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Posted: 4/17/2017 1:53:55 PM EDT
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Have a Mossberg 935 that was damaged, the top few threads were flattened, when I sent it in to have the camo refinished. Can this be fixed?
Sending it back to Mossberg, just wondering if I should be expecting a new shotgun/receiver in return. Screw hole on the left side. 
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I meant starting the tap from the inside to pick up the undamaged threads at the bottom of the hole and then end up cleaning up whatever threads are still present at the top of the hole.
The top of the receiver is probably too thin for a threaded insert and it's doubtful that they come in a small enough size (I've seen Heli coils down to #8 in standard and M4 in metric), I'm figuring these screws are probably smaller than that. For max strength, you only need a threaded hole the diameter of the fastener, you won't even get close to max strength in aluminum (I'm assuming that's an aluminum receiver) anyway so it's possible there may be enough thread to retain the scope mount with a little thread Locker for insurance. If all you lost is the top thread, you might be OK still. I hate to suggest welding it up and redrilling and tapping as an option since that will kill your refinish job. A new replacement receiver is going to involve a trip to an ffl but you probably already know that. - Chris |
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Why not D&T for an oversized screw/bolt. Â It might be a bit of an ass pain possibly for any mount you might put on (will need to have the slot enlarged, but you could DIY that pretty easily with some needle files), and would mean you wouldn't have to refinish the receiver again.
As far as a Helicoil goes. Â IIRC Mossbergs have 4 screw holes on the top of the receiver. Â So if your scope base is one piece and has multiple hole slots it's just providing some more holding power. Â I'd not be too concerned about a helicoil in there. Â Maybe add a touch of loctite to the coil when you install, tape around it on the receiver, and file it flush. Â |
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Quoted:
I meant starting the tap from the inside to pick up the undamaged threads at the bottom of the hole and then end up cleaning up whatever threads are still present at the top of the hole. The top of the receiver is probably too thin for a threaded insert and it's doubtful that they come in a small enough size (I've seen Heli coils down to #8 in standard and M4 in metric), I'm figuring these screws are probably smaller than that. For max strength, you only need a threaded hole the diameter of the fastener, you won't even get close to max strength in aluminum (I'm assuming that's an aluminum receiver) anyway so it's possible there may be enough thread to retain the scope mount with a little thread Locker for insurance. If all you lost is the top thread, you might be OK still. I hate to suggest welding it up and redrilling and tapping as an option since that will kill your refinish job. A new replacement receiver is going to involve a trip to an ffl but you probably already know that. - Chris |
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Quoted:
Why not D&T for an oversized screw/bolt. Â It might be a bit of an ass pain possibly for any mount you might put on (will need to have the slot enlarged, but you could DIY that pretty easily with some needle files), and would mean you wouldn't have to refinish the receiver again. As far as a Helicoil goes. Â IIRC Mossbergs have 4 screw holes on the top of the receiver. Â So if your scope base is one piece and has multiple hole slots it's just providing some more holding power. Â I'd not be too concerned about a helicoil in there. Â Maybe add a touch of loctite to the coil when you install, tape around it on the receiver, and file it flush. Â Apparently I am too rough on the little snowflake during turkey season. 
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Thread size 1 and M2 up.
The often odd pitches in guns can be tough though. heli-coil catalog ETA: You can use epoxy and a screw with release agent to get some thread back. Find a long enough screw to go all the way though. Coat it with release agent. put some epoxy with a Drive the screw from the inside till it sticks out. Wipe away as much of the epoxy as you can that squeezes out. After it seats the release agent will prevent the epoxy from sticking to the screw. Remove the screw, clean up the top of th ereciver carefully and see how it foes. |
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ETA: You can use epoxy and a screw with release agent to get some thread back. Find a long enough screw to go all the way though. Coat it with release agent. put some epoxy with a meat filler in the stripped area with a toothpick. Drive the screw from the inside till it sticks out. Wipe away as much of the epoxy as you can that squeezes out. After it seats the release agent will prevent the epoxy from sticking to the screw. Remove the screw, clean up the top of th ereciver carefully and see how it foes. |
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Ok now that's a nifty trick I've never thought about. Quoted:
Quoted:
ETA: You can use epoxy and a screw with release agent to get some thread back. Find a long enough screw to go all the way though. Coat it with release agent. put some epoxy with a meat filler in the stripped area with a toothpick. Drive the screw from the inside till it sticks out. Wipe away as much of the epoxy as you can that squeezes out. After it seats the release agent will prevent the epoxy from sticking to the screw. Remove the screw, clean up the top of th ereciver carefully and see how it foes. |
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I really don't think losing one thread here is gonna be a deal breaker.
BUT... If you want to repair it, combining chasing/reforming the thread, with the epoxy (and powdered aluminum) replacement thread ideas would do the trick. Finish with blue loc-tite. If that sounds too involved, maybe check the chart for the next size up, English or metric, and drill and tap up. As long as scope mount has enough meat. All good ideas here. |
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If it were my fault that the threads stripped, I wouldn't have even posted this thread. They sent the gun back to me like that.
It is shipping off to Mossberg tomorrow. If it comes back with another QC failure, it will be sold and either a SX4 or SBE will be replacing it. |
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You may need a longer 'nut tap' to reach from the inside.
They have a longer reduced shank so you can tap multiple nuts and slide the finished ones up the shank and then remove the tap from the holder and drop all the nuts off at once. The alternative would be to unscrew each nut down the tap. The extra shank length lets you also do things like reach inside side walls to tap a hole at the bottom of a recess. And of course ALWAYS buy two taps. They usually arrive for little increase in P&H. If you save money and only have one it is more likely to break before the job is complete. They must 'talk' to each other and when they realize they are alone decide to break on the next job. It is actually more likely that with only one tap the excess caution we try use leads to it breaking. Just buy two. If you break one buy two more. For very small taps they may come in packages of three or five like smaller drill bit. They do the same thing. Your #80 (0.0135)drill bit WILL break if there is not a spare handy. Let alone anything in the #81 to #104 (0.0031 in) range. On aircraft work we would often have to braze a long extension on a tap to reach a hole. We did more than one that was 3 feet of extension. |
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Easy fix, at most 5min of work depending which method you want to use to fix it.
Method 1: Run an "oversized" 6-48 tap from the inside out. These are specialty gunsmith taps and screws made to cut a little oversized for repairing damaged 6-48 threads, so you do not need to enlarge the screw hole of your scope mount. Method 2: Recut all four screw holes on the receiver to #8-40, making the mounting significantly stronger and keeping all the screw sizes identical and standard. Note these would need the scope base to either be modified or one made to use #8 screws. Method 3: Countersink the top to remove the damaged threads, and use a longer (1/2" long) #6-48 screw and cut it to fit the full hole depth. Most screws provided with bases are too short anyways and usually only engage 1/3 of the threads on the thick Mossberg receivers, so this will also make a stronger mounting. Personally I'd charge $10 for fixes 1 and 3, and $20 for 2 because of the extra screws. |
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Easy fix, at most 5min of work depending which method you want to use to fix it. ... Method 3: Get that longer screw from your local gun shop that also mount scopes (its a common size), or order one from Brownells, then run the longer screw into the damaged hole from inside the receiver. The the longer screw will easily clean out the threads on that aluminum receiver (no tap needed). Then cut the screw to length as mentioned above. *Current production Mossberg shotguns (since 1996 IIRC) are drilled/tapped 8-40, earlier guns were 6-48. |
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