect points?

Lapping compound is like industrial toothpaste.
It comes in all different grits.
When you have two parts that won't fit together, you put a little lapping compound on one of them and work the two parts against each other.
In the case of threads, you start threading in until it gets tight, then loosen. Then tighten. Then loosen, then tighten.
As the abrasive cuts, the parts will fit together better and better.
You keep doing that until they fit like you want them to.
Then you clean off all the lapping compound with brake cleaner and compressed air.

You can get 320 grit at autozone.
It is used for lapping valves into their seats on a cylinder head.

[LTC-J]

I thought I'd do a quick update on this.

You can get 320 grit at autozone.
It is used for lapping valves into their seats on a cylinder head.

ar15barrels, thank you VERY much for this advise. Worked like a charm.

Actually spent more time trying to find the stuff than using it.

Literally, I put a bunch of it on the buffer tube and then just tried to thread it into the lower. Very little force was required. Just a lot of rotating back and forth very similar to tapping a hole.

I also spent more time trying to clean off that foul smelling stuff than using it.

[ar15barrels]

Worked like a charm.

Actually spent more time trying to find the stuff than using it.

Literally, I put a bunch of it on the buffer tube and then just tried to thread it into the lower. Very little force was required. Just a lot of rotating back and forth very similar to tapping a hole

It does not take much.
If you had asked, I would have told you to use about as much as a green pea and spread it around in the first three threads of the lower.
Then work the tube in and out until it threads in far enough.
Brake cleaner works real good at removing the lapping compound when you are done.

I'm glad it worked out for you.