The last page from the spare parts book.
Fowler referred to ropes by their construction rather than diameter: 24,30 or 36 wires, which broadly correspond to 5/8” diameter (K class engines), 3/4” diameter (BB engines) and 7/8” diameter (AA and Z classes). The Superbas and Z7 engines on very heavy work had a 15/16” rope with 36 wires.
The drum should have space for X number of coils plus a bit less than half a rope’s width, so the coils of the next layer sit in the angle between the coils already on. It spreads the load so the rope isn’t being crushed along a single point of contact, and assists good coiling as the rope naturally wants to drop into that space. The gap at the top is just less than half a rope’s width so it holds that coil out a little bit, which assists the next coil to ‘snap’ down into the right place. It’s reassuring to hear the sharp snap noise regularly as the engine is pulling, it tells you that the rope is probably coiling properly, bearing in mind you can’t see it while driving.
Mike Goodman wrote a good series of articles in the SPC journal about ropes. I’ll try to find them.