Bill,
A 6X48 will not screw in and neither will a 6X40. The factory slotted plugs screw in and out easily. They are of course very small and best I can tell with my thread gage,they are either 6X42 or 3.5X.6.
Butch
Butch:
I don't know if someone retapped the holes on my 513T, but I came run a 6-32 tap into mine with no problem. I'll dig around and see if I can fine some on the old plug screws and see if they still fit.