Round receiver scope base screws

[Steven69]

I have one of what I suppose is a "contract" L579 receiver which has the "gunsmith" Leupold/Redfield scope bases machined to fit the action. I lost two of the base screws (the barrelled action is in the white to be sent off for BlackNitride finishing). They are #6-40 fillister head hex socket screws with a head diameter of 0.192" going into counterbored holes in the bases measuring 0.195" in diameter (screw OAL is 0.443" and threaded shank is 0.323"). The bases were machined so many years ago that I can't recall the details any more but it seems like the screws were not modified, except perhaps for length, by the gunsmith who machined the bases for me.

All of the #6-40 fillister head screws I've found have larger diameter heads, like 0.22", so maybe the screws I have had the heads reduced somewhat. I will have to wait till tomorrow to contact Brownells to see what they say.
Are these likely standard screws that were just machined for length, or do the head/counterbore dimensions sound special as well? I'm afraid the gunsmith who did the work originally (back in the late 80s or early 90s) probably isn't around to check with.

Thanks!

 

[icebear]

There are fillister head and cheese head screws made with smaller diameter heads. Check a specialty fastener supplier for "narrow-head" screws. You could also chuck a standard size screw in your power drill and turn it down with a mill file. It's funky but I've done it.

 

[Steven69]

Thanks. Turns out they are #6-48s; ordering some from Brownells. They said the head diameter is 0.197", so maybe I won't have to grind them after all.

 

[Steven69]

When I grind them to length, should the threads go the full threaded depth of the receiver bridge? The screws that came with the bases originally (two of which I lost, which occasioned this post) only go about halfway into the bridge (bridge thickness is about 0.340"; original screw threaded length into bridge is about 0.172").

I thought if I ground the new ones flush with the receiver inner diameter there would at least be less space (a really tiny amount, admittedly) for dirt to collect.

 

[paulsonconstruction]

When I grind them to length, should the threads go the full threaded depth of the receiver bridge? The screws that came with the bases originally (two of which I lost, which occasioned this post) only go about halfway into the bridge (bridge thickness is about 0.340"; original screw threaded length into bridge is about 0.172").

I thought if I ground the new ones flush with the receiver inner diameter there would at least be less space (a really tiny amount, admittedly) for dirt to collect.

Rule of thumb for screws that attach bases or sights to a rifle is 3 threads minimum. More than that is obviously better. As long as they don't interfere with the bolts operation or bottom out on the barrel threads, they can't be too long.

 

[Steven69]

Rule of thumb for screws that attach bases or sights to a rifle is 3 threads minimum. More than that is obviously better. As long as they don't interfere with the bolts operation or bottom out on the barrel threads, they can't be too long.

Thank you. It took me a couple of minutes to realize that bottoming out the base mounting screw on the barrel threads puts a compressive force on a screw that is meant to have only a tensile load (ignoring the shear component from recoil for the moment). I've got five threads (turns) into the receiver before the front mount screw touches the barrel threads, so I can get the minimal three threads in. My FAT wrench instructions say you want around 18 - 20 inch-lbs of torque on a lubricated #6 screw in a scope mount. I figure wet blue Loctite would take care of the lubricated part. Or I could use epoxy on both the screws and where the mount surface matches the receiver. It's a hunting rifle in .250 Savage, so not a huge amount of recoil.

So my new question is whether grinding the screw , say, four threads give me enough room to get my torque in the proper range or do I need another way of calculating the proper screw length ?
Thanks in advance.

 

[paulsonconstruction]

There is a special place in hell for people that use Lock-Tite on guns & it is the number one reason gunsmiths cuss at their customers! Please reconsider doing that, as there is no need for it & should you want to remove them at a later date it will save you, or future owner, a HUGE headache. The threads you have are the threads you have & if the screw bottoms out before the base is tight you will have to grind the screw shorter to fit. This is very common on almost every rifle. The other base screws could also need to be ground shorter if they protrude into the action & interfere with the bolt lugs or body. You need to check that EVERY screw is the proper length. I have NEVER used any thread locker or a "torque" wrench in over 55 years of turning gun screws & have NEVER had a screw come loose on a scope base or ring. Torque wrenches are just the current fad among gun "techies" & have no useful purpose on my work bench. However they are used on action screws for specialty benchrest rifles where there is metal to metal contact, but that is more about consistent accuracy than something "coming loose". Using a torque wrench where there is wood to metal contact is a fool's errand. BTW, keep the epoxy away, as that would be a total disaster. People have been turning screws on firearms for hundreds of years without chemical "lockers" & torque wrenches. Stop trying to reinvent the wheel. Just food for thought.

 

[Steven69]

There is a special place in hell for people that use Lock-Tite on guns & it is the number one reason gunsmiths cuss at their customers! Please reconsider doing that, as there is no need for it & should you want to remove them at a later date it will save you, or future owner, a HUGE headache. The threads you have are the threads you have & if the screw bottoms out before the base is tight you will have to grind the screw shorter to fit. This is very common on almost every rifle. The other base screws could also need to be ground shorter if they protrude into the action & interfere with the bolt lugs or body. You need to check that EVERY screw is the proper length. I have NEVER used any thread locker or a "torque" wrench in over 55 years of turning gun screws & have NEVER had a screw come loose on a scope base or ring. Torque wrenches are just the current fad among gun "techies" & have no useful purpose on my work bench. However they are used on action screws for specialty benchrest rifles where there is metal to metal contact, but that is more about consistent accuracy than something "coming loose". Using a torque wrench where there is wood to metal contact is a fool's errand. BTW, keep the epoxy away, as that would be a total disaster. People have been turning screws on firearms for hundreds of years without chemical "lockers" & torque wrenches. Stop trying to reinvent the wheel. Just food for thought.irst

Paul, I appreciate your comments but would respectfully disagree in part. First, I agree that I can dispense with the Loctite. The chances of me hunting with this rifle have declined over time and it probably will never get shot enough to loosen the screws, so I can pass on that. Were I starting this project over I would use a conventional Sako action with the integral dovetail mounts. At the time, I wanted the lines of a round receiver in the worst way. I'm not so picky now as I was then.

I have not mounted enough scopes over the years to be able to confidently tighten (and not over tighten) #6 screws by feel, so I like to use a torque wrench. I also put Scotch tape inside the rings to prevent marring the scope tubes (and I'm sure there is a better material for doing that out there somewhere).

The screws I purchased from Brownells are extra long and are being ground to a length slightly less than the receiver ID at each point. I put a drop of red fingernail polish on the one above the barrel threads and will grind the screw to the point where the nail poish dot looks untouched when fully tightened.