Importance of truing threads?

[mram10]

After a month of playing on my new lathe and reading how most smiths says the threads are better loose than tight, why is it important again?
1. If shoulder and receiver are square, they will mate when torqued down, correct?
2. Does it correlate with centering the bolt face (firing pin) to the chamber?

 

[Shawnba67]

The threads will try to center up on each other if allowed thats why looser doesnt seem to have any issue. If reciever threads arent parallel then barrel will not center up parallel in them

 

[carlsbad]

"loose is better than tight" means to me, tight is bad. if they are too tight, then they will bind and your high spots will center the joint instead of the taper. if your shoulder is square and your threads are nice 60 degree triangles, then tightening the threads together will cause the tapers of the threads to reach the lowest energy state which is centered.

A properly fitted thread should thread on easily by hand and wiggling it will reveal minor play, but not much. No play, is too tight. a lot of play, is not good for a high strength joint. for a 6br or a 223, I guess a sloppy fit would be fine, but why be sloppy. Slight play and easy hand fitting and you're good to go.

--Jerry

 

[mram10]

Copy all guys, thx.
I made a jig that has a 1” 1-1/16 x 16 tpi that screws into the action with a .700 hole in the center. Then machined a rod that tapers from .700 to .703 so it holds nice and snug in the rear bolt thingy (no idea what they are called). I run a tap to clean up the threads first. I figure if the jig fits snug and doesn’t bind, the threads are centered enough. Is that a safe assumption?

 

[Tucker65]

If you want to see how bad your threads are just chuck up your threaded jig in the lathe, dial it in so it's running true and screw your action on it. After you see how much the receiver runs out you'll know why the threads get trued up on a lot of them.

John

 

[tclaunch]

If you want to see how bad your threads are just chuck up your threaded jig in the lathe, dial it in so it's running true and screw your action on it. After you see how much the receiver runs out you'll know why the threads get trued up on a lot of them.

John

This +

 

[JRS]

Quote:

"Fitting barrel(s) to your action with the closest fit thread possible. This guarantees that the centerline of the bore is as close as possible to the centerline of the action. Thus, the cartridge is not forced into the center of the chamber by an inconcentric action/barrel fit".

 

[butchlambert]

Quote:

"Fitting barrel(s) to your action with the closest fit thread possible. This guarantees that the centerline of the bore is as close as possible to the centerline of the action. Thus, the cartridge is not forced into the center of the chamber by an inconcentric action/barrel fit".

Just my opinion JRS, but vee threads and a square shoulder will center it, even if the threads are not tight.

 

[Eddie Harren]

Just my opinion JRS, but vee threads and a square shoulder will center it, even if the threads are not tight.

Be careful there Butch. Remember who you are talking to.

 

[JRS]

Be careful there Butch. Remember who you are talking to.

The quote is from a well respected professional. If you want to debate his method and reasoning, take it up with him.

 

[GenePoole]

You never know until you check. I built a tool that is roughly a 2 inch long cylinder about 1.25 diameter with a 1.0625x16 thread on one end. All turning/threading operations were done without removing the workpiece so everything is true to the spindle bore. I set up my actions using a Manson ground dowel and bushings in the raceway as is standard practice. Thread in the tool and put an indicator on it. You'd be surprised how far they are out; eccentricity of the threads, squareness of the face, axial alignment, etc. You would think that it wouldn't even be possible to get them this far out unless they do every operation in a separate workcenter in a 3-jaw chuck.

I've seen threads off by 0.009 from centerline, 0.002 off axially (over a 2 inch run), and the face off by 0.002 from perpendicular. Integral lugs are surprisingly square. They must do those in the same operation in which that they cut the raceway..

I wish I had some pictures or video. Perhaps I'll add that to my todo list.

 

[RWO]

Quote:

"Fitting barrel(s) to your action with the closest fit thread possible. This guarantees that the centerline of the bore is as close as possible to the centerline of the action. Thus, the cartridge is not forced into the center of the chamber by an inconcentric action/barrel fit".

Whomever said this never had a college level course in engineering mechanics. The important thing is to have a solid connection between the receiver and barrel. Smooth "V" threads and a square and smooth receiver and barrel shoulder providing near 100 % contact is the best we can do. It's just like a threaded lathe spindle. The chuck threads and spindle register don't center the connection until the chuck is tightened against the spindle shoulder. I remember reading about a few cases back in the day where Mauser receiver rings (case-hardened)
were cracked by thermal expansion of the barrel when they were fitted very tightly. Besides, I don't think a few thou separation between the receiver and barrel centerlines makes any difference as long as connection is solid. JMHO.

