You can save yourself some time and buy premade sideplates, but here is how I made two sets of them for about $10. They are slightly oversized for a final fitting in the area of the rear filler and the bottem edge. You need about .040" overlap above the stock tang, trigger group, and rear filler since the receiver is round. You could in theory measure the stock tang and trigger group heigth and just make it .040" taller, but it would leave you no room for error. I first made a rough template from some thin cardboard (a pepsi box) that had a square corner and long straight top. I mounted a trigger group in a demilled stock tang and mocked up a rear filler onto the front lug of the trigger guard and rough measured it a little under 8" from the rear of the sideplate to the inside front of the rear filler. I then removed a stock tang from demill receiver by knocking out the rivets with a drift that was about 1/2 the diameter of the rivet. Hit them hard or you'll just expand the rivet more and you'll have to drill it out. Then I traced the demilled tang section on the cardboard. Once I had my rough template I cut two, 2"x9" strips from a 9"x9" section of .080" 4130 sheet steel from wicks aircraft. I squared up the two pieces and tack welded them together at the lower left end that would end up as scrap and the right edge. I traced out my template on the steel and cut the bottem profile. When I was happy with it I cut the end of the stock tang area out of the sideplates, leaving the rear filler weld to hold the two sideplates together at this point. Next I clamped the stock tang in its spot and aligned it so it was square by measuring the distance from the bottem of the little shelf right in front of the charging handle guide to the top of the sideplate (with the .040" overlap on top) and making a mark about 5" forward of this same measurement (about a half inch). Then I took a straight edge and placed it in the cocking handle shelf of the stock tang and adjusted the stock tang until the straight edge was on the mark I had made. I put another clamp on the tang and sideplates and drilled the sideplates through the 3 rear stock tang holes with a 5/32" drill bit. Then I cut the rear filler area of the sideplates off at 8" which will leave me about 1/8"+ for final fitting. I will not cut the rear filler rivet holes or trim the sideplate length until after I repair the rear filler.
Bolt modifications part one
The bolt in the suomi kits needs to be modified from its closed bolt/fixed firing pin to a closed bolt/ hammer or striker design. I sort of followed the directions shown in the shotgun news article SGN-9. I started by cleaning the bolt and removing the felt wad (if any) in the recoil spring hole. Next before you do anything else REMOVE THE EXTRACTOR before it gets ruined in the following steps. Do this by taking a small screwdriver about the same size as the width of the extractor and put it in the extractor groove. Then slowly push the extractor away from the boltface. When you can see the pin that retains the extractor then lever the extractor forward out of its pocket and out. Then I drove out the firing pin retaining pin located about 3/4" behind the front of the bolt. I drive them out from the top as it appears they were driven in from the bottem.
The bolts as they come are too hard to machine with high speed steel cutting tools, so like in the sgn-9 article I annealed my bolt. I use the "fires of hell" brush torch from harbor freight - Harbor Freight Tools - Quality Tools at the Lowest Prices . I got mine on sale for about $25 and they have a cheaper version without an ignitor that goes for $20 on sale. Hang the bolt by a small wire and heat it until its a dull red color, then let it air cool or throw it in the grill set on high and slowly bring down the temp. Then I drill the firing pin channel from the rear with a lengthened 11/64" bit to a depth of just behind or to the firing pin retaining pin hole. Now you can drive out the firing pin from the back with a lengthened 1/8" drift. It should come out easily since when you drilled it you also releived some of the pressure holding it in.
In order to use the original bolt you also have to make it so that an unmodified bolt cannot fit inside your gun. This is usually accomplished on tube guns by reducing the bolts diameter or installing a blocking rail in an original diameter receiver and milling a matching slot in the bolt. On this build I reduced the diameter of the bolt by turning it between centers until it slid freely inside my receiver tube. If you go with the 1.5"x.188 wall tubing like I did the bolt will need to be turned down to 1.115"-1.118" as shown by the two middle bolts in the picture of the 4 bolts. IIRC the prexis bolts need to be 1.170"-1.175". If you decide to cut a slot in the bolt a great spot for the slot is where the rear sight groove is in the bolt already. If you choose this route then just mill the slot the rest of the way to the rear of the bolt. I slightly deepened the one in the photo just for good measure. I did this to the top bolt in the bolt pictures.
