Gunsmithing Finding unknown Torque value of a screw?

[want2learn]

This is somewhat of a hypothetical question

....let's say i have a rifle that is shooting great and i want to establish and record the torque values for future reference so that if the screws ever loosen i can return them to this previous established optimal setting.

Is there a reliable/accurate method to establish the torque of said screws having no other source (no owner's manual, no published source, no manufacturer to contact etc.) with the exception of the rifle in front of me?

can one use a non digital torque driver and slowly march up the torque ratings until the torque driver finally catches?

 

[308pirate]

This is somewhat of a hypothetical question

....let's say i have a rifle that is shooting great and i want to establish and record the torque values for future reference so that if the screws ever loosen i can return them to this previous established optimal setting.

Is there a reliable/accurate method to establish the torque of said screws having no other source (no owner's manual, no published source, no manufacturer to contact etc.) with the exception of the rifle in front of me?

can one use a non digital torque driver and slowly march up the torque ratings until the torque driver finally catches?

The closest you can come is to make a witness mark from the bolt to the part under it, break the bolt loose with a std wrench, and retighten to the witness mark with a torque wrench that records peak torque.

A click torque wrench will not work. Breaking the bolt loose with a torque wrench will not work either.

Thats as accurate as you're going to get unless you start spending serious cash for the kind of torque analyzer and transducers that I've used at work.

 

[dimar1492]

Using a torque driver to see how much it takes to loosen the fastener will not be accurate. Break away torque is usually less than the tightened torque value. Usually around 15% or so, but there are obviously variables.

You are about to go down a rabbit hole that will provide little to no benefit.

 

[want2learn]

thank you both....appreciate the information

 

[308pirate]

Break away torque is usually less than the tightened torque value.

It really depends on the type of joint. Hard joints typically have higher breakaway torque compared to the tightening torque. I agree with what you say about it all being dependent on any number of factors and about this being a rabbit hole that many will get lost in.

The fact of the matter is that threaded fastener tightening is a very complex subject and dabbling into it as a layman without understanding the physics of what's really going in is going to lead to some seriously flawed conclusions, which will then be spread throughout the gun internet world as gospel.

 

[Powder_Burns]

Break away torque is usually less than the tightened torque value. Usually around 15% or so, but there are obviously variables.

God I wish that was true. I have a collection of broken bit sockets, impact sockets, and breaker bars.

 

[308pirate]

We measure torque because it's easy and cheap and good enough for most applications.

What really matters is clamping force generated by the fastener. There's instrumentation to measure that directly, but it's financially out of reach of the average person. Plus the fact that you need to have the engineering knowledge to understand what you're seeing.

The problem lies in the fact that the linear relationship between bolt strain and tensioning force is altered by joint conditions.

 

[dimar1492]

I totally agree with you guys, break away torque can get wild. Especially with corrosion or deformed threads. I was more or less picturing the op's particular application on action screws. Where most of the time, I believe my statement to hold true

 

[Powder_Burns]

tap and die days, my favorite

 

[gunsnjeeps]

The best I can offer is that in a Savage you can torque the front action bolt then play with group size with the rear screw.

Out of curiosity, what action and stock?

 

[seanh]

It would probably be best just to Simply research or call the action manufacturer... Ask for the torque specs. I'm not sure if the torque spec would change based on chassis / stock but it may be worth researching torque specs for what you have.

 

[Supersubes]

In the case of receiver screws, if the rifle is sensitive to screw torque, the action isn’t supported. Properly bed the action, and then it won’t matter what the screw torque is.

 

[want2learn]

all interesting points, thank you.
i don't have a specific rifle in mind.....more of an academic interest.

It came about because on my Sig SSG3000 i've seen printed different numbers for securing the barrel in the receiver. this led me to consider that i have some old C&R rifles and had no idea what numbers i should try for and figured best to duplicate where they're currently set at...but of course i have no idea what they're set at as the rifles are very old (carcano, krag etc.).

 

[...]

most screws on a gun have a "standard" torque value.....

Scopes rings are usually around 15 in-lbs.
Scope bases are usually around 20 in-lbs.
Action screws are usually around 65 in-lbs.

as an engineer, for any screw i didnt know the torque value of, i would consult a screw torque chart for a baseline, then i would cross reference that to torque recommendations from similar parts/ manufacturers.

Screw Torque

https://www.fastenal.com/content/merch_rules/images/fcom/content-library/Torque-Tension%20Reference%20Guide.pdf

Torque Table Standard Bolt Sizes SAE

The following suggested tightening torques provide an excellent starting point for determining torque requirements. Remember, you may need to vary these numbers somewhat based on the individual joint or the amount of fastener lubrication.

www.engineersedge.com

 

[gunsnjeeps]

I've always heard 65 pounds for most bedded rifles. Savage plastic "Tupperware" stocks are 25 pounds, Savage Palma laminate is 35 pounds, both from Savage. As far as tuning with action screws see below.

You are being redirected...

