I recently noticed that my m42 28mm f/3.5 wasn't focusing at infinity either. I have a split screen and couldn't get the image to "line up" at infinity. I used this link
Takumar Super Multi Coated 35/3.5 disassembly - wiratama.net to help me with the disassembly and adjusted it to work correctly.
Follow kh1234567890's advice.
What I did to get the front ring off:
Cut thin pieces of electrical rubber insulating tape (not vinyl) and placed them on the front ring.
Used a plastic bottle cap that just fit inside filter ring. (This clears the glass in the center so you don't scratch it)
Hold the lens tight and push down hard while turning.
Good Luck!