Step by step information for building a Yugo underfold AK
I am posting this as a reference for anyone who is building a Yugo underfold AK. I will try to be as detailed as possible in all of the steps using pictures to show as much as possible. I have found that the Yugo underfold (And RPK) kits differ from most standard AK kit in several factors:
The Yugo has the humps on the front trunnion
The receiver dimensions are marginally smaller
I am doing this as a screw build.
I started my build on a receiver blank that I custom formed to fit the Yugo dimensions and designed a template specific to the underfold layout.
The first step is to cut out the mag well opening and position the trigger guard/mag catch unit.
Drill the corner holes for the mag well and the cut along the lines to connect the holes.
Position the mag catch plate and mark the holes for drilling and tapping.
I drill and tap the holes for 8-32 screws. I use .25" screws and you will need to harden the holes the same way the FCG holes are done.
Here is some info on heat treating the holes: http://www.akfiles.com/forums/showthread.php?t=1296
Next you will need to file and fit the mag well. Remember! Only file fit until the mag can just be pressed in. You can always file it more later but you can not add metal back on.
Next you will fit the front trunnion. The first thing to do is reduce the top rails to fit the trunnion. Again, cut the rails a little too big and file to fit.
Cut the rails back about 2" and insert the trunnion. Compare the location of the trunnion to the mag. This will let you mark how far back the rail cuts need to go.
You will now need to file the rail cuts so that the trunnion fits tight.
Next you will need to locate, drill and tap the lower trunnion screw holes. Doing these first is the easiest since they can be seen from the mag well. I drill and tap them for 1/4-20 screws .25" long.
Once you have these holes completed remove the trunnion and measure for the other holes using these as the base line. Start with small pilot holes and fit the trunnion to see if they line up. If they do just complete the hole with the proper bit and tap.
Next you will fit the top cover to mark for the rear trunnion location.
Once the rear trunnion location is marked you will need to notch the rear of the receiver and the rails to press the trunnion in place and fit the top cover.
Now you can locate, drill and tap the rear trunnion screw holes. If you have the receiver stub that came on the kit you can use it as a template to mark the screw holes. Once the holes have been drilled you will need to tap them for 1/4-20 .25" screws.
Next you will need to trim the top rails and fit the carrier and bolt. Depending on the receiver blank you use you may need to cut the rails down or you may just have to file to fit. The bolt carrier has a minimum clearance of 24.75mm. You will also need to do the cutouts for installing the bolt and carrier. In the first picture the blank on the left has been reduced to fit the carrier. The second picture shows the notches you need to add for the bolt and carrier.
You are now about half way to a completed rifle!
At this point I tried fitting the front hand guard I got from Choate and found that it was too short. I tried several other hand guards that I have and it appears that the Yugo rifles are about .50" longer in this area.
The next step is to complete the bottom cutouts. The trigger port can be done for a double or single hook trigger. The 3 small holes down the centerline behind the trigger port are for the trigger guard and pistol grip mount. These should be drilled for 8-32 screws (They will need to be hardened). The large hole is for the pistol grip bolt. I have found that it is not necessary but you may want to add it as it is part of the original design. The 2 small holes to either side of the grip frame are access ports for the assembly of the folding stock.
Now for the fun! Next you will want to do the underfold cutouts. Even though I have the cutouts on the template I prefer to use the receiver stub sections for these cuts to ensure accuracy (I use them on opposite sides so remember to put the pin wings on the right). I drill under sized holes and then hand file and dremel them to size.
You will want to start with pilot holes and then complete them with a file bit or a dremel.
Here is a list of the part that I will be referring to in the assembly process. The first thing is to slid the stock on the receiver and insert the left lock in place.
Slide the hinge body in from the left and screw the locking ring on 2 turns.
Insert the right lock and push in the first retaining pin. As the pin seats push in on the hinge body so the retaining pin goes into the vertical slot on the right side of the receiver. This is what stops the whole assembly from rotating.
Finger tighten the locking ring and push a retaining pin through. This must be done vertically. Insert the spring and plunger into the hinge body making sure that the hole in the plunger is vertical.
Move the right and left locks to the open position and push the plunger all the way in. With your third hand push the last retaining pin through the left lock, the plunger and the right lock. The folding stock is now installed.
Almost done. Next you will drill the FCG holes, the center support holes, the safety selector holes and the recoil spring retainer holes. If you are using a bonded template it is advisable to recheck the dimensions before you do any drilling. The following dimensions should be checked on both sides of the receiver. These are measured from the center of the hole to the top of the rail minus the thickness of the receiver.
Center support .75"
Hammer 1.10"
Trigger 1.38"
Safety .55"
The AK design allows for some "Slop" in the dimensions but I believe that accuracy counts.
You will want to start by center punching all of the holes and may want to do a small pilot hole to ensure the bit does not "Wander" and end up with an oval instead of a circle. Once the holes are drilled you will need to file fit the parts.
With all of the holes and slots in place it's time to check for the fitting of the parts. The first problem I found was that the 1/4-20 button head screws interfered with the underfold stock. The head of the top forward trunnion screw will need to be modified for the stock to close completely. The second issue was the screw on the right side top would not allow the safety selector to seat properly. This will need to be modified also. Other than these 2 issues the whole kit fit and functioned well.
Next you will need to install the spring latch that locks the recoil spring in place. This was added to the rifle to stop the top cover and recoil spring from popping off when firing a rifle grenade. This will require a modified screw driver as pictured. Start by inserting the plunger and spring into the left side of the receiver.
Tighten the nut from the right side and the assembly is complete. Her is a shot of the recoil spring plunger modification.
Next you will want to install the lower rails. Both the right and left rail are supported by the center support and the right rail is supported by the hammer pin. The rails will need to be fitted to run in the bolt grooves. The ejector will need to be hardened . This can be done before or after it is installed, but I have found it easier to do before. The rails can be screwed or welded. If you are welding them a spot welder is the easiest but a TIG or MIG welder can be used also. DO NOT USE AN ARC WELDER.
From here it's just a matter of trimming the receiver ends to be flush with the trunnions and installing the 5 US made parts. Every build I have done has required some minor (Or major) tweaking to get the rifle functioning properly. I hope that some of this information has or will be of assistance with your project.
For information on my Yugo receiver blank click here.
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