I lost the chuck key to my old Makita cordless drill. It's got a handy clip on the drill that holds the key, and only this exact style key will fit in the clip. Jacobs still sells a key with this model number, but pictures show they have changed the style. I was thrilled to find these and they fit perfectly.
This key didn't fit my MC1 chuck. The pilot is too large. I think it (was) about 3/16. Needs to be .156". It appeared that the gear pitch was a match so I filed the pilot down to fit the hole in my chuck. Works fine now.
Dennis
Unlike others listed wrongly as 'Jacobs 1/4" X 1/4" ,' but are actually 1/4" X 3/8" and do NOT fit, this is an actual Jacobs KG1 1/4"X1/4" key that fits EXACTLY.