Are there any identification marks on the drill chuck in the Tailstock? That might make finding a suitable key easier.

If it a Jacobs chuck, the Type number will make finding a compatible key much easier.

If not, the important dimensions will, be

Diameter of the pin that goes into the hole. (You could undamaged shanks of drills to measure this ).

This will enable you measure / calculate the distance from the centre of the pin to the bottom of the teeth on the chuck body., and hence the OD of the gear on the key.

Armed with these dimensions you will the have a better chance of finding a key with the correct size pin, and gear diameter. You will still have the problem of estimating the number of teeth on the gear on the key, but hopefully this will be reasonably constant DP or Module from make to make. You may be able to make a fair jab at measuring the pitch of the teeth on the chuck body, which may equate to a rack.

Spare keys are available from

If you can find a key where the gear is correct, ,but then pin is over or undersize, all is not lost.

By driving out the T handle, the key can be gripped in the chuck and turned down to the required size, before replacing the T handle. I have done this on at least one key to make it useable in a chuck where the key was missing.

If undersize, you merely turn up a sleeve that can be pressed onto the key to give the required larger pin diameter. If need be, press on an oversize sleeve and then grip the key in the chuck to turn the OD down to what is required.

Howard