Typically these are 2.5 - 3mm allen screws with flat (normal) threaded head. I don't know the threading, and it looks more coarse than standard metric for a 3mm or 4mm screw. But I've never checked exact threading. The $10/set Shimano Acera has philips screw adjusters, but the $8/set Origin8 Vbrake has 2.5mm allen. Go figure. At that price. I just replace the arms. But if you know a hw supplier, that might be a better source.