Building The Suomi M-31 Semi Auto
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The Parts Kit



Well, now that you have your Suomi M-31 parts kit, you need to go through it and make sure you have all of the parts. You can use the accompanying photos to help you see what you should have.

I would recommend sorting the parts into some kind of logical order and store them in labeled zip lock bags so nothing gets lost.

It's a really good idea to spray clean everything metal with carburetor cleaner and then with WD40 or some such protective oil to prevent rust. Put everything in a good sturdy box as these parts are going to be in and out of it a lot over the next few months.

If you have something such as the trigger group or other involved group of parts, I suggest that you take a few digital photos as you take it apart so you can refer to them later. It's also a good idea to start a notebook for plans, drawings, ideas and words of advice to yourself. Also, keep all receipts in there so you can prove to the Feds that you bought this thing before any new gun ban goes into effect! With this last thought in mind, remember to date your entries.

First off - if you have a receiver tube in your possession, even an unfinished Prexis undersize ID tube, you MUST start by grinding (carefully, don't damage the bolt face) the fixed firing pin off of the front of the bolt. By having a complete, original bolt and a receiver you are in violation of federal law. You have the parts to assemble a machine gun and the simple possession is grounds for prosecution!! Believe me, people have gone to jail for this. By removing the fixed firing pin you have defused this issue.


From the original manual



Disassembly of Kit Pieces:

The first chore you have is the complete disassembly of all of the sub assemblies. This can be quite a job as many of the parts are held together with steel pins that were driven in. With 70 years to get rusty, they can be tight!

A few tricks for finding these pins and removing them are:

1. Use different light sources to look at the area where the pin is supposed to be. Look from different angles to try to spot it.

2. Polish the area of the pin lightly with 400 grit wet or dry sand paper. Sand a little and look; repeat as needed.

3. Lightly cold blue the area. The two steels may be a bit different in grain and hardness. They may blue differently. Use only a little bluing on a soft cloth and rub it in, then wipe off with a water wet cloth. Repeat if necessary.

Try combining #2 and #3. Lightly polish the blued area to see if the bluing penetrated the gap between the pin and the part.


And just where are all of these pins? Let's go hunting!


Front Sight – First, decide if you even need to remove this part. This one was a bear on my gun! It was very easy to find the pin but it was stuck like the blazes. I ended up drilling it part way out from one side and then driving it out from the other. I heated the area a bit with a torch after drilling and added some penetrating oil for good luck. Be careful to support the off side real well. I find that a small ¼ inch drive socket gives good support and a hole for the pin to be driven into.




This photo shows the hole in the barrel shroud after the pin and sight were removed.











Moving to the rear, next comes the front filler. There are two pins here. The big issue is, how well was your receiver torch cut. Some are lucky enough (like me) to have the front filler undamaged. Here is a photo ( by Tom Radant) of a removed front filler so you can see where the pins were. This one was damaged when De-Milled and the small damaged area was welded in and had yet to be filed down.






There is a two pronged stub that's welded to the receiver, and the legs of the stub fit into the two wide grooves in the front filler. Gently drive the filler down off of these legs.


The large round hole ,shown to the right, is for the barrel shroud take down lever rivet. It should have been removed already by drilling off the head on the lever. (Later you will have to rework this lever and substitute a screw here.)






Next, comes the rear filler. Most rear fillers have been heavily trashed during the Demil process. Numrich Arms has some 80% complete rear fillers as of this date (11/2007). Try at - http://www.e-gunparts.com/ and look up under Suomi.



Here are the location points for the two rear filler retaining pins. As you can see, they may have trashed the filler in the Demil process. The one on the right is a Numrich 80% filler. It is un-drilled.


The small mag release hole is often vaporized. In the rear filler section, I talk briefly about reconstructing your old one, if it's possible. Although, with enough welding wire and time, anything is reconstructible!







Moving on to the rear of the receiver, there are three pins that hold the stock tang in. Carefully drive out all three of these pins.







Then pull down and rearward to remove the the entire assembly.


In this picture the Fire Control Group (FCG) is shown in the receiver. I'm not too sure why I had it in there, but please disregard it right now!









Last, if they came assembled, take the cocking/charging block off of the charging rod. And you're done taking things apart.













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