H2munitions Posted February 4 Report Share Posted February 4 Another war time kit build. The lower was on the max end of specs. I've seen it with a lot of the thompson upper/lowers fitting tightly or loose once assembled. I refit this lower to the receiver so that solved the issue but you can see whoever was issued the gun fought it and beat up the lower with a metal hammer/tool to disassemble. This bent the magwell rail to the point magazines wouldn't slide into the frame. Measuring a few NOS condition frames show the magwell slot is .195-.200 tall. This one was visibly bent inward and measured ~.182 Our fix was to machine a mandrel, heat the magwell rails with torch and drive mandrel into magwell. The fix was a success and I thought other thompson enthusiasts would find this helpful/interesting! 1 Link to comment Share on other sites More sharing options...
Jim in Texas Posted February 7 Report Share Posted February 7 Thanks for sharing. Please tell me more. Did you have to harden the mandrel or was mild/annealed steel sufficient? Did you heat the magwell rails to a specific temperature or color before driving in the mandrel? Link to comment Share on other sites More sharing options...
reconbob Posted February 12 Report Share Posted February 12 (edited) This is a clever fix, but I think could accurately be described as a blacksmith approach. If correct tools are not available I guess any port in a storm. I do not think it is a good idea to randomly heat a trigger frame red hot, because the line between heating and burning is a fine one. If the steel is over-heated or "burnt" it will become brittle. I have seen many frames like this that are so battered that I assumed it was not an attempt to remove the frame from the receiver, but an attempt to destroy or demilitarize the frame. A better safer fix is to clamp the trigger frame in a Bridgeport type mill as shown and use a standard 3/16" wide x 5/8" keyseat cutter. The T- slot in the trigger frame is 0.190" x 0.660" so frequently one pass thru the T-slot is all that is needed. Bob Edited February 12 by reconbob 3 Link to comment Share on other sites More sharing options...
H2munitions Posted March 8 Author Report Share Posted March 8 On 2/12/2026 at 12:21 PM, reconbob said: This is a clever fix, but I think could accurately be described as a blacksmith approach. If correct tools are not available I guess any port in a storm. I do not think it is a good idea to randomly heat a trigger frame red hot, because the line between heating and burning is a fine one. If the steel is over-heated or "burnt" it will become brittle. I have seen many frames like this that are so battered that I assumed it was not an attempt to remove the frame from the receiver, but an attempt to destroy or demilitarize the frame. A better safer fix is to clamp the trigger frame in a Bridgeport type mill as shown and use a standard 3/16" wide x 5/8" keyseat cutter. The T- slot in the trigger frame is 0.190" x 0.660" so frequently one pass thru the T-slot is all that is needed. Bob I agree 100% in fact those exact words about blacksmithing were spoken as this unfolded 😅. I can't remember why I didn't think to order/use a keyseat cutter. Especially knowing now there is a standard size that fits well. Thanks for the education! Link to comment Share on other sites More sharing options...
H2munitions Posted March 8 Author Report Share Posted March 8 On 2/7/2026 at 2:37 PM, Jim in Texas said: Thanks for sharing. Please tell me more. Did you have to harden the mandrel or was mild/annealed steel sufficient? Did you heat the magwell rails to a specific temperature or color before driving in the mandrel? Just a piece of mild steel flat bar. We heated it just enough that a mini sledge would drive it in. The first attempt it wasn't quite hot enough actually. The magwell rails were red/orange. It's a lot of material to move. It didn't take very long to heat. Drove the mandrel out from the other side with a punch. Link to comment Share on other sites More sharing options...
68coupe Posted March 8 Report Share Posted March 8 Would not a woodruff key cutter be enough, or is there too much material lost to support a mag? I'm genuinely asking. There has been a recent new poster that has had a similar problem. Please answer for that individual. Can a good set of files take care of the problem? V/R, Karl, 68coupe Link to comment Share on other sites More sharing options...
H2munitions Posted March 8 Author Report Share Posted March 8 41 minutes ago, 68coupe said: Would not a woodruff key cutter be enough, or is there too much material lost to support a mag? I'm genuinely asking. There has been a recent new poster that has had a similar problem. Please answer for that individual. Can a good set of files take care of the problem? V/R, Karl, 68coupe Bob posted in detail about using a keyseat/woodruff cutter above. If I were doing it again that's what I would use. Link to comment Share on other sites More sharing options...
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