FX FX liner lock nut (jam nut) sanity check -- how tight?

[stovepipe]

I know the answer to the question "how tight should my jam nut be" is going to depend on liner length, caliber, fpe at the muzzle, whether the rifle is using o-rings as spacers between the liner and the barrel housing or a carbon fiber spacer instead. There are probably a lot of other things to take into consideration that I'm not even thinking of. And I know that a lot of the answers are going to be given in terms of how many turns after hand tight rather than torque specs -- either one is fine with me. I just need a sanity check.

I've got an fx maverick 30 cal sniper with a 1:18 twist liner and a carbon fiber spacer between the liner and the barrel housing. I'm shooting nsa 54.5 grain slugs at about 950 fps, but might go to 980 fps. Either way that's in the 109 fpe to 116 fpe range. The liner is trying to spin the slug to the right, but if the jam nut is too loose, then the liner will slip (spin) to the left. I want the jam nut to be tight enough to prevent the liner from slipping, but I don't want to bend my liner. I can't really tell if the liner is slipping or not and there doesn't seem to be any way to know for sure.

Currently I'm tightening my jam nut 1/16th of a turn after fully hand tightening, but I'm wondering if that's really enough -- is 1/8th of a turn OK without bending the liner?

The FX motto "not too tight -- not too loose" causes irreversible insanity.

stovepipe

 

[Firewalker]

I cheat:

I have a 5# lead weight with a carabiner and I loop that over my wrench and allow that mass to torque the jam nut.

Is this right? Is it wrong?

 

[stovepipe]

I cheat:

I have a 5# lead weight with a carabiner and I loop that over my wrench and allow that mass to torque the jam nut.

Is this right? Is it wrong?

That's not cheating. It's thinking outside the box to get a repeatable amount of torque on your jam nut.
The question is, how many fpe is your rifle putting out at the muzzle and is that amount of jam nut torque enough to keep the liner from slipping (spinning left) and not so much that it's bending your liner? Also what caliber and liner length?

stovepipe

 

[Firewalker]

I actually do this with all of my liners, I have the 700mm .30 on their now, 900fps @ 50gr NSA, 90fpe?

 

[Damuffin]

I would say snug, if its put under to much tension it will deflect or crush. Finger tight plus an eighth to quarter turn.

 

[stovepipe]

I actually do this with all of my liners, I have the 700mm .30 on their now, 900fps @ 50gr NSA, 90fpe?

This actually helps. Real info. Do you happen to know the radius of the point at which you hang the weight? I can esimate the torque from that.

stovepipe

 

[stovepipe]

I would say snug, if its put under to much tension it will deflect or crush. Finger tight plus an eighth to quarter turn.

I know I'm going to drive people crazy with my responses -- sorry. What caliber and length of liner do you have? And how many fpe is your rifle putting out? Do you think a 1/8 turn after hand tight is enough to bend the liner? If I was totally confident that 1/8 turn after metal-to-metal hand tight contact on my 30 cal 700mm liner would not bend it, then I would start getting closer to 1/8 turn vs the 1/16 turn I'm currently applying.

I know it sounds like I'm over-analyzing or being paranoid, but the fx statement in the manual that says approximately 1/4 turn after hand tight feels wrong when you have clean smooth jam nut and barrel housing threads -- when I hand tighten, I feel the jam nut touch the liner like an instant stop. No slop.

stovepipe

 

[JimBentley]

Well, the old car mechanic once told me to tighten until it strips, then back it off a quarter turn.
I hold the wrench in the palm of my hand, near the box end, and just snug it. I've never seen any difference in accuracy out to 100 yards.
They do not need to be over tightened!!!

 

[Damuffin]

how about screwdriver tight. Not wrench tight. Your just trying to couple the tubes to have consistent harmonics.

 

[jps2486]

FX should publish this information.

 

[stovepipe]

how about screwdriver tight. Not wrench tight. Your just trying to couple the tubes to have consistent harmonics.

You are back to exactly what I'm trying to avoid -- the FX motto "not too tight -- not too loose". Now tell me exactly which model of screwdriver I should use to guarantee that I'm not too tight to bend my liner and I'm not too loose to allow my liner to slip left as it tries to put a right-hand spin on a 54.5 grain slug going 980 feet per second.

stovepipe

 

[bigHUN]

How about these?

 

[stovepipe]

How about these?

View attachment 477735

I need one of those. Is that a custom part, or is there a common name for that bit?

stovepipe

 

[stovepipe]

How about these?

View attachment 477735

And I already have the torque driver -- so I just need the bit. Then I need to know what torque value to use on the jam nut.

stovepipe

 

[Damuffin]

3 ft/lbs or 36 in/lbs
You can purchase a 1/4” torque driver for perfection.

 

[Airgunmann]

How do you think it's slipping ? Instead the slug is spinning it ? All I've ever done on the barrel nut was snug but thats relative to who your talking to
Any tightening at all is making the barrel flex some place or another

 

[nervoustrig]

The liner replacement document from FX says
"Screw on the Liner lock (1) and tighten ~a quarter turn"

Copy attached.

View attachment liner-replacement-eng.pdf

 

[Vana2]

Interesting discussion.

For wheel nuts on an aluminum rim I use a torque wrench, for everything else I tighten "by feel".

Don't know, but I can feel (through the tool) when the faster is approaching maximum stress or when tight is tight enough without going too far.

Guess I developed that sense while tapping holes or threading parts. Taps are hardened and are quite brittle. Depending on the material, percentage of thread and the depth of the thread it can be very easy to break the tap in the hole and really mess things up. The tap will flex a tiny bit before it fails, learning how much you can "force" the tap applies directly to feeling how a fastener is reacting to the load.

My father taught me to tap by giving me a bunch of drill bits sized from too big to too small, a piece of steel and two taps. His instructions were: I'm going for a coffee, when I get back I want to see a series of tapped holes and one of the taps broken. Essentially he was saying: learn the limits.

I should get one of those nice gunsmithing torque driver sets though.

Cheers!

 

[Revoman]

You're looking for one of these....

FX Airguns Liner Lock Removal Tool

FX Airguns Liner Lock Removal Tool Easily remove and install your FX STX Barrel liner lock with this tool. Insert this tool into any tool that accepts 1/4" drive to quickly remove the liner lock from your STX barrel. Only for .177cal through .30cal. The .35cal liner lock uses a different style...

www.capefearairguns.com

Mike

 

[Ghostranger169]

I've only done this with the FX Crown STX barrel system when indexing the barrel. I've found that if I tighten until I can't twist the liner by hand, they will not move/twist/slip thereafter. If you tighten them too much, you will start to shred the outermost O-rings in the stack.