A foreword. I only bought this gas tube because I wanted to mount a red/green dot scope on my SKS bullpup. The iron sights are useless on a bullpup.

If you are planning to purchase a gas tube scope mount for your SKS, I have a few words of advice.
When you receive your tube, 1st and foremost, check to see if your piston fits snugly inside the tube. The one I received was 30 thousandth too large, which means all the gas will bypass the piston, and the tube will NOT seal around the gas block.

I tried contacting the company that sells them with no luck.

Therefore I decided to fix the problem myself. Since the tube's inside diameter is way too large, I had to make a piston from a 1/4" drill rod and an AK piston. I drilled a 1/4" hole dead center of the AK piston, then cut the piston head off of the shaft.
With the 1/4" drill rod cut to the correct length, I welded them together using 56% silver solder.

Now for the excessive tube's ID. Back when I was making AK pistols, I modified a 5/8" shallow well 12 point socket to push the front sight bases on. The modification consisted of using a Dremel sanding drum and grinding the inside to a taper.

****MAKE SURE YOU PUT THE MODIFIED PISTON IN THE TUBE BEFORE PROCEEDING WITH THE NEXT STEP****

I secured the SKS gas tube in a vise, and heated the front red hot with a MAPP torch. Holding the socket with a pair of vise grips I put the socket on the gas tube and hit it with a hammer. This lessened the diameter of the tube. After checking the fit after each heating, the tube now fits tight on the gas block. After installing the tube, I test fired the SKS with 20 rds........It now cycles 100%!!!!

A lot of work to fix a product that shouldn't need fixing, but I needed this type of scope mount.