A new to me K31 had the odd problem of having the trigger stick at the second stage (after "taking up the slack"). I'm posting my repair procedure in case someone else comes across this problem.
Since there was lots of old grease in the bolt I took it apart and degreased it with varsol, and then lightly oiled it. I tried using lappîng compound on the sear surface and pulling and releasing the trigger (many times). This helped, as the problem now happened only rarely, and if the trigger was released deliberately to cause it to stick.
Finally, I found a slight defect near the very edge of the sear, and removed it with an arkansas stone by slowly smoothing out the surface and feeling with a fingernail (do not change the angle/shorten the sear). This burr could only be felt on the way back up the sear, hence why it was catching the trigger group on release. Not a single problem since...
Hope this helps anyone that gets a rifle with a similar problem. Attached pictures are of the position of the trigger where it would catch, the disassembled bolt parts, the sear surface that was polished, the trigger group sear release that was polished, and the reassembled rifle.
https://www.dropbox.com/sh/8jrwo1lp03aei56/AADhR4iaDkzc4x1DOW1vXZM9a?dl=0
Since there was lots of old grease in the bolt I took it apart and degreased it with varsol, and then lightly oiled it. I tried using lappîng compound on the sear surface and pulling and releasing the trigger (many times). This helped, as the problem now happened only rarely, and if the trigger was released deliberately to cause it to stick.
Finally, I found a slight defect near the very edge of the sear, and removed it with an arkansas stone by slowly smoothing out the surface and feeling with a fingernail (do not change the angle/shorten the sear). This burr could only be felt on the way back up the sear, hence why it was catching the trigger group on release. Not a single problem since...
Hope this helps anyone that gets a rifle with a similar problem. Attached pictures are of the position of the trigger where it would catch, the disassembled bolt parts, the sear surface that was polished, the trigger group sear release that was polished, and the reassembled rifle.
https://www.dropbox.com/sh/8jrwo1lp03aei56/AADhR4iaDkzc4x1DOW1vXZM9a?dl=0
