Screw build
Doing a screw build will produce a nice looking and durable rifle if care is taken to properly thread the parts and secure the screws. It is best to do the build from the front back. This will ensure proper fit and placement of the parts and allow for adjustment of the mag well if needed.
Front trunnion
Insert the trunnion into the front of the receiver. It may be necessary to trim the top rails to get a proper fit. Mark the top rails to show how far back they will need to be trimmed.
You can use the Dremel and files to do the trimming. The receiver sides should set flat against the trunnion sides.
Depending on the receiver used the trunnion rivet holes may need to be located and drilled. There are several ways to do this. The first option is to use a "Post-it" note. Place the note on one side of the trunnion and use a marker to show the hole location. Insert the trunnion into the receiver and lock it in place with a "C" clamp and drill the rivet holes. You can drill all the way through to do both side a the same time. You can also make a locator jig to find the holes.
Once the trunnion has been fitted and the rivet holes drilled in the receiver you will need to re drill the holes in the trunnion using the #21 drill bit. Next lock the trunnion in the bench vise on it's side and tap the 10-32 threads. GO SLOWLY! Use the cutting oil and back the tap out after every turn to clear the channels. Repeat for the other side.
Inset the trunnion into the receiver and using Locktite, tighten the screws. Place a magazine into the mag well. The mag catch on the back of the magazine should set flush with the bottom of the receiver. It may be necessary to file the back of the mag well for a proper fit.
Trigger guard
Place the selector stop/spacer on the bottom of the receiver flush with back of the mag well. Be sure to locate the tab on the right side of the receiver. With the selector stop/spacer in place insert a magazine and check the rivet holes. If you had to file the back of the mag well it may be necessary to adjust the hole placement. This can be done with a round file. Starting with the rear screw insert an 8-32 screw through the trigger guard and receiver. Using Locktite, secure the screw from the inside with an 8-32 nut. Now slide the selector stop in place and do the last 4 8-32 screws.
You can use 8-32 nuts on the front of the trigger guard or you can buy one of the tapped screw plates that are available on the chat boards. Once the screws are in place and secure insert a magazine to be sure that it will lock in place.
Rear trunnion
Inset the trunnion into the receiver. It may be necessary to trim the top rails for proper fit. This can be done with the Demel and a file. Position the trunnion in the receiver and check for proper hole alignment. If the receiver does not have rivet holes use the post-it note method as with the front trunnion. Lock the trunnion in place with a "C" clamp and drill through the same as for the front trunnion. Once the trunnion has been fitted and the rivet holes drilled in the receiver you will need to re drill the holes in the trunnion using the #7 drill bit. Do this from each side and only about 1/2" deep. Next lock the trunnion in the bench vise on it's side and tap the 1/4-20 threads. GO SLOWLY! Use the cutting oil and back the tap out after every turn to clear the channels. Repeat for the other side. Insert the trunnion into the receiver and using Locktite, tighten the screws.
Lower rails
Rail caliber modification
The right lower rail that comes with most commercial flats is set up for the 5.45x39mm rifles. The magazines for these rifles are narrower than the ones for the 7.62x39mm cartridge. The tab on the bottom front of the right lower rail is set up to center the narrower 5.45x39mm magazine and will not allow the 7.62x39mm magazines to be properly seated.
To use this in a 7.62x39mm rifle the tab needs to be reduced by half. This can be done before installation with a grinder or after installation with the Dremel or a file.
This method makes use 6-32 button head screws. With this process you will install one rail at a time. Start with the left rail. Place it inside the receiver and run .200" drill bit through it from the right side. Using a 1/4" drill bit as a spacer position the top of the lower rail level with the transition ledge on the front trunnion. Lock the lower rail in place with the vise grips using a piece of scrap metal as a pad on the outside of the receiver to avoid marring the surface. Use a marker to highlight the side of the receiver and score a line 1/2" down from the top of the rails.
You will now need to mark and drill holes through the receiver and rail for the screws to go through. Tap the receiver and rails for 6-32 screws. At this point you will need to heat treat the rail and holes as outlined in Preparing the receiver .
Apply JB Weld to the back of the lower rail and set in place. Insert the screws from the outside and tighten them down. The JB Weld will form a bond between the rails and the receiver and will also fill in the threads of the screws to lock them in place.
Repeat for the right rail.
Center support
The center support gives added rigidity to the receiver and stops the hammer from falling forward when field stripping the rifle. Trim the 1/4" OD tubing or rod to 1.17". Drill and tap the ends for the 8-32 screws. Position the center support between the lower rails and start a screw from each side. It will be necessary to use 2 allen wrenches to tighten them. Be sure to use Locktite on both screws.
Trimming the top rails
The top rails need to be trimmed so the bolt carrier and bolt will drop in and move freely. This will entail making several cuts in the top rails and filing them down. Starting at the rear trunnion you will need to mark and cut out the bolt carrier slots. Be sure to cut these under size and file to fit!
Once you have it so the bolt carrier will fit into the slots you will need to trim the rails full length. Use the Dremel with a sanding wheel to do this. Go slowly and check the bolt carrier fit often!
When you have reduced the rails to the point that the bolt carrier can be forced by hand all the way forward finish trimming the rails with a hand file. You want to get to the point where the bolt carrier will move smoothly back and forth just by tilting the receiver.
At this point insert the bolt into the carrier and install it into the receiver as far as it will go. If necessary mark the rails on each side for the wide parts of the bolt and use a file to make cutout for the bolt to drop into. You will also check to see if the bolt will pass the ejector on the left lower rail. If it binds you will want to use a file to reduce the ejector so as not to effect the heat treating.
At this point the receiver is complete and should look like this:
CYA message: This web page is posted for informational and educational use only. Any other use of the information on this website is solely at the user's risk.