Thursday, September 4, 2008

SKS trigger job

Bit the bullet. Took the trigger group of my Yugo SKS apart to

  1. figure out how the thing works
  2. reduce weight
  3. eliminate crunchiness.
Yesterday, I was successful in all three endeavors. What a goofy design. The sear rides on a pair of rails and is pushed by a bar, horizontally, levering off the trigger. The disconnecter pushes this bar vertically, thus allowing/disallowing engagement of the trigger bar with the sear. This design almost cries out for crunchiness because there is so much contact surface between the sear rails and the sear that any particles or non-smoothness will be amplified by the full weight of the hammer spring bearing down on the sear pushing it into the rails.

Nevertheless, smoothing/stoning out the top surface of the rails and the sear/hammer engagement surfaces did wonders for the crunchiness and shortening the sear spring and the hammer spring by one loop reduced the weight of the trigger quite nicely. The pull is still long, but is now uniform and smooth. I can imagine that the length of pull could be reduced by taking some material off the hammer edge that engages with the sear -- another day perhaps.

I was *quite* surprised at the outcome. The before and after difference is noticeable.

Let's do a step-by-step tour.

After removing the trigger group from the rifle, locate the latch stop pin near the front of the trigger group. This pin also serves as a group retention pin, sliding into recesses in the receiver to hold the trigger group in place. This pin must be driven out. One can start it out with a few taps of a brass hammer and then complete the driving out with an appropriately sized punch.

Once the latch stop pin has been removed, the latch stop, the sear spring and the sear can be removed by sliding them out the front of the trigger group housing on the rails they ride on.

Next, we need to remove the hammer from the trigger housing. Fortunately, this can be done without disassembling the rest of the trigger group. Merely, hold the hammer in a vice so that you can push the trigger group down against the weight of the hammer spring. The hammer must be in the decocked position before an attempt is made to remove it. The hammer hinges in two recesses, one on either side of the housing.

Once the hammer is pushed past these recesses, it can be removed.

We can now see the rails on which the sear rides.

Since the sear is pushed down against the top portion of these two rails by the full weight of the hammer spring, the top portion of the rails must be polished to remove any burs and non-uniformities. We do this with a ceramic stone.

Next we polish (again with a ceramic stone) the two engagements surfaces between the sear and hammer. Here's a view of the engagement surface on the sear.

And here's a side view of the two engagement surfaces.

Like with the stoning of any sear engagement surface, care must be taken to not introduce high spots by stoning the left and right sides non-uniformly. Don't remove any more material than is necessary to achieve a mirror smooth surface. The leading edge of the hammer surface can be rounded a little to spread the weight of the hammer weight over a larger radius.

Finally, I cut one loop off both the hammer and sear springs with a pair of wire snips.

The edges should be ground flat against the coils of the springs.

Reassemble and give it a go. I will be taking this SKS to the range this weekend to test whether the smoother trigger affects accuracy.


Weetabix said...

I have a buddy who's been talking about exploring something like this for a while. I'll pass this on to him.

I look forward to the range report.

Did you gauge the trigger weight before and after?

Felix Estrella said...

Felt the trigger weight by fingerspitzengefuhl only. ;-)