Opinions on spiralock threads.

[Alex Wheeler]

I have had some barrels cut with this thread. I don't know how I feel about it. Seems it relies on a slight deformation of the crest of the thread. Opinions welcome.

 

[Scott Harris]

Sounds like a great idea. I looked into getting some sprialock threading inserts and it was going to be quite pricey. Gave me pause, but I'd like to try it out.

 

[dalewoolumjr.]

Had a customer ask me to thread a barrel this way. The engineering data from Stanley says this is for the female thread. So I refused to cut the barrel this way. Would be interesting to see data with the profile on the male thread.

Dale

 

[Erik Cortina]

It's for the female thread. It would be interesting to blueprint a Remington 700 action with this thread profile.

 

[Alex Wheeler]

Well, some very good br smiths are using it on the male side. I don't see how it matters if its on the male or female side.

 

[jonbearman]

What is the advantage of this thread over a std thread? Is this thread meant to be torqued to yield type threads that are used on modern internal combustion engines and it costs a lot more money for the three thread gages to check that it is spec and you have to sign a confidentiality agreement before they will give you the true specs to make the threads correctly. I think it is a farce as class 3 barrel threads are way more than what is needed to tighten a barrel properly . I don't buy this. It is just overkill. If you want a perfect thread contact Loctite makes a special compound to make the threads stay pit. It is not 271 ,it is a green color and cant think what it is called and comes apart without heat.

 

[mikecr]

I doubt it was ever about gun barrels staying tightened, as in not-loosening.
I believe this came out of Vaughn's testing where tenon thread unloading under fire affected results. A problem reduced by Spiralock threading and solved by a Savage barrel nut. This, analogous to improved mechanical connection(thread loading) of a stud over a bolt.

Spiralock causes more threads to be loaded through interference fit. A savage nut causes more threads to be loaded as it's pulling all threading into fit -and away from the boltface. That is, pre-tensioned in the direction of firing stress.
Standard barrel tightening(like a bolt) only loads the first few threads. The remaining threads are pulled toward the boltface side of pitch -where they would unload on firing anyway. Vaughn didn't like this, and it's seen as a weakness in a lot of 'important' applications.

 

[Alex Wheeler]

I doubt it was ever about gun barrels staying tightened, as in not-loosening.
I believe this came out of Vaughn's testing where tenon thread unloading under fire affected results. A problem reduced by Spiralock threading and solved by a Savage barrel nut. This, analogous to improved mechanical connection(thread loading) of a stud over a bolt.

Spiralock causes more threads to be loaded through interference fit. A savage nut causes more threads to be loaded as it's pulling all threading into fit -and away from the boltface. That is, pre-tensioned in the direction of firing stress.
Standard barrel tightening(like a bolt) only loads the first few threads. The remaining threads are pulled toward the boltface side of pitch -where they would unload on firing anyway. Vaughn didn't like this, and it's seen as a weakness in a lot of 'important' applications.

I have to disagree with you on a couple points. A stud does not have an improved connection over a bolt. More consistent torque, yes. Tensile strength and the amount the fastener is stretched is what dictates clamping force, it makes no difference if its a bolt or stud. The Savage nut will have the same issues as a shouldered barrel. The first few threads in the action take most of the load as well as the first few in the nut. As far as I am concerned the nut offers zero benefit to a shoulder. I also doubt that we are torquing barrels tight enough to be actually stretching the tenon.

The spiralock thread seems to equal the load across more of the threads. I don't really care for how it does this. I wonder if it is necessary. Its my opinion that you could turn the threads off the tenon, just leaving a few to torque the barrel on with and you wouldnt know the difference.

 

[mikecr]

There is a unique attribute to consider in this connection(gun barrel).
Here, instead of a outward transient force being exerted to a bolt head/nut, it is exerted to the threaded length. In other words, it is not the head/nut clamping force that holds a barrel in during firing, it's purely thread contact with the action.
Therefore, the more thread contact, the better the connection, regardless of tightness of the barrel(tenon stretch).

In this sense a stud type connection is absolutely better. While there is high stress in a stud's nut pull(inside the nut), there is less/more evenly divided stress seen at the internal action mating threads themselves, and more internal contact points are less distorted from machined pitch, given the same load. This is the function of Spiralock threading as well.
With a stud type barrel connection all threads pre-loaded remain loaded as the transient is outward to the threaded length.

With a bolt type barrel connection the first few threads are distorting to stretch the remaining threaded length. We're pulling those 'other' mostly unstressed threads toward the bolt face side of action pitch. This is just opposite of ideal with an outward transient to the bolt, as here even more of the threads will unload on firing(tension is lost instead of gained).

The difference between bolts and studs in all applications, is that bolts pull threading inward, and studs pull threading outward.

 

[Alex Wheeler]

How does a tenon know if it has a shoulder or a nut on the other end pulling it?
We used studs in race engines for more consistent torque. You were not fighting the twist/spring of a bolt. That is the benefit of a stud.

