SKF RM30 Imperial Bearing 3-3/4" ID 12:1 Tapered Bore x 8-1/4" OD x 1-3/4"SKF Self Aligning Ball Bearing 3 3 4" ID to 4" ID Nominal Tapered Bore (1: 12) x 8 1 4" OD The smaller side (3. 750") is the nominal bore size. The larger side (4. 000") is the mouth of the taper. This style is not meant to mount directly on a straight 4" shaft its designed to mount on an adapter sleeve that matches your shaft size (commonly 3 1 4" or 3 1 2" shafts for RM30K). Type: Self aligning double row ball bearing tapered bore Bore: 3. 75"