Hoffman Ball Bearing 3-3/4 to 4" Tapered Bore x 8-1/4" OD x 1-3/4" (SKF RM30K)Hoffman Self Aligning Ball Bearing 3 3 4" ID Nominal Tapered Bore (1: 12) x 8 1 4" OD (Equal to SKF RM30K). 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