Pretty much every ak should have this. - KNS Precision Adjustable Gas Piston System

This is a no-brainer. Every AK should have one of these installed. You'll think to yourself "but I kinda like how mine works, it's not actually that bad, it's not super over-gassed..." and that's cool. But then you'll shoot an AK with the KNS piston installed and you'll catch fire from ordering one so fast. The difference is yuge, and it's real. It will have more effect on some than others, so keep that in mind - barrel length, etc is going to play a part.

Oh, and installation can be a little bit scary, but it's actually pretty easy to do with simple hand tools. Tuning it can be a bit of a pain, and take a lot of trial and error, but once it's tuned you're good to go.

All told, hands down, I would consider it a must-have accessory for the discerning AK guy.