Home Forums Hunting & Shooting Gunsmithing 6-48 vs 8-40s

It it really worth it to retap a base for the 8-40s if your not working with a heavy kicker or mounting an overweight scope?

Im killing time while on the mend and need a project to keep my busy. Im looking the hardest at an older Rem M7 that is the old three screw base, figure it could use the most support.

I have seen rifles break stocks and scopes in nasty falls but never seen a base move. It it just a marketing ploy or is there an actual benefit to maximize durability.

In a word...No.

The only time I've done it was to cure a stripped hole.

that kind of what I was thinking guys, I guess Ill head for big 5 and get some more pellets and head to the backyard for some practice instead.

If you really want something to do while mending why not start by learning the layout of your keyboard...paying particular attention to the keys that apply punctuation.

Just kidding!

takn pane killrs wil do dat 2 a gi...

John Ricks drilled out and retapped my 375AI. He had to cut a little more clearance in the bases. Had some accuracy issues and swapped scopes to test. The rear base came off with the scope as soon as the front was loosened. The cut had apparantly been made a touch deep and broke out the shouldered area around the hole.
art

NO-keep as is!!

I think that all of the Custom Action makers use 8-40 screws for their base screws. It looks good and is a lot stronger than those little #6 screws. Do it if you like it.
I'd go to a lot of trouble to align the new holes up properly. Use an endmill to open up the existing thread because you will probably moving the holes(s) a small amount. Then drill to the proper tap drill size. Tap on location in the mill.
Opinions vary,

Thanks for all the input guys. I will probably pass on it for now. I have one rifle that takes the 8x40 and when you look at them side by side with 6x48 is shocking how much bigger they are. Im suprised more makers dont just go with the 8x40 as a standard. Im leaning towards it being a "feel good" move rather than a necessary job for a hunting rifle.

I watched a gunsmith index one of my Model 70 actions in a milling machine, then rebore the holes with an endmill. I was surprised how far out of line they were, both side to side and spacing.

I have it done on any rifle that is getting any major work done, since it is a minor job while the gunsmith is into it.

I use 6x48's. The exceptions are: 1) When I'm asked. 2) When the original 6x48's are off center or angled and enlarging to 8x40 can straighten them out.

There are many instances where the 8x40 might be more beneficial but also some where it can be detrimental. One place in particular where I avoid the use of 8x40's on my personal guns is on the rear bridge of a Mauser. The bolt guide slot leaves the bridge pretty thin. The thickness typically runs between .080" and .100". On those receivers on the thin side 6x48 will give you 4 threads while 8x40 only 3. While on the thicker side it's 5 vs 4.

Most of the 6x48 screws that I have seen broken or sheared off were not a result of the size of the screw being inadequate for the job. More often it is a result of improper tightening, poor fitting bases,dirty threads, and the biggest one... poor alignment of receiver holes to base holes. This last one places a huge amount of stress on the screw when it is tightened down and can also deform the threads. I've also seen a couple of 8x40 screws give way when one or more of the above conditions exist.

The other culprit is using the same screws over and over again and having them fatigue due to repeated stretching from torque.

Good designers calculate the potential forces the joint will be subjected to when deciding the size of the fasteners to be used...and they also calculate in a "over-kill" factor. However...they also assume that the two pieces will fit and align properly and that they will be assembled and tightened properly.

How do the factories drill their holes for bases? I have seen one that was off enough that a one piece base would not align, with the front two screws in the rear screw was off about 1/2 the screw hole width. I dont recall the solution but I remember joking with the guy that we could open it up for a 1/4 screw to line everything up. I really hope he did not try it.

Rick, great point on using new screws to maintain the correct thread fit.

What about silver soldering the bases to the receiver?

I admit to being a "belt and suspenders" kind of guy on some issues. I usually bed the bases to the action for a perfect fit but do use a release agent so there is no adhesion is assist, just perfect fit and alignment.

I have never taken the time to learn to silver solder enough to get pretty results that I would trust.

How do they do it?...Not very well most of the time!

All the machining, drilling and tapping is done prior to heat treating and that is one of the primary reasons things end up a bit of of alignment since steel has a tendency to move around during heat treating. Also...receivers are not done individually. They are fed into multi-station machines that do several at a time. Great for fast mass production, but not the best way to get things precisely aligned. They do this primarily because hardened steel is very tough on cutting tools...especially fragile taps being run by machines rather than by hand.

Same is true for most scope bases. They are mass produced and the tolerances are designed to fit the widest possible variances in receiver holes and diameters. Again, this is about the only practical and economical way to produce these parts...but it also doesn't make for real good fit many times.

Considering all this I am continually amazed at how well things line up on mass produced factory products.

You could do that...but there really isn't a need for it if you mate the base[s] to the receiver and properly attach it with screws.

If you have a really heavy kicking rifle and you mate it to a really heavy scope that is mounted really high then it is smart to use larger 8x40 screws just for some added strength.

All I've been trying to say is that just substituting larger screws isn't going to do much if all the other factors I listed aren't dealt with first. After dealing with those you may find that the larger screws aren't needed.

good post, switching to 8x40's is part of getting perfect scope ring alignment, and not a 'solution' in itself, especially if all one does is open up the existing holes that are not in perfect alignment

Some folks add a third 8-40 on both ends of the receiver.