I've got a 1983 O'Day 34 and I need to replace the masthead halyard sheaves. What diameter sheaves do I need? From information on the Rig-Rite website I've determined that I have the NG-51 mast and think I have the NG-1 masthead rig. (There are three sets of two sheaves each, side-by-side. and a pair of rollers for spinnaker halyards.) What I can't determine is the diameter of sheaves that I require. There are both 50 and 60 mm diameter sheaves. (I'll get the aluminum versions.) According to a rep at Rig-Rite there is no way to tell except to measure them and it DOES matter which ones you use. It's tough to measure them with the mast still in the boat and I'd like to pull that mast, replace the sheaves and restep the mast in a single day to avoid extra marina charges. Of course, I could order BOTH sizes and return the ones I don't use but that would cost me about $45 in restocking charges.