What’s your go-to way to model screw threads?

I’m trying to engineer a replacement for my boss’ weedwhacker’s gas cap.I can model the shape more or less ok, but the internal threading is proving to be a pain.Admittedly, I have been using simple tools so far like Tinkercad and 123D Design.Is it time to graduate to a more advanced modelling platform? http://ift.tt/29ngImc