October 14, 2011
George Spafford is building a replica of the Yugo M70B rifle using a stub parts kit. A new installment will appear here every Friday
Fabricating the Front Trunnion = Part 1
Now with a milled stub, the original receiver was a milled - not sheet metal. The stubs actually give you what you need to make the trunnions! If you compare the stub to an AKM trunnion, you can see the heritage and also where the steel needs to be removed.
The receiver sub was cut with a bandsaw just behind the shoulder. The length of the Yugo stub is now the same as the comparison Hungarian. The next step will be to equally grind down both sides with a groove to accommodate the sheet metal receiver.
When the Soviets moved to a stamped sheet-metal receiver, they still needed the forged trunnions for strength and they wanted to minimize tooling changes otherwise. The inside dimension of an AKM is 1.25". By cutting the stubs to length and milling or grinding the portion of the stub that sits in the receiver to the correct width and length, you are actually creating the needed trunnions from scrap metal. With that said, to be safe, I would recommend still counting the part as foreign made for the sake of 922r just to be safe.
Drilling the Trunnion Holes
To allow for mocking up the build, I decided to use screws as fasteners for the trunnions. True chest beating arguments have ensued on the Internet over the use of screws versus rivets. The fact is that the only proven fastener for the AKM under full-auto combat use is the rivet. Now with that said, many semi-auto civilian AK rifles have been assembled with screws that are locked in place with Loc-tite high temperature thread locking compound and not had any failure.
This article will use screws, but the reader could change the design to use rivets or even later remove the screws one at a time and do solid plug welds with a MIG or TIG welder. As for myself, I decided to do my first screw build and took the time to plan for a result that was as strong and reliable as possible. I had a good secondary motivator as well — the milled front receiver did not have the interior recessed locations for rivets to be squished because the receiver was not designed for rivets. Rather than mill them into place, it further prompted me to try screws.