For this build the front and rear drivers have the standard 12ba crank pin as provided by Slaters with their wheels. The centre axle has had the 12ba cheese head screw replaced with a 10ba countersunk screw. This is to allow the crank pin bush to be tapped 10ba and an additional bush to be added and again tapped 10ba so that they can be screwed in opposite each other to house both the couple and connecting rods (on this particular kit the extra bush was available because a pair of smaller bushes had been provided for the front axle to ensure clearance for the crossheads).
As well as tapping the crank pin bushes 10ba, I also tap the wheel itself allowing the screw to be tightened and retained.
I must admit for my own builds I tend to replace all my crank pins with 10ba and blacken the wheels but I was told not to for this build.
You can see the difference in screw sizes from the back of the wheel.
I also drilled and tapped the return crank itself but it will be soldered to the bush once I have cut it to the correct length and made sure that the crank is in the right orientation.