Can anyone give the definitive answer to the question of which way should the angle grinder abrasive wheel disc be fitted?    Any manufacturer's guidance I have read states it must be fitted correctly but does not say if the spindle locking nut should be against the unblotted side of the disc or the metal insert with expiry date on it.

Been n contact with a number of disc manufacturer's and suppliers with conflicting answers!   Some say it does not matter, other it should be against the metal insert, others against the blotter side.   On-line images tend to show all grinders with the information label facing upwards towards the user, ocasional ones not.

It would be helpful to know what the reason would be to say one way or another.

Don't have a definitive answer but as an Abrasive Wheels Trainer, I was always told the labelling should be face up becausea grinder should not normally be set down on it's back with the whole disk exposed because of the risk of activating the trigger when manouvering it into position to be able to use it. The label should be viewable in the open arc of the guard. When using a recessed wheel on an angle grinder for example the label will always be on the "bump" side and the nut in the recess to prevent damaging loads, since these wheels are self tightening (to an extent) the pressure as the nut tightens can cause a burst. 

A bench or pedestal grinder though is different and a flat wheel can be mounted either way as it will normally have a blotter on either side and the load will be equally distributed.

Hi, blotters arn`t required on wheels smaller than 400mm, centre depressed wheels don`t have blotters. Type 41 wheels do have blotters and can be fitted either way. Hope this helps.