[Espeefan]

3x.5mm is correct. If the tap is binding up on you, don't worry. It is the right size. It's probably just a minor tolerance difference. As long as you are threading it in nice and straight, it will not ruin the existing threads. A drop of thread cutting oil, on the tap or in the hole, is a good idea. You'll need some oil anyway, when you start to tap new holes. The drill size for a 3x.5mm tap is 2.5mm. Hope this helps.