December 09, 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
Installing the Rear Trunnion
Here is the unique Yugo M72 rear trunnion. An M70 trunnion would have a spring loaded crosspin that blocks the recoil spring base's channel until depressed. This prevented the heavy Yugo cover from jarring loose under the extreme recoil of a rifle grenade launch. Since I never plan to launch rifle grenades, using a M72 trunnion that lacks the retaining pin was not a concern.
I bought an aftermarket RPK rear trunnion, as they are readily available from a number of sources including Apex and R Guns. I drilled its two rivet holes out with a #3 bit and then used the AK-Builder trunnion jig to locate the holes for it. Honestly, my only regret with this jig is that I didn't buy it sooner.
In terms of screws, I'd definitely recommend using at least 1/4" screws in back to resist the recoil they will take. You can run a smaller tap and screws, but the connection would be too weak, in my opinion.
Tapping the rear trunnion using the drill press to ensure the tap is vertical to the work piece. With a 1/4" tap, you can get it started, but the torque required will likely make you choose to use a hand tap wrench partway through.
Next, with the #3 holes drilled into the rear trunnion, I tapped the holes in the same manner as the front trunnion using my drill press to hold the tap square as I manually inserted. Note — do not apply any Loctite yet. You'll want to do that after testing. I'll mention this again later, but when you do want to finalize the screws, use Loctite Red 2760 as it is impact resistant and makes a very solid bond.