3/4 Scope Base Screws Enough?

[BigTimeMediocrity]

Hey,

So I had my new Tikka T3X (.308) build all together, and the other night my buddy and I went to level my glass and torque everything down. My scope base was on only finger tight, and we went to torque everything, and the front most hole on the reciever to mount a scope base stripped the #### out of the screw.

When I originally bought the rifle, the smith at the place I bought it from mounted my old Bushnell on two small sections of picatinny he also mounted. He torqued the living #### out of them (I thought they were threadlocked on when removing, they weren't). I think in doing so he damaged the threads on the rifle itself. In the meantime we just put in the 3 working screws, with threadlock, and torqued them to spec. We figured since it was a .308 it should be fine, and I shouldn't have to get that one screw rethreaded. Anyone have 2c they could throw in?

Also the base has a built in recoil lug


Thanks!

 

[Potashminer]

Not sure that I fully followed you - you are saying the threads on the screw got chewed up and stripped by the front hole in your receiver? I have repaired similar, but was almost always that the threads in the hole were stripped by the screw, not the other way around. If your receiver hole threads are damaged, might want to simply "chase" them with a "bottoming" tap in the original thread - likely 6-48, but I do not know what is on a Tikka T3X. If the hole in the receiver is stripped, likely have to drill with a #28 drill and re-tap it to #8-40. I think that size is used on many Remingtons - pretty sure that is the size on my Remington 788, which came with a 243 barrel, but is now a 308 Win. Then the hole and recess on the base will also have to be altered to fit the new, larger size screw. The 788 uses only three holes to hold the scope mount bases. I think most one piece "rails" used on Mauser and other actions use three screws as well - typically two into the front receiver ring, and one into the rear bridge.

 

[guntech]

3 out of 4 screws will hold a rail... if they are tight and not bottomed out...

 

[BigTimeMediocrity]

Not sure that I fully followed you - you are saying the threads on the screw got chewed up and stripped by the front hole in your receiver? I have repaired similar, but was almost always that the threads in the hole were stripped by the screw, not the other way around. If your receiver hole threads are damaged, might want to simply "chase" them with a "bottoming" tap in the original thread - likely 6-48, but I do not know what is on a Tikka T3X. If the hole in the receiver is stripped, likely have to drill with a #28 drill and re-tap it to #8-40. I think that size is used on many Remingtons - pretty sure that is the size on my Remington 788. Then the hole and recess on the base will also have to be altered to fit the new, larger size screw. The 788 uses only three holes to hold the scope mount bases. I think most one piece "rails" used on Mauser and other actions use three screws as well - typically two into the front receiver ring, and one into the rear bridge.

Yeah you got it right. Scope base has 4 screws that attach it to the receiver. I think they damaged the thread on the front most hole in the receiver. I don't have a drill press and I'm not sure how comfortable I would feel trying to tap that on my own. Do you think I can shoot it without hitch?

I've sent a few emails trying to get someone to call me, but smiths in NB seem to be thin pickings and I can't come up with anything..

 

[Potashminer]

Screws or bolts into threaded holes have some slop, or you could not turn them in. So when they have pulled down whatever they are holding, the one face of the threads on the screw are against the corresponding face of the threads in the hole, unless the screw is bottomed out in the hole, as was pointed out in Post #3. The "torque spec" then causes you to apply a bit more torque to create a small bit of "stretch" within the threads - will not vibrate loose, and is holding tight. Threadlocker can lock the thread so it does not come loose, but does not make it hold "tight" - the torque that is applied does that.

If yours is the very front hole over the receiver, it is directly above the chamber. Often, if you clean out that hole completely, you are looking down at the barrel tenon threads. So, do not want to drill into the barrel tenon - often done as a "through hole" when the barrel is removed, or should be done as a "blind hole" if done with barrel in place. Not a lot of material thickness there - but enough to get enough threads for the screw to hold tight and not shear - typically want to be about as deep as the screw is wide. Then when cutting new threads, want that first thread to be sharp - can not really afford to waste any threads on a wobbly or crooked start. Sometimes, the hole will be finished with a flat bottom bit, to get a flat bottom hole, then grind the last taper off a tap to be able to get a last thread right at the bottom.

If I ever could free-hand start a tap straight, I no longer can. I have started a number of taps "straight" by swapping out the drill bit that drilled the hole, from the drill press chuck, inserting a starting tap, and lowered the quill and turned the chuck by hand. I have since acquired a Forster jig, which uses hardened bushings to hold the drill "straight", then swap out the bushing for the one sized for the tap, so it is held "straight" as well.

I would go with Post #3. Too many commercial rifles are made from factory with only three mounting holes - often in much harder "kickers" than a 308 Win. If three screws are tight and holding and not bottomed out, I doubt that your rail will move. Parker Hale A29 side mount bases, which is used on some 7.62 NATO conversions, only use two 6-48 screws to hold their top plate to the side mount - the scope mount rings attach to that top plate - essentially is the "rail". Several used ones that I received were stripped, though - the side mount is just aluminum, not steel, and I believe the previous owners got too carried away with "gudintight" - I have re-drilled and re-tapped several to #8-40 and they seem to hold fine.