Wrong Threads on a Buffer Tube?

[MossbergMan]

I purchased a complete lower kit from Palmetto State Armory. It had a Magpul buttstock and mil spec buffer tube (or so I thought).
Long story short, the buffer tube will not screw in to my mil-spec lowers. It seems to be a tighter screw pitch. I should have suspected something when the castle nut wasn't screwed on more than 2 turns.
Anyone else run into this problem????

[SloChicken]

All receiver tubes are threaded the same 1 3/16" x 16 tpi. There is no difference in the thread height, thickness, width, or diameter. Unless it was off spec to begin with. The diameter on the stock end of a mil spec tube is 1.15" , the diameter of a commercial tube is 1.17".

The castle nut is probably hard to thread on because the channel (key way) that was machined in the bottom of the receiver tube deformed some of the threads, that or the tube got dropped on the ground causing the end to egg out or bend...

(edit) the thread height/taper on a mil-spec threading is a little higher than on a commercial thread, but again, diameter and pitch are the same. If you are having continuing trouble you can get a die and chase the threads or just use a little oil on the threads of the castle nut and it should work itself in.
That said, typically they are smooth fitting and should be easily spun with just a couple of fingers ...

Good luck!

[blazeaglory]

OP, Heres a little diagram, but I dont think it will help your problem...Sounds like you might have a stripped/deformed thread issue or something else. Buffer tubes, mil spec or not, will both screw into mil spec lowers. Mil spec buffer tubes are just made better.

[/img]http://i655.photobucket.com/albums/uu272/Wiringguy/milspecthread-1.png[/img]

[Arkangel]

Sounds like your tube may be defective or the threads have been damaged.

[G21Shooter]

It's damaged or out of spec. Get your self a Vltor buffer tube and move on.

[wchutt]

Have seen three buffer tubes from Palmeto with bad threads at builds over the last two months. They must have had a bad batch.

[ROOS_HAPPENS]

Check the hole in the lower for out-of-roundness. My buffer tube wouldn't screw into my TM 80% lower. Male buffer tube thread measured 1.144" pitch diameter when checked with a thread micrometer. It's within tolerance & should've fit. Print callout for the female buffer thread on the lower is 1.1461/1.1519" pitch diameter. I don't have a plug gage to check that internal thread unless I make one.

Minor diameter (bore) callout is 1.120/1.1283" on the lower. Mine measured oval @ 1.1185/1.122" when checked at 90 deg. intervals with a bore mic, I'm guessing probably due to spring back distortion after the lower was removed from the milling vise/fixture when it was being machined. The out-of-round condition is the probable cause for lack of fit.

I suspect we're all expected to own taps to fix something which should've been checked with go/no-go gages at the manufacturer.

By the way, this is NOT intended to slight TM, as my lower is otherwise beautifully machined.

[m16]

Find a local machine shop and ask if they can chase the threads on the receiver extension.

[ar15barrels]

Get some 320 grit lapping compound and lap the tube into the lower.
It works every time...

[ROOS_HAPPENS]

Randall, I just tried lapping the thread, and it worked out great! I had some 280 grit Clover valve lapping compound laying around, and in less than 5 minutes the fit was damn near perfect. Thanks for the idea!

[ar15barrels]

Randall, I just tried lapping the thread, and it worked out great! I had some 280 grit Clover valve lapping compound laying around, and in less than 5 minutes the fit was damn near perfect. Thanks for the idea!

I would have thought that lapping threads together with clover would have been obvious to anyone that owns pitch diameter mics...

[tal3nt]

Ownnnedddd

[ROOS_HAPPENS]

I would have thought that lapping threads together with clover would have been obvious to anyone that owns pitch diameter mics...

Oh, NICE snark! ...but many of us make stuff with shiny, sharp pointy cutting tools & leave playing around with oily mud to others.

Seriously though, I've found thread mics indespensible at work. Years ago, I bought an inexpensive Polish VIS 0-1" set from KBC in Fullerton, and in one of my better Ebay snags I stumbled across a mislabeled "Brown & Sharpe mic w/points" auction, which turned out to be a B&S/Tesa thread mic with a complete set of anvils. Ebay auctions where the owner has no idea what they're selling are always gold.
I still think the core of the issue is that companies need to pop a go/no go gage into a part before sending it off. Mine may have been checked in the machine after thread milling, but in a relaxed state the bore went oval.

[ar15barrels]

Oh, NICE snark! ...but many of us make stuff with shiny, sharp pointy cutting tools & leave playing around with oily mud to others.

Oily mud is great when the shiny parts don't fit together!
I have a full set of 0-1" starrett thread mics and use a couple of them almost daily.