The bolt is not the problem, the threads have been stripped out on your radiator. There is just a small section of thread remaining as the bolt has a plain, waisted portion near the head. Therefore when the bolt was overtightened, and the threads were stripped, a small section of thread remained. You don't need to replace the bleed screw/bolt with a different type of plug. You need to repair the damaged threads in the radiator so that the bolt can tighten the lower face of its head against the sealing washer/radiator. That's what provides the seal.