November 04, 2011
By George Spafford
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 Trunnion — Part 3
When compared to an AKM trunnion, it was clear the stub needed to be cut behind the left shoulder and then ground to 1.25 inches below the "shelf" where the barrel pin was pressed in. The planning and marking of the trunnion is critical. If you are not square, you can end up with a trunnion that is unsightly or even dangerous. Take your time and literally measure twice or even four times before you ever cut.
Angle iron is securely clamped to create a guide to cut the grove for the sheet metal lip to press into. By doing this first, it also creates a straight guide edge.
To show this could be done, I pressed out the barrel pin and barrel to get them out of the way as previously mentioned. From there, I used my Harbor Freight bandsaw to cut the front trunnion to length, scribed the lines with a square and then hand ground the trunnion to the correct width. I did this with a 4.5-inch grinder to do the rough work and then files for the fine work. I took care not to get the metal too hot and ruin the heat treat.
One side of the trunnion has the initial rough grinding done. The straight edge is there to protect the finished upper shelf of the trunnion from being nicked by the grinder. Yes, a mill would have been easier, but using hand tools seemed like more of a challenge and it worked out just fine.
I only trimmed enough from the bottom to slide the new trunnion in, as the "ears" would be needed to secure the milled lower handguard. Sure, I could have used my mill but this handwork method seemed like a more basic approach that I had to try. It took a while but the results came out to my liking.
All the fine shaping was done with coarse and fine files. Yes kids, in this age of power tools, good files still come in handy, as you can see in this photo, a pair of Nicholsons. The machinist's square and piece of scrap receiver were used to check progress. To fit into the receiver, a little shaping was done on the bottom of the trunnion as well.
To give a solid surface behind the sheetmetal receiver, the milled stub's lightening cut was filled with JB Quick Weld and then sanded flat. So far, by the way, this "filler" has held up just fine, no doubt because there isn't any load on it. The ends of the stub were ground to create slight angles to mimic the AKM trunnion. The result was a perfectly useful trunnion that would fit in a standard AKM receiver.
If someone had a rear receiver stub, the same basic principles would apply. Cut it down to size, grind to a 1.25-inch width, locate the holes and drill them out. I'd recommend using 1/4" screws given the recoil the rear trunnion would need to absorb.