I tried to remove it and it broke. Can anyone tell me if this is a special bolt or regular one? Despite the head of the bolt broke off the radius arm is still stuck in place. I suppose it should come off? I've never removed the radius arm before and have no clue how it is designed.

Edit: OK, the arm came off and now I understand its design. Does anyone know what size/thread is this bolt?

 

It is a special bolt. The thread is 7/16-20 UNF, about 1.5" long. You could probably use a regular bolt. Use loctite on the threads, both to lock the bolt and to keep water out of the threads. The original bolt has a drilled head for lock wire.