One of the problems in building the Japanese 0-4-0T is that the crankpins are
very small (about 1.0mm diameter) screws with a plain head, i.e. round, with no slot to stick a screwdriver in. To get around this I made a small collet-type spanner which gripped the head of the pin and allowed me to easily screw them in and out while test fitting the rods. It took a while to make but also saved me hours of frustration. And lost crankpins. It's a piece of 2mm silver steel with the end turned down to ease access, drilled to the diameter of the head on the lathe and with the end slit and pinched slightly to grip the pin.
The photo shows one of the pins in the spanner/wrench/whatever-you-call-it. It would be fairly easy to make one from plastic tube.
