new threads

[clodbuster]

Putting the scope base on my Mossberg 835 ultimag I completely boogered the threads in one hole.  It seems to have an alloy receiver.  Can this be repaired?

[John Traveler]

When scope mounting holes are damaged, the normal repair method is to drill out the damaged hole and re-thread in the next larger size.  Before you do that, you might try running a tap into the damaged threads.  It just might clean up enough to work.

If your original hole is 6-48, it would be drilled and re-tapped to 8-40, for instance.

If you have one or more other undamaged holes, you might be able to just use Locktite thread sealant and loose the security of the damaged thread hole.

HTH
John

[303Guy]

You might also be able to re-tap the damaged thread to a metric thread which is only slightly larger than the original, still using loc-tite for added security.  But first try running an original thread, bottoming tap, in it.

[BoarHunter]

You might also be able to re-tap the damaged thread to a metric thread which is only slightly larger than the original, still using loc-tite for added security.  But first try running an original thread, bottoming tap, in it.

True, 6-48 have a diam of 3.5mm so you can use a M4, the pitch being 0.7mm no need to drill (hole for M4 is typicaly 4 - 0.7= 3.3mm). I have done it before.
Besides it is easier to find these tap and screws than the specialized 8-40.
If the hole is through, you could even find a self taping M4 to avoid the tap purchase. A source for these screws can be an old PC, printer, copier etc...

[The Gamemaster]

You already messed it up once, you came on here for help.  You don't seem to have any mechanical abilities- since you already messed up the first set of threads.

You would have to buy a tap and a tap wrench and maybe even a drill blt, a drill guide and the oil to tap the hole.

You would be 1000% better off  to take it to a qualified gunsmith and have the gun fixxed professionally.

It would require a Bridgeport type mill and a gun vise.  All of the tools required to keep the hole perpendicular to the action of the firearm and the skill to perform this task without messing up.  The new screw - which could be bought at most any gun shop - still has to be the right length which leads you to other things that could go wrong.

TAKE IT TO A GUNSMITH!

[gunnut69]

Chasing the threads may work and it may not, depends on the damage. It is also possible to buy an over size 6x48 tap and screws for just such a fix. they are available from Brownells.. Can't help with the metric but it too sounds possible.. Forgive  Gamemaster, he only knows the one response.. Good luck..

[The Gamemaster]

Now you think about it - Gunnut 69

Threads are based on being able to keep at least 75 - 85% of the material in the hole to make the threads to hold the screw.

If you use a hand drill and you wobble even just one time, you will remove too much material and the hole is no good.  The more times you drill it the bigger the hole gets.

A true hole machined is not done in one pass.  You usually have to make at least two holes. the first being the pilot hole and the second to make the hole true and to size.  The whole time you want to keep the hole in the same place which means that you want to have the gun mounted in a vise.  You want both the gun and the vise to be level with the drill.  You want no more than .0005 of run out in the spindle.  Something you cannot get from a regular drill press.

Then you need precision drill bits, not the kind you get in home depot - because they are very critical on how your work is going to turn out.

When you change screw sizes, you are also going to have to re-drill the bases to a larger size and might also have to reestablish the taper in the bottom of the scope mount base for the larger screw.

You are going to need a bottle or tube of PINK - small screw Loctite.  Not RED or BLUE!
Pink is small screw service removable.  The rest of them, once you put it in, you will not get it back out.  Loctite is not to be used in place of threads.  It will only hold the screw and not the material that the screw holds.

Now let's look at what the screw does.

Machine shop theory tells us that for a 1/4 inch bolt, with a grade 8 fastener, you have to screw that bolt 1/4 of an inch into steel to get it's maximum amount of strength.

For an example if we take a 6/40 screw, which is about a drill size of .1130 or a number 33 drill.
Compare that to a 1/4 -20 bolt that has a drill size of number 7 drill bit or .2010

We are talking something half as big, A M4 screw,having to hold something 100 times it's size, a scope mount and a scope.

Now you look at a M4 - which has a pitch of .7 and a diameter of .157 and uses a drill in English of .1299 or rounded off .130 which is a #19 drill size.  We use A #19 is .1285 - because that is the closest size available.