RWO

 

[JRS]

Whomever said this never had a college level course in engineering mechanics. The important thing is to have a solid connection between the receiver and barrel. Smooth "V" threads and a square and smooth receiver and barrel shoulder providing near 100 % contact is the best we can do. It's just like a threaded lathe spindle. The chuck threads and spindle register don't center the connection until the chuck is tightened against the spindle shoulder. I remember reading about a few cases back in the day where Mauser receiver rings (case-hardened)
were cracked by thermal expansion of the barrel when they were fitted very tightly. Besides, I don't think a few thou separation between the receiver and barrel centerlines makes any difference as long as connection is solid. JMHO.

RWO

"Whomever said this never had a college level course in engineering mechanics"

 

[dcali]

Food for thought:

How much does barrel concentricity to the action matter after you cut out a feed port, gas port(s), bolt stop, etc, bolt to to a stock, and then hang a 2 pound scope off the top? Why are you after concentricity to the action's outer surface? Seems arbitrary.

Edit: It was pointed out that it's the action centerline, not the outer surface that matters. I accept that, but it still strikes me as arbitrary. if the action is parallel to the bore, that's all that's required. Am I wrong?

 

[shortgrass]

Just my opinion JRS, but vee threads and a square shoulder will center it, even if the threads are not tight.

Threads are not an alinement device, they're a fastener, they hold 2 or more parts together.

 

[STS]

Threads are not an alinement device, they're a fastener, they hold 2 or more parts together.

Alinement may not be a threads primary function but the unmistakable fact is that a "V" thread will center on the common centerline of the two mated components. Thread fit isn't all that important either. The biggest danger in thread fit is galling a thread that is too tight. I've not ever had it happen but have seen it and it's a huge disaster.
Early in my benchrest shooting I had a gentleman fit a Hart barrel on a rifle. He screwed up the threading and the threads were so loose that when you would break the barrel loose from the action face the barrel would fall to the bottom of the barrel channel of the stock. I just knew that barrel was ruined. It turned out to be a smoking hot barrel that won many aggregates. It's not that important.

 

[treeman]

After a month of playing on my new lathe and reading how most smiths says the threads are better loose than tight, why is it important again?
1. If shoulder and receiver are square, they will mate when torqued down, correct?
2. Does it correlate with centering the bolt face (firing pin) to the chamber?

Least play possible but be able to thread together by hand. In other words a nice fit. I'm not sure this matters or not but i always knock the sharp points if the threads off with a file.

 

[carlsbad]

Least play possible but be able to thread together by hand. In other words a nice fit. I'm not sure this matters or not but i always knock the sharp points if the threads off with a file.

Good plan. Better plan is to start with the right diameter that there will be no sharp points. you can look up standards for major diameter. For example 1" 12tpi thead majore diameter is 0.9868 to 0.9982. I generally choose something like .995-.996 for 1" threads.
--jerry

 

[STS]

I'll do my best to summarize why I true action threads. Most of us here know (or are learning) that a rifle is more that a bunch of individual parts assembled into a workable unit. Things have to work together to get any accuracy out of the final product. We discuss clearances when chambering a barrel but there are considerations that are almost never mentioned. For example we had a thread recently where bolt nose clearances were mentioned from less than .005 to as much a .010 if I remember correctly. The exact numbers are not all that important relative to this subject. What is important is where the barrel is relative to the centerline of the action which controls where the bolt nose lives. If we have an action with threads that are .010 off the centerline of the bolt bore then the barrel counterbore and bolt nose may (almost certainly will) come in contact with each other which has been shown to cause flyers. There has to be enough clearance in this area and if the barrel is off center you really never know exactly if the clearance is adequate.
I was given a near new fluted Hart barrel by a friend that said no matter what he tried it wouldn't shoot. I surface ground a recoil lug so that it would headspace on one of my rifles and it shot superbly. I sent it back to him and told him to put some dykem on the bolt nose and shoot it. The bolt nose was contacting the counterbore and causing the problem. This action didn't have the threads trued, only the action faced off. The counterbore was opened up and the flyers went away.
I also sleeve every bolt so that it stays on center in the action. This will keep the trigger sear from pushing the bolt out of center and lifting the top lug off it's seat in the receiver. These techniques are pretty labor intensive but once they are done I never have to do them again for that action and always have yielded excellent results.

 

[carlsbad]

STS. Good post.

Sleeving bolts would be much less time consuming if we could buy premade split sleeves.