Is this unusual?

Like I said, I need to be able to brake. The line that runs to the back is shot so I'm trying to not use it, and only have front brakes. Until I can bring the car to the shop and have the line replaced, of course. So I'm trying to plug the rear port underneath the valve. There must be a bolt at Ace Hardware that fits, in fact it's supposed to be a 1/2 - 20, according to info online. But it doesn't, only half of the beginning threads catch and it wants to be crooked. Since I don't want to ruin the port threads and end up needing a new valve too, I need a bolt that actually fits perfectly. 1,000 psi, after all.
I'm just pissed that no bolt will fit it because no one online says that it's some special threading or that it's metric. They do say however that a 1/2 - 20 is what's supposed to fit, and it almost does but not quite.