I know some of you already know about this, but I never saw a write up on it, so I'm going to attempt one.
First you need a Door lock cylinder kit from Autozone, O'reilly's, Advance, etc. for a 1990-'94 Sentra, they run about $20, a lot less than what the dealer wants
note: this write up is for people who's locks have broken the inner cylinder that holds the arm.
Also, I will not cover changing out the tumblers.
Tools needed:
10mm socket(to remove door handle)
dikes
needle nose
small screwdriver
wire brush
Dremel/hacksaw (not needed but makes it a little easier)
hammer
small center punch
The S13 lock is on the left, and the Sentra is on the right, as you can see they are mostly different.
First thing we want to do is remove the C-clip and arms from the 2 cylinders. Save the arm from the 240sx
Now take the return spring off of the outer cylinder
Now we need to remove the Stainless cover from the front of the Sentra lock. I used a Dremel to slot it, then I pried it off with a pair of dikes. Below you can see the one removed on the left, and a new one on the right
Now slide the inner cylinder out
You can pitch the the Sentra outer cylinder, arm, and front cover, all we need to use is the inner cylinder, return spring and c-clip
Now for the S13 cylinder. Remove the stainless cover by prying the two pinched tabs out. Be careful not to mangle it up too bad as we will need it later.
Slide the cover off, and the broken inner cylinder, out of the outer cylinder. Set the cover and outer cylinder aside.
As you can see, the inner cylinders of the S13 and Sentra are Identical
Now take your handy Dremel with a wire wheel and clean the crap out of your S13 outer cylinder. A .50 cal muzzleloader or 12ga. barrel brush works too.
Now slide your Sentra inner cylinder into your 240sx Outer and pop the stainless cover on. Use a punch or something similar to re-crimp the Stainless cover.
Put the return spring back, put the arm on the inner cyl. and finally the C-clip, Now you have a functional lock cylinder and you don't have to spend 90 bucks for a new one at the dealer!
I appreciate feedback, let me know if anything was confusing or if I need to ad pictures, and I hope this was helpful.
thanks!