I will add more bolt info soon on how to slot the bolt and drill it for the firing pin.
Last edited by moleman; 05-16-2009 at 08:21 AM.
Reason: add torch link
Bolt mods part two
In order to use the bolt with a hammer or striker system you need to cut a slot in the bolt. The slot on the top of the bolt is .390" so I shoot for this width. The slot needs to come to about .675"-.7" away from the step in the bolt, and about 1.5" from the rear of the bolt. On my first one I drilled/filed out the slot. It wasn't fun, but it was "do"able. On the bolts Ive done since then I milled the slot and squared up the front with a file.
Next I remove the full auto feed lip by either filing it off or milling it off. Then I take a small file that has no cutting grooves on one end and the front corner on that side rounded. I file the two sides of where the feed lip was until a 9mm case will just slide up into the bolt face. Then take the sharp edges off of the bolt where the feed lip was cut off.
Next I like to set the firing pin protrusion to be limited by the large step in the firing pin so it does't peen the weaker firing pin hole in the bolt face. I take a 27/64" drill bit and slowly deepen the recoil spring hole, stopping to check often how close the firing pin tip comes to the bolt face. When The firing pin tip is close to the bolt face I measure how close it is with a set of dial calipers. The firing pin needs to extend .035"-.40"above the bolt face. I don't cut it all at once, and always make sure the inside of the bolt is clean before I measure the firing pin protrusion. When you get the protrusion between .035"-.040" you're done with this step.
Next I tap the firing pin hole with a #12-28tpi tap so that .7" or less of a 1" bushing will screw into the hole when tight. If you go over this measurement you will have to drill the bushing from the rear to the .7" measurement or the firing pin will stop there and give you a false firing pin tip protrusion. The danger here is that if you don't realize it you could drill through the bolt face with the next size up drill when setting final firing pin protrusion and wreck the bolt.
You can screw in and red loctite a #12-28 screw and drill the next two steps, but a better way is to drill the bushing first. I made a simple fixture to hold a machine screw so I could drill it with a 1/16" bit half way from the front and a 7/64" bit about halfway from the rear. Then you can locktite and screw in the bushing until its tight. I then chuck the bolt up in a lathe and remove most bushing above the bolt face except for a couple thousands. I then file the bushing flat with a small flat file that I've rounded the head on.
Your firing pin will now fit in the bolt but will be below the bolt face. When the locktite has set, verify that the bushing extends no more than .7" into the bolt from the boltface. You can use a 1/16" bit to get an idea of how deep the firing pin tip is in the bolt, and how much the last step in the bushing needs to be drilled in order to get the proper firing pin tip protrusion. Now is a good time to drill the clearance in the firing pin channel for the return spring. I use the original suomi sear spring or you can use a 7/32"x1.75"x.028' wire diameter spring. drill the bolt from the rear with a 1/4" bit to a depth of about 4.95" measured to the drill bit tip. This will let you step the rod you used to lengthen you 7/64" bit to reach further into the bolt. We've already set how the max firing pin protrusion, but the larger 7/64" hole in the back of the bushing now needs to be drilled deeper from the back with a lengthened bit to reach the .035"-.040" we set the protrusion at earlier. use the same clean the bolt, measure, cut as we did to set the initial headspace. Stop and check often until you reach the correct firing pin tip protrusion.
In the next step I'll finish the bolt.
Last edited by moleman; 05-21-2009 at 04:36 PM.
Final bolt mods
There's not much left to do to finish the bolt. If you're going to use a section of sten gun recoil spring you just need to drill the firing pin retainer hole. On the first bolt I did, I copied the setscrew method in the sgn-9 article. I really didn't like the setscrew and even though it hasn't worked loose in about 1000 rounds I'm still concerned that it will. I have since switched to a pin on the side of the bolt like an ar15. To simplify things I made a jig to spot the retaing pin location. The center of the hole for the pin reguardless of the type you use needs to be about .350"-.360" back from the step in the bolt. If going with the setscrew just center it in the slot on the top of the bolt and tap it shallow so that the setscrew is in there tight. I made a jig out of .080" sheet steel scraps from the sideplates and a 1/4" rod. The jig fits in the extractor groove and needs to be peened/dimpled so that it fits in there snug with no wobble. The extractor groove is tapered at the end and your jig will have to not contact this area to seat correctly. I cut a step in the jig so it would locate the drill hole .350" back from the bolt step. Then I drilled a section of 1/4" rod with a 3/32" bit and squared it up and welded it to the flat stock. Double check the squareness and install the jig backwards in the bolt with a drill bit installed. Adjust the jig until the drill bit just misses the original suomi firing pin body as it is slightly larger than the part of the ar15 firing pin we're trying not to hit.