 

[want2learn]

Wow, great information, i'll check out the links just now.....thank you all

 

[308pirate]

all interesting points, thank you.
i don't have a specific rifle in mind.....more of an academic interest.

It came about because on my Sig SSG3000 i've seen printed different numbers for securing the barrel in the receiver. this led me to consider that i have some old C&R rifles and had no idea what numbers i should try for and figured best to duplicate where they're currently set at...but of course i have no idea what they're set at as the rifles are very old (carcano, krag etc.).

There is absolutely no correlation between whatever torque the SSG3000 barrel screws need and what some old milsurp rifle needs to hold the receiver on the stock. None.

In any case, I seriously doubt the exact action screw torque matters in a milsurp rifle that at best could do 2 - 3 MOA when new.

 

[OutThere]

God I wish that was true. I have a collection of broken bit sockets, impact sockets, and breaker bars.

Same here. Along with the cuts, bruises, and colorful language that goes with them.

 

[Powder_Burns]

Had to skip the range this weekend, seized 1/2”x13 csshcs bolts on friday had me swinging a 4lb sledge against an impact driver until I missed and hit my support hand. Even after a 36” breaker bar, 800 ft-lb impact, and the impact driver didn’t do it, had to cut the steel strap they were bolted to and use a 24” pipe wrench to unscrew them.

Same scenario on monday resulted in the heads shearing off and had to drill them out and re-tap the threads.

 

[308pirate]

Had to skip the range this weekend, seized 1/2”x13 csshcs bolts on friday had me swinging a 4lb sledge against an impact driver until I missed and hit my support hand. Even after a 36” breaker bar, 800 ft-lb impact, and the impact driver didn’t do it, had to cut the steel strap they were bolted to and use a 24” pipe wrench to unscrew them.

Same scenario on monday resulted in the heads shearing off and had to drill them out and re-tap the threads.

 

[Powder_Burns]

Yeah, problem is the bolts being counter sunk no penetrant reaches the threads. I’ve soaked them overnight with pb and threads are still dry the next day. Even heated the fasteners with mapp gas, no dice.

 

[atvman400]

Yeah, problem is the bolts being counter sunk no penetrant reaches the threads. I’ve soaked them overnight with pb and threads are still dry the next day. Even heated the fasteners with mapp gas, no dice.

I will admit I'm not sure what a CSSHCS is. Assume it's some type of socket head cap screw. Either way you don't want to heat up a fastener with external threads like a a screw. You want to heat up what it is threaded into to expand it. You actually want to cool down to shrink the fastener with external threads.

 

[army_eod]

Somewhere out there is a chart that shows you pretty much all of the standard torque values for fasteners.

 

[atvman400]

mcameron posted one to Fastenal above but that is the recommended torque to get a set clamp load. Most of the time you do not want that much clamp load. You put that much clamp load on an action screw that is in a cheap stock and you will thread the action screw right up through the stock. Being an engineer I can tell you there is no such thing as "standard torque values". Every application is different. Do you torque your lugnuts on your cast aluminum wheels to the same torque as your steel wheels? I hope not but maybe you do.

 

[Deleted member 10043]

Don't worry about it.

 

[OutThere]

Ah, yes. "Kroil- the oil that creeps". I've had a can for 15 years and it's still over half-full. I also use a drop on my adjustable sights every time I clean my guns.

 

[LRI]

This is somewhat of a hypothetical question

....let's say i have a rifle that is shooting great and i want to establish and record the torque values for future reference so that if the screws ever loosen i can return them to this previous established optimal setting.

Is there a reliable/accurate method to establish the torque of said screws having no other source (no owner's manual, no published source, no manufacturer to contact etc.) with the exception of the rifle in front of me?

can one use a non digital torque driver and slowly march up the torque ratings until the torque driver finally catches?

There is a way, however, it requires some homework.

1. What is the material of the receiver?
2. What is the material of the fastener?
3. What is the thread diameter?
4. What is the thread pitch?
5. What is the percentage of thread flank engagement? (the tough one)
6. Are any lubricants being used on the threads and/or head of the fastener?


From here, the problem gets "mathed" and you get an answer.

For the bulk of this stuff:

A carbon steel screw in 1/4-28 typically tolerates around 55 inch lbs.
Stainless in the same pitch likes 40-45

Metric stuff in the 6mmx.75 range runs right at 40.

This is assuming the rifle is either a chassis or pillar bedded. Squishy stocks can be more troublesome.

C.

 

[Powder_Burns]

I will admit I'm not sure what a CSSHCS is. Assume it's some type of socket head cap screw. Either way you don't want to heat up a fastener with external threads like a a screw. You want to heat up what it is threaded into to expand it. You actually want to cool down to shrink the fastener with external threads.

Yeah you’re right about the thermal expansion deal. I was just grasping at straws at that point. Air chisel didn’t do it either.
csshcs is a counter sunk socket head cap screw, bane of my existance. Fuckin’ Archimedes!