fixing stripped pistol grip screw hole in plastic M&P 15-22 receiv

[jaredr]

finally picked up a S&W M&P 15-22 a few weeks ago and really like it. Went to swap out the stock grip for an aftermarket part (tangodown) and as I am tightening the grip screw down, I realize it's not very tight and not getting any tighter... I back the screw out and discover that the hole in the plastic receiver is littered with small bits of black plastic from the pistol grip screw hole so I had obviously stripped the plastic threads in the receiver. Didn't think i was using much torque at all but apparently it doesn't take much.

Question: what's the best way to fix this? My thought was:

1. to coat the inside of the hole with a thick layer of marine-tek,
2. let it dry for about 5-10 min (until it get's tacky)
3. spray the grip screw with cooking spray (as a release agent)
4. clamp the pistol grip in place so it is flush against the receiver (will probably use a pipe clamp)
5. once the pistol grip is clamped into place, screw the pistol grip retaining screw all the way in and let the marine-tek cure around the screw (clamp will hold the grip in place rather then relying on the un-cured marine-tek)

Not sure that this will be good for lots of swapping the pistol grip on and off but best I can think of for a secure but non-permanent way to reattach the pistol grip?

Appreciate any feedback or suggestions for alternatives.

thanks,

jared

[kmca]

Do you have enough room to tape it to the next largest hole? If you do, I"d consider installing a threadsert.

[jaredr]

I thought about trying to install something like a helicoil or thread insert but there's really not a whole lot of plastic left on either side of the existing hole.

[kmca]

These only add 1/16 to the diameter:

http://www.mcmaster.com/#standard-threaded-inserts/=mson1s

[GunCat]

You might completely fill the existing, stripped out hole in the lower receiver with a plastic or metal plug that is drilled and tapped for a smaller screw . The standard grip screw is 1/4, but a 10-24 screw would likely hold your grip on with no problems. Use appropriately sized washers on the new, smaller screw to keeps its head from pulling into the 1/4 hole on the pistol grip.

[jaredr]

Someone on another forum had suggested putting some JB weld in the hole, coating the bolt with wax/cooking spray/makeshift mold release and then putting the screw in while it's still wet. After full curing time, you should be able to unscrew it with a little bit of elbow grease.

I think this is the optimal route for me. helicoil/thread insert or just drilling and tapping to the next size up is what I would normally turn to first but there's less than 0.10" of plastic on either side of the hole so I'm very leary of removing any more material. Even trying to thread in an insert that is close to the current ID and would accept a smaller diameter bolt would (i think) be dicey as there's just not much material there.

this is so far my only complaint about the M&P 15-22 - i think it was a mistake to use the standard AR 1/4" 1/4"x28 bolt to secure the pistol grip to the plastic receiver given how little material that leaves on either side of the bolt hole. i'm going to see if I can find a #10 sized bolt with anything coarser than the standard 24tpi. would almost prefer something with the super-exaggerated threads of a sheetrock anchor but I suspect that is overdoing it...

thanks for everyone's suggestions.

[Poppa Bear]

I would Knurl a piece of brass, then drill and tap it to accept a 10-24 screw. Liberally coat the inside of the existing hole in the receiver with epoxy, and then apply a layer to the brass. Press the brass into the receiver with the screw inserted into the brass. Carefully unscrew the screw and use it to gently screw the grip back into place for proper alignment. Apply pressure to the grip to keep it properly aligned without using the torque of the screw to hold it in place. Give it 24 hours to cure and then remove the grip and 10-24 screw. Carefully drill and tap the 10-24 hole to the correct size. The knurled brass at this point should be well fused to the plastic of the receiver, and the thin layer of brass will give you a good thread to start the screw with so that cross threading can be avoided.

My thought is that the screw started to make its own threads and chewed through the original threads. Being plastic it is easy to miss whereas if it were metal you would feel the extra effort needed to start the screw and would have backed off and started it again until you felt it go into position easily.

[Poppa Bear]

I could also drill a couple of holes at an angle at the front and rear of the existing hole and force some epoxy into them prior to fitting the sleeve. They would form an anchor to the sleeve and the epoxy coating it which will assist in keeping everything in place.

You could also drill a couple of holes through the sleeve and insert pins covered in adhesive through the sleeve and into the receiver at an angle which would also assist in keeping everything stable and in place.

[Jaxshooter]

You can also fill the hole completely with JB Weld or Marine Tex and let it dry and redrill hole. Run the appropriate tap in the hole and use the original bolt.

[jaredr]

Resurrecting this thread for the stunning conclusion.

Short answer: used JB weld, worked fine.

