It is a metric screw, I have seen people retap the thread for standard thread but it uses an M4 screw that is 16mm long and of graphite reinforced nylon. A specialty screw store may well have them in plain nylon and possibly a bit shorter which would work. You could also silicon it on temporarily as silicon will not bond well to nylon and could be peeled off easily later. The screw is part 6080.641 and costs $1.13, if you need the shaft as well it is part 6080.64 and costs $7.22. We have both in stock and the quickest easiest way to order is on tunze.com, just be sure you are on the US part of the website so the order is immediately sent to me.