Both BB's need a spacer if fitted to 68mm shells,no spacer normally needed for 73 mm shells.The standard supplied spacer is around 3mm wide,although Shimano did produce other sizes,often needed when fitting e-type derailleurs.XTR had thick carbon mounting plates,XT/LX thinner steel.
Goes on the non drive side,but spacers can be used on either side in some cases in order to adjust the chainline.If you were to use a 108/118mm Shimano road octalink BB for example.Never done it personally but is possible on some setups.
68mm shell,M952 BB,Standard spacer..
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