Barrel Threads--Tight or Loose?

[HTSmith]

I have two different R700's set up as switch barrels. They were built by different Smiths but both shoot good and both Smiths have built match winning rifles. With one rifle I break the receiver loose with my wrench and screw it off by hand easily--same going back on. With the other I break it loose and have to use my breaker bar all the way off--very tight--same going back on.

Is there an accuracy advantage to tight fitting threads? What sort of fit do you strive for? Thanks for your input.

 

[338 Mollett]

If you have to use a tool all the way something is wrong ! Check to see if the front screw on the base mount is hitting .If not it is a poor fit .

 

[338 Mollett]

And by all means keep those threads greased.

 

[6fatrat]

Agree with both replies.

 

[Jesswil61]

If the threads are too tight the alignment is based off the thread, in order for the action face and barrel face to line up square there must be enough give in the threads to allow that to happen. Also if the tread is too tight it could affect the tensioning/ torque of the face, tensioning of a threaded joint is what holds it in place not torque.

 

[tobnpr]

^^^^
This.
Threads just serve to bring the parts together, shoulders align them. If you literally need a breaker bar all the way on or off- threads weren't cut correctly. If you have a 1"-2" mike and wires you can check barrel tenon OD and thread pitch on the barrel.

 

[BoydAllen]

I would rather have a loose thread than a tight one. V threads self center when tightened, and I want the barrel to square up on the shoulder, without being in a bind.

 

[Walt Krafft]

Your tight thread will gaul and become stuck. Not if -but when. When it does go back to the loose thread gunsmith.

 

[HTSmith]

If you have to use a tool all the way something is wrong ! Check to see if the front screw on the base mount is hitting .If not it is a poor fit .

The base screw could have been it. I took it out and filed it down. The threads on the barrel I took off don't look to be damaged. Yes I used a touch of synthetic bearing grease on the threads. (BTW what's the best lubricant to use here?) Thanks for everyone's help.

 

[jelenko]

How about anti seize compound?

 

[std7mag]

Anti sieze.

 

[gunsandgunsmithing]

Your tight threads are way too tight and it needs to be corrected before putting the barrel back on. There's a high degree of likelihood that it will gall. Anti seize and thread cutting oil are your friends here. Hopefully, it has plenty of anti seize on it now, but soak in thread cutting oil and don't force or fight it when taking it off. Get it fixed before it ruins the action ans bbl. A tight thread should never be forced. The only interference thread is a pipe thread. All others have clearance. Yours don't, if it literally takes a breaker bar to take it all the way off.

 

[hoz53]

I have two different R700's set up as switch barrels. They were built by different Smiths but both shoot good and both Smiths have built match winning rifles. With one rifle I break the receiver loose with my wrench and screw it off by hand easily--same going back on. With the other I break it loose and have to use my breaker bar all the way off--very tight--same going back on.

Is there an accuracy advantage to tight fitting threads? What sort of fit do you strive for? Thanks for your input.

and the only thing i might add-- is one of them a stainless receiver?

 

[Dgd6mm]

Question, to be clear here, should you be able to screw the barrel in all the way by hand before using your torque wrench?

 

[jelenko]

Yes!

 

[HTSmith]

and the only thing i might add-- is one of them a stainless receiver?

Nope--regular blued steel.

 

[tobnpr]

should you be able to screw the barrel in all the way by hand before using your torque wrench?

Yup. This goes for ANY threads- including class 3 threads for a muzzle device. And not just "by hand"- if you've got to firmly grip a receiver, brake, etc. just to spin it on- something is wrong.

Sometimes a burr can be raised at the thread crests when cutting them. Might hit the crests lightly with a fine file, thoroughly clean/flush the threads with some brake cleaner/compressed air to make sure no foreign crap is down in the roots (including the receiver). Then apply some anti-seize before re-assembling.

Best bet is nickel anti-seize. Copper is fine for plain carbon steel, but if you have a SS barrel nickel is preferred to avoid potential galvanic corrosion.

 

[gunsandgunsmithing]

Yup. This goes for ANY threads- including class 3 threads for a muzzle device. And not just "by hand"- if you've got to firmly grip a receiver, brake, etc. just to spin it on- something is wrong.

Sometimes a burr can be raised at the thread crests when cutting them. Might hit the crests lightly with a fine file, thoroughly clean/flush the threads with some brake cleaner/compressed air to make sure no foreign crap is down in the roots (including the receiver). Then apply some anti-seize before re-assembling.

Best bet is nickel anti-seize. Copper is fine for plain carbon steel, but if you have a SS barrel nickel is preferred to avoid potential galvanic corrosion.

Typical threads have a specification for the flat on top. They should not be left sharp. The spec is normally that the crest WIDTH be equal to 1/8th of the thread pitch. So, a proper thread of 1-1/16-16, having a .0625 pitch should have a flat width of .0078, with a min/max of .0055-.0133 for a class 3 and .0047-.0133 for a class 2
The top and bottom of a thread does nothing and it should have clearance. All it can do is cause problems.

I don't think this is the op's problem, as it would've already galled. But it is a common mistake among even very good self taught gunsmiths. It's explained in Machinery's Handbook

 

[AckleymanII]

If the top of the thread(crest)on barrel, digs into the bottom(root) of the thread in the action, you are in real trouble with galling and bad thread mating issues. Muzzle threads for muzzle breaks and cans are not to be forgotten in the issue of gauling.

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[DaveTooley]

Loose threads mean square shoulders. I use full profile threading inserts now but when I used a 60 degree threading insert I always turned the tenon diameter undersize by at least .005"-.007". I got to where I could gage the width of the flat on top of the threads to know when to check the fit.