Longer answer with pictures:

Decided to go the JB weld route versus a thread insert. Figured it might be challenging to try a “do over” on the M&P 15-22 plastic receiver if I wasn’t happy with the results, so wanted to first run some test cases and see what kind of thread strength I might be able to get trying to create new threads out of JB weld. Just saying that product name sounds like a bad omen (“no problem, we can fix this with JB Weld!”), but things actually worked out well…

Another reason to try a few test cases was because I needed to choose a bolt size to use. There wouldn’t be enough clearance between the existing hole and the original ¼” bolt to for fully formed threads to develop in the adhesive. I would need to either expand the hole (not really an option as not enough material left in receiver) or choose another size fastener.

I didn’t want to do my experimenting with fastener selection on the actual receiver, so this was another reason to test this approach on some scrap plastic before getting epoxy all over my M&P receiver.

I had some scrap 0.75” delrin rod in the shop, figured I would use it as a test medium and try a couple of different thread pitches to see whether there was any difference in the strength of the “JB Weld threads” that would be formed around the fastener hardware. Testing approach was to try a couple of different size fasteners and for each fastener:

1. drill a hole in a piece of delrin that would be at least 0.05” oversized from the fastener to allow the JB weld to bond to the delrin and have enough material for full threads to be formed by the fastener,
2. Using a burr grinder, score up the interior of the oversized hole and then also grind out two deep pockets on opposite sides of the hole to help the JB Weld to bond securely to the interior of the hole in the delrin plug
3. fill the cavity in the delrin with JB Weld, spray the fastener with cooking spray (as a mold release) and then screw the fastener into the hole full of JB Weld
4. let the delrin plug cure overnight
5. secure the delrin plug in a lathe chuck to hold it in place, remove the fastener (backing it out of the delrin plug) and then test thread strength by screwing the fastener back into the plug and trying to overtighten the fastener in the newly threaded JB Weld plug using a torque wrench
6. keep increasing the torque and record how much torque was required before the threads sheared or the JB Weld insert broke free inside the plug and started rotating inside the delrin.

Figured I would try this with a #12 sheet metal screw (approx. 12 tpi), a #10 x 24 bolt, and for the heck of it would try the original ¼” x 28tpi bolt. I knew I wouldn’t be able to actually use the original bolt in the expoxy repair, but figured I would try it as a test case and see whether the 28tpi threads appeared to be as strong as a deeper, coarser thread.

I started with the #12 sheet metal screw – this looked to me like the best option as it had the deepest threads. I set up the test with the #12 screw and called it quits for the night, figured I would set up the other two the next day.

Came back after 18 hours or so, chucked the delrin plug with the #12 screw in the lathe to secure it, back the screw out (broke loose of the epoxy very easily) and then started screwing it back in using a torque wrench. It’s an inexpensive Midway USA tool but figured it would be sufficient to just give me a relative idea of how strong the JB weld threads turned out. Once the wrench would start to slip, I would just increase it by 5 or 10 inch/lbs and start tightening again.

I worked up to 60 inch/lbs and the threads were still holding fine. I was satisfied with that and didn’t see a reason to bother with the other tests as I wasn’t looking for more than 60 lnch/lbs to secure the pistol grip

Went ahead applied the same approach to the M&P15-22 receiver and it worked fine.

All in all, I’m pleased with how it turned out. I like the M&P15-22 and think it’s a good value for the $$ but probably not a great platform for the endless tinkering and new parts trials that you can do with an aluminum receiver.

[cas]

Glad it worked out.

Reading the thread I was going to suggest attacking from the other direction, epoxying a stud into the receiver and using a lock nut inside the grip.

[jaredr]

Reading the thread I was going to suggest attacking from the other direction, epoxying a stud into the receiver and using a lock nut inside the grip.

Now that is a darn good idea and one I had not thought of at all!

[jaredr]

re-resurrecting this thread for a necropost update. After about 500 rounds, my rube goldberg fix has worked loose. there are some good suggestions in this thread, but I think i'm going to quit while I am ahead and get this back to S&W so they can fix it before I maul it to the point where they may (rightfully) claim i've created more problems then there were to begin with... time to break out the crummy drop-in atchisson conversion while this thing is in transit

[sm096]

It usually happens when you swap out your pistol grip, here’s the deal. Same thing happened to me and it was driving me crazy tried all bunch of stuff. I finally realized that , let’s say you replace it with a Magpul grip. If you look closely the Magpul grip it a bit thicker than your factory grip so when you screw in the screw the screw doesn’t go all the way in. So what I did was I put in the screw without the lock washer and it fit perfectly. It’s in there right and have not had any problems with it. Or just get a screw that’s a bit longer and you can put the lock washer on. Fixed