My heater (different style) had a 5/16 motor shaft, the replacmeent 12v motor I ordered indicated it came with a 1/4 - 5/16 sleeve but it was not included so I returned it and bought one with a 5/16 shaft. I am guessing that the blade type fan used a 1/4 shaft and the squirel cage type fan used a 5/16 shaft and some of the 12 v conversion motors came with the spacer to make them viable for both. rather than drilling out the fan for the 5/16 shaft I would replace the motor with one with the 1/4 shaft.
