So how was it connected was it glued to shaft, or was it pressed on?
The fan is on a brass shaft with hole in it? There’s a stainless shaft comes out of motor, stainless shaft fits in brass fan shaft just not sure how stay together as had come apart when took gun apart. I’ve tried glue but no clue if it’ll hold
The brass bush sounds as though it is worn. You will need a new bush or you could try some loctite thread lock.
The bush and fan will be whats known as 'interference' which basically means push fit.
I've just pulled the top off a Senco gas nailer to see how they do it - in that case there appears to be an interference fit boss on the end of the motor shaft with raised tabs (sort of castellated). The fan is pressed onto that and the tabs are peened over to lock it in place.
Looking at the exploded parts diagram for the Hitachi NR90 on the Miles Tool & Machinery site the Hitachi guns are quite different with two sleeves and a nut (presumably Loctited in place) to hold the fan on the shaft
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