Archmark Posted May 7 Report Share Posted May 7 I recently acquired as a spare the Thompson SMG M1 bolt shown on the left in both of the photos below. The other bolt shown is the typical M1A1 bolt shown for comparison. Both are marked with an 'S' on the rear end (recoil spring side) and with a 'P' on the left side of the bolt. My question concerns the approximately 3/16" wide "ridges" on either side of the top and left side of the bolt body that project approximately .25 mm above the rest of the bolt body. The M1 bolt currently in my M1A1 does not have these "ridges" and is configured exactly the same (except for the firing pin assy.) as the M1A1 bolt shown, smooth and flat on all four sides. I'm curious as to why this "new" M1 bolt differs from my "old" M1 bolt. I can't see any reason that it wouldn't work just as well and may even run slightly faster since there is less surface area on the top and left bolt surfaces sliding against the upper and lower receivers. What do I have and THANKS... Link to comment Share on other sites More sharing options...
Got Uzi Posted May 7 Report Share Posted May 7 (edited) That bolt is what is called the “sand cutter” bolt. The slots were supposed to allow for dirt and debris to clear out and not shut the the guns down. They don’t make the guns run any faster-not enough material was removed to make it a “speed bolt” Edited May 7 by Got Uzi 1 Link to comment Share on other sites More sharing options...
Archmark Posted May 7 Author Report Share Posted May 7 THANKS for the information, GotUzi. I figured that there must have been an explanation for the difference in the bolts, but I couldn't figure out what it could be. Your explanation makes sense, so the "sand cutter" bolt must have been an early short lived "experiment" in the original M1 bolt design, since the M1A1 fixed firing pin bolt design came out fairly soon after the initial M1 bolt design. I've never seen the "sand cutter" design on a M1A1 bolt, just the M1 bolt that I recently acquired. Thanks again... Link to comment Share on other sites More sharing options...
Got Uzi Posted May 7 Report Share Posted May 7 I might still have an original m1a1 sand cutter bolt laying around. Link to comment Share on other sites More sharing options...
Archmark Posted May 8 Author Report Share Posted May 8 Uzi, I did some searches using the term "sand cutter" & came up with your discussion of the bolt from several years ago & one from 10 years ago. Looks like the modification was atributed to the Brits as a "field expediant" change... Link to comment Share on other sites More sharing options...
Bridgeport28A1 Posted May 8 Report Share Posted May 8 If these were a British "field expediant" change then they would have been milled after final heat treatment. Would it have been possible to mill a M1 or M1A1 bolt after heat treatment. Reconbob always talks about how hard WW2 bolts are. Link to comment Share on other sites More sharing options...
Archmark Posted May 8 Author Report Share Posted May 8 Good question. The depth of the milling is so shallow on two sides of the bolt, that I wonder if such minute removal of material would affect the hardness of the bolt. I've only seen this process on the M1 bolt, but Got Uzi said that he has a M1A1 "sand cutter" bolt... Link to comment Share on other sites More sharing options...
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