I have found the stock coupler screws on my Spectrum 0-6-0T to be weak and generally unsatisfactory.  #4 brass machine screws work much better, plus there is a choice of drives.

I have a DC version of the 0-6-0 tank switcher, and it is one sweet runner; just great for slow-speed switching, and small.  Many here may already know this, but I have found a #4 machine screw to be superior to the stock fastener for the couplers.  This is just a great little locomotive.