'clearance' for a plain bearing in such an application should target for 0.002 - 0.003" per inch of shaft diameter. THE most important factor is to 'polish' the shaft to a 'mirror polish' .... if you dont polish you risk 'transfer' of the plain bearing surface material TO the shaft by abrasion. Delrin, a common rudder shaft plain bearing material, is notorious for 'transfer' if the rudder stock surface is not @ a high polish.