When working on a Honda CRM 250 I needed to disassemble the engines water pump from the casing to replace the seal.
To do this the impeller needs to be unscrewed from the shaft. There is a hex on the nose of the impeller which can be gripped with a spanner or socket, the other side is not so easy. A lot of people simply jam a screwdriver in between the teeth of the plastic gear and the aluminium casing but I wasn’t happy that this would allow me to unscrew the impeller without damaging the gear. If you look closely in the photos you can see some cracks where the gear is pressed onto the shaft (apparently quite common on these engines).
The answer I came up with was to 3D-print a socket which would fit over the entire spur gear spreading the load and reducing any chance of damage. I tapped the two holes in the back of the socket to fit a flat steel bar as a handle.