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Moleman or anyone else, How would you go about repairing the threads on the end of the receiver for the end cap to thread back on?
I can't help you there I builld my suomi's on reduced diameter receivers I make. I'm sure there's someone here that can help you out though.
Hello barrel guy,
If you can't find a useful repair section I'd weld in the ruined area of the threads using an inside aluminum plug as a chill block mandrel. Use plain steel filler or also a piece of plain steel to fill in the missing area.
That aluminum plug is a press fit mandrel and should also serve as a lathe center to shape and re-thread the new material via single pointing. Pick-up the original (metric ??) thread pitch and continue it into the repair area at only perhaps 5 thou initial depth of cut lest it blow the tip off your threading bit. Hard spots resulting from repair welding are best tackled with a carbide threading insert tool.
Being a barrel mfg you will know lathe single point threading very well. This technique will help repair threads on other torched items. Happy machining !!
The last .4" or so of the receiver is threaded 20TPI at around a 1.455" outside diameter. Most of the threads on this gun are standard for some reason or the metric equivalent. The trigger guard screw is a #10-24 IIRC.
Thanks Viper and Moleman! Doing the single point thread cut was all I could think of doing.
After converting a few suomi trigger guards to use the remington FCG, I've thought of a few ways to make the trigger group parts a little better. The main parts of the conversion are a front and rear block to hold the hammer, sear, disconnector, and hammer spring in alignment. I had previously used a .090" spacer on the left side of hammer pivot pin and a .080" disconnector on the other side to keep the hammer more or less centered. I changed the front trigger guard block to have a .090" shelf to act as a spacer where as before I milled it away. This will also give a little extra meat to hold the hammer pin. On the rear block I raised the center section up .085" and then milled out a .085" deep pocket for the sear to sit in. This was to keep the sear better aligned and to make the rear block stronger. In the pic the new style front block doesn't have the rear filler lug machined into it yet, and the rear block doesn't have the upper right fork trimmed for the disconnector. A original suomi front block is shown at the top for comparison. The blocks are made out of 1018 steel and I case harden mine, even though untreated is probably fine as remington now uses plastic for some of their trigger housings and the originals are aluminum. I have a couple that weren't heat treated and they seem fine. I machined these out of a 3/8" thick bar, but a 5/16" cold rolled bar is only a few thousands under and can also be used.
Here is some more pics of the trigger groups blocks ready to be riveted into the suomi trigger guard. Also shown are the trigger bar and mounting block to the trigger. The original suomi safety is modified by cutting it at the first detent groove to shorten the upper bar. Then it is installed along with a bb and ar15 disconnector spring that fit into a recess in the back hump of the rear trigger guard block. Work the safety back and forth about 100 times so the bb makes a rub mark on the top of the safety. Remove every thing and make a small dimple at each end of the wear mark with a 3/16" diamond ball dremmel bit. You just have to make a slight dimple, about the size of a pencil lead to get the safety to snap into fire and safe. Any more and the safety will be hard to work. The bottem photo shows the safety in the fire position. In the safe position the trigger is prevent from being pulled because the upper rear section of the safety blocks the trigger via the tab sticking down on the trigger bar mounting adaptor The upper rear portion of the sear where the original coil spring would rest needs to be trimmed until the sear no longer sticks past the back of the trigger guard. Make sure it is trimmed well enough so that in the fire position when the sear is the to the rear it will not touch the charging handle block. Then make a groove or lip to retain the double torsion sear spring.