That means on either side of the middle of the hole you have  13 1/2 thousandths of material removed from the original piece when you thread it.  If you wobble .010 - the hole is now a .140 and is now 9/64'ths of an inch and no longer has enough material inside of the hole to be able to leave you enough material to thread it and put a M4 screw into it and now you are screwed.

Take it to a gun-shop and let them be liable.

[John Traveler]

After reading Gamemasters detailed discourse, I must admit to agreeing with him.

We in the gunsmithing/advanced hobbyist category easily forget that what almost comes automatically for us does not always hold true for many gun owners.

Just think of the many (gasp!!!)  times that someone has brought a gun to you for repair because the owner's attempt at repair made the situation worse than it was if he had brought it in before trying to fix it himself.

For the modest cost of having a gunsmith repair the buggered up sight mounting hole, the owner avoids all the hassles (and hazards) of a homemade repair.

Clodbuster, without intending any slight or insult to you, the easiest course is to have a real gunsmith/machinist fix it.

HTH
John

[clodbuster]

I am tending to agree that self fixing is not the best course and never did really.  I'm one of these people who needs to understand what a pro would do to fix something for me.  You know, go watch a few triple bypass operations before mine is done but not by me.
Actually a better solution exists than all the suggestions.  That is: Fill the bunged hole with JB weld and repaint the camo.  Then buy a rifled barrel with the cantilevered scope base and sell my old iron sighted one.  Ten times easier than resetting the scope every deer season and I can do it myself   ha!!!

[BoarHunter]

Now you think about it - Gunnut 69

Threads are based on being able to keep at least 75 - 85% of the material in the hole to make the threads to hold the screw.

If you use a hand drill and you wobble even just one time, you will remove too much material and the hole is no good.  The more times you drill it the bigger the hole gets.

A true hole machined is not done in one pass.  You usually have to make at least two holes. the first being the pilot hole and the second to make the hole true and to size.  The whole time you want to keep the hole in the same place which means that you want to have the gun mounted in a vise.  You want both the gun and the vise to be level with the drill.  You want no more than .0005 of run out in the spindle.  Something you cannot get from a regular drill press.

Then you need precision drill bits, not the kind you get in home depot - because they are very critical on how your work is going to turn out.

When you change screw sizes, you are also going to have to re-drill the bases to a larger size and might also have to reestablish the taper in the bottom of the scope mount base for the larger screw.

You are going to need a bottle or tube of PINK - small screw Loctite.  Not RED or BLUE!
Pink is small screw service removable.  The rest of them, once you put it in, you will not get it back out.  Loctite is not to be used in place of threads.  It will only hold the screw and not the material that the screw holds.

Now let's look at what the screw does.

Machine shop theory tells us that for a 1/4 inch bolt, with a grade 8 fastener, you have to screw that bolt 1/4 of an inch into steel to get it's maximum amount of strength.

For an example if we take a 6/40 screw, which is about a drill size of .1130 or a number 33 drill.
Compare that to a 1/4 -20 bolt that has a drill size of number 7 drill bit or .2010

We are talking something half as big, A M4 screw,having to hold something 100 times it's size, a scope mount and a scope.

Now you look at a M4 - which has a pitch of .7 and a diameter of .157 and uses a drill in English of .1299 or rounded off .130 which is a #19 drill size.  We use A #19 is .1285 - because that is the closest size available.

That means on either side of the middle of the hole you have  13 1/2 thousandths of material removed from the original piece when you thread it.  If you wobble .010 - the hole is now a .140 and is now 9/64'ths of an inch and no longer has enough material inside of the hole to be able to leave you enough material to thread it and put a M4 screw into it and now you are screwed.

Take it to a gun-shop and let them be liable.

Only one screw has been "screwed" up.

Besides for an M4, he does not need to drill, just tap or even he can use a self tapping screw. This set up will be as strong as the 6-48 it replaces.

I have done it many times because outside of the US, you can't virtually find these screws, taps etc...

