Rally wheel center cap bolt size

originals were 1/4 dia thread cutting into pot metal. I have substituted 1/4 machine screws and used a tap to make full threads deeper in. It issue to be aware of is the bolts head size. The wheel mounts against the drum/hub at those 2 rings stamped in it. The bolt head have to fit between and below those rings.
A hex head requires a socket to fit between the rings. A phillips or other center driven head allows a larger diameter head.