 

[Syncrowave]

Interesting.

 

[mikecr]

-A shoulder does not pull on anything.
It's the breech end threads that are pulling. First 3 threads assuming ~75% of the load, with no more than 6 effective at all(if tight enough). This is a standard in choosing bolt thread length.
-A barrel nut does pull.
It pulls all threads in the action(at the same time) against their pitch. Even if you had 1,000 threads on a stud, it's nut would easily(hand turn) pull them all into contact at once.

Totally opposite ways to achieve the same preload. But this is a unique condition in that it's not the action that's loading the threads. On firing the action is not pushing against the barrel shoulder(the bolt head). Firing is applying pressure against the chamber(the bolt itself) (away from the boltface), that's loading(or unloading) threads in the action.

 

[Alex Wheeler]

Like I said, I dont care how you load the threads in the action, use a shoulder, a nut, or just pull on the barrel the first few threads still take most of the load. A nut on the other end does not change that.

 

[Road_Clam]

I do CNC machining for an industrial severe service custom pump company and we frequently apply both bolts and studs as fasteners. When it comes to the "which is better bolts or studs" it all depends on the necessary precision torque application. When you torque a bolt you load the threads, and you get linear stretch. With bolts you also get radial twisting of the entire bolt shank. When you torque studs you are evenly loading both the bottom and top threads which almost eliminates the shank twisting. Radial twisting of the shank can lead to inconsistent torque readings. I prefer to use a small amount of anti-sieze on my bolts/studs to minimize the torque inconsistencies. However I am not speaking from the precision gunsmithing applications, but more from powersports applications. Interesting reading in regards to heat and deformation causing accuracy inconsistency.

 

[jfseaman]

I am most interested in stability of the tenon into the action.

Standard thread, well we know it works.
Acme and versions, got one. 6BR and 17Rem switch barrel bench rest rifle. The 6br is now into the mid .2s. More development needed.
Spiralock, looks interesting for stability of the tenon. Would require a commitment to recut the threads of an action, barrel it and test.

Of course there are lots of other factors. Action flex under whip (I do like barrel block setups), compression/tension compromises on barrel weight on the action. Etc.....

 

[Scott Harris]

you can spiralock the barrel threads....not sure why people think you have to do the action.

 

[dalewoolumjr.]

you can spiralock the barrel threads....not sure why people think you have to do the action.

Where do you buy inserts? Stanley only lists inserts for the female form. If you flip the insert wouldn't it cut the ramp on the wrong side?

Dale

 

[jonbearman]

Think about this, if you have to sell it , there maybe some who wont want these overkill threads. This idea will work but why when you have over 1 1/2 times the thread diameter buried in the receiver which is perfect for the total yield strength of the barrel threads in the first place and when torqued we don't even come close to yield strength of the fastener which is the barrel threads.

 

[BoydAllen]

Greg Tannel uses Spiralock threads, and has done so for some time. I think that he would be the best available source of information for this application. Harold Vaughn documented his work on barrel receiver connection movement in his book Rifle Accuracy Facts. I have a copy, and have read it. I also had a number of phone discussions with the author, before he passed away. In addition to this thread form, he also mentioned tapering threads to achieve the same goal. My early Viper has a slight intentional taper to its threads. It is tighter at the back, and I think that that helps distribute the load down the barrel shank. Those of you who have done no specific research, or testing as to what happens at the barrel/action joint when a rifle is fired might want to get a hold of a copy of his book.

 

[Outrider27]

In addition to this thread form, he also mentioned tapering threads to achieve the same goal. My early Viper has a slight intentional taper to its threads. It is tighter at the back, and I think that that helps distribute the load down the barrel shank. Those of you who have done no specific research, or testing as to what happens at the barrel/action joint when a rifle is fired might want to get a hold of a copy of his book.

The classic representation of the load in individual threads in engineering text books is the type of curve referred to as an inverted bathtub curve, which, of course, is a great ilustration of what we would like to be going on, but way off, as already mentioned in posts above. The actual stress is mostly in the first 3 threads from the bearing surface of the nut/shoulder, with the bulk of that in the first thread, then a rapid taper to near zero for the rest of the threads. Harold Vaughn's idea of adding a slight taper to the threads at the back is both effective in spreading out the load more evenly and simple (at least in concept; machining the taper takes a bit of exrtra talent and attention ). Personally, I would be more inclined to Vaughn's taper solution than the use of Spiralock threads. Another very simple method used to make the thread load distribution more even is to take a tapered reamer and remove about half of the thread height of the first thread in the female half of the joint. Over the years there have been a lot of attempts to bring things closer to the idealized inverted bathtub load curve, but among the more simple concepts, the tapering of the threads at the back has got to rank very high, and it's a lot easier than odd thread forms.