With that done, color the area where the hole needs to be in sharpie marker and flip the jig back around and make a mark with the bit to locate the holes location. The bottem or down hill side of the mark you made in the sharpie coloring is the center of the holes location as only the uphill side of the drill bit contacts the bolt. Don't try to drill the hole using the drill guide because it will wonder off center as the hole isn't a straight on cut on the round bolt, and when you break through to the inside the drill bit will break when it hits the opposite side. To prevent this I made a snug fitting removeable plug and pushed it into the firing pin recess where the retainer pin hole would be. This way the drill acts as though you're drilling a solid piece of metal. I mount up the bolt in a drill press or mill and make a plunge cut deep enough for the retainers head with a 1/4" end mill. Then I put a center drill with a 3/32" tip and drill and taper the bottem of the hole slightly. Then drill through the bolt with a 3/32" bit and remove the plug. Debur the hole and test fit the firing pin with its return spring. Use the receiver endcaps recoil spring guide to push the firing pin in enough to get the retaing pin in.
If you're using a sten recoil spring you're done. If you're using a two stage recoil spring then drill the recoil spring hole in the back of the bolt with a 17/32" or 9/16" bit to a depth of about 3-1/4" and bevel the hole slightly to remove the sharp edge. Another option for a two spring recoil setup would be to drill the bolt recoil spring out with a 1/2" bit to a depth of 1.9" then turn a tight fitting button to fit inside the recoil spring hole with a hole for the recoil spring guide rod. You will have to shorten the original recoil spring quite a bit and use a a section of a sten type spring with a rubber bumper that will limit the recoil spring guide from ever hitting the back of the firing pin.
Here's a picture of a bolt with a retainer screw and one with the ar15 type retainer. The jig pictures show the profile and proper firing pin clearance. I've also included a picture of the plug for drilling the retainer hole and a 3/8" tapered shaft I used to center the drills bit on the location mark.
Last edited by moleman; 05-22-2009 at 10:03 AM.
Thanks for the great writeup.
There are several options for the trigger group such as ar15, m1 garand and carbine, striker fired, and my favorite the remington sear and hammer. One consideration with converting any trigger system to fit the suomi is the width of the narrow suomi triggerguard. The remington 870, 1100, 11-87, 11-48, 11-58, 742, 7600, 7400, 552 ect... use a very narrow hammer and sear. I used an 870 on my first suomi build and I believe this is a 742 trigger guard that I'm using for this build. It could be for any of the rifles I suppose, but it was incomplete when I bought it. You really only need the hammer, hammer spring, and sear out of the remington trigger group. The rest will not be used in the build.
Here is a picture of a completed trigger group using 870 parts, an unmodified suomi trigger guard, the donor remington trigger group, and a block of steel milled to 1.206" tall, approx 4-1/4" long and .315" thick. I will be cutting the new hammer and sear mounting blocks from this steel block.
wow, please keep posting details. this will help me IMMENSLY when i get around to building my SUOMI M31's
can i legally keep the shorter barrel, but extend the barrel shroud?
im cheap and dont want to buy another barrel. i will probably do one of my kits as a SBR and not have to wory about 922R or the barrel issue...
Originally Posted by par0thead151
I just lengthened the barrel with a section of heavy walled 3/4" tubing and welded it on. I think you would want to get an actual letter from the baft as to whether or not you could just lengthen the shroud. My gut feeling on it would be no, as the barrel is still too short and you could replace the shroud with a normal length one at any time. I think you'd have to weld the lengthened shroud to the barrel and receiver for it to even be considered though.
Originally Posted by moleman
good point, i never thought of welding to the barrel.
i assumed i would have to buy a new barrel, in which case it would almost be worth SBR'ing all my suomis then as thats only 200$ whereas a bew barrel, and us parts and all that jaz would probably cost more than 200
No Hope For Me
I never measured my barrel but aren't they only a couple inch (3 or 4) under the 16" mark?