In some cases like front sights which are not subject to stress, I just used an M3.5 in place of the 6-48, the diam is OK, the pitch being 0.6 compared to the 0.529 of the 6-48 makes only a few thread engages then it blocks. Not as bad as it sounds as so it holds firm !

The real issue is that it is about time manufacturers give up on this flimsy way of mounting scopes, 4 tiny screws, and use strong dovetails like Sako, CZ and others.
Integrated Weaver base would be the ticket.

BTW: You should go metric, it would make thing so simpler, no fraction, no letter gage and dramatically cut on inventory in drills, taps, screws etc.... All companies that have done it were amazed at the savings which can run as high as 30% 

http://ts.nist.gov/WeightsAndMeasures/Metric/mpo_fact.cfm

[Brithunter]

BTW: You should go metric, it would make thing so simpler, no fraction, no letter gage and dramatically cut on inventory in drills, taps, screws etc.... All companies that have done it were amazed at the savings which can run as high as 30%   

   That's for profit pure and simple. They care not for quality! The SI metric system is not all it's cracked up to be limits are way too big and poor fit of screw and nut is the result even when made to psecs and guages. Check out teh slop in a properly cut 6mm thread and then that of say a 1/4 UNF. The UNF thread fits the metric slops about  Also the bumph about not having to have special threads with the SI system is just that. It don't work which is why if you have a vehicle that has metric threads then you cannot just buy a bolt for it as it most likely will not be a standard SI pitch as guess what they don't work  .

    I used to make the bolts for the Buggatti owners club as the company I worked for had the contract. We had to have special taps and dies made as well as chasers becasue none of teh trheads were standard. How about 11x1.0mm that's an 11mm bolt with a 1.0 mm pitch. Give me the old Imperial threads any day of the week 

[gunnut69]

The hole doesn't require machining, it is already extant.. it requires a tiny bit of reaming and threading. God knows how many receivers have been drilled and tapped using a jig and drill/tap bushings.. Fixing threads in a receiver is one of the things a gun owner can do. With a bit of info and tools this is safe.. In the case of the metric thread or the over sized 6x48 only a tap and screws are needed.. We certainly do NOT need .0005 inch accuracy on threads for a scope mount on a slug gun! Many of us started gun smithing because we couldn't afford custom shops and machine set up.. I would seriously doubt that were this gun taken to a working gun smith that he'd set it up in a mill.. There are thresholds of danger and complexity that would force a reference to a gun smith, re-tapping a hole in a slug gun receiver is not one of those.. IMHO of course..

[Slufoot]

You are going to need a bottle or tube of PINK - small screw Loctite.  Not RED or BLUE!
Pink is small screw service removable.  The rest of them, once you put it in, you will not get it back out.  Loctite is not to be used in place of threads.  It will only hold the screw and not the material that the screw holds.

Hello Gamemaster, I have been through 2 different Loctite training courses and the information you have provided is what Loc-tite would recommend. However my experience with Loctite 222 (pink) has not been good for use on scope bases and I have even used it with (7649 Primer N) the primer that Loctite recommends.
Micheals of Oregon bottles and sells what they refer to as Gun-Tite in small tubes, they have been selling this stuff for years and it is Loctite 242 (blue).
As long as you use scope base screws with good heads and a properly fitting tool to install and remove them you will have no problem removing scope base screws that have been treated with Loctite 242 (blue).I have been using this for years on several guns and I have never had any trouble removing a screw. If someone uses a sloppy fitting tool while trying to remove a screw and buggers the head then you may have to heat the screw with a soldering iron to get it out.
I told the instructor in my last Loctite traing class that I tried 222 (pink) and used (7649 Primer N) on my scope base screws for my 22/45 Ruger pistol and the screws worked loose. Now I shoot this gun alot, it's not uncommon for me to put 550 rounds through it at one shooting session. He told me to use 242 (blue) for this application and it would hold, I did and it's still solid after several thousand rounds.
I do use 222 (pink) for certain applications on guns such as the small slotted set screws that hold the setting on some peep sights and rear sights but for 6-48 and larger with good heads I will use 242 (blue).

GOOD SHOOTING!
Slufoot