I pulled the driven gear off of the water pump drive shaft with a small 3 jaw puller.  I wanted to reverse the gears so the undamaged side of the teeth are being used.  The problem was this collar on the back side of the driven gear.  The solution was the 10mm washer which I drilled and filed to fit over the shaft, then filed flat so it was the same thickness as the shoulder (.256 from the other side of the gear to the face of the shaft I think).