I replaced the Masthead sheaves this past weekend on my 1992 Hunter 33.5. The main haulyard was binding, but the Topping lift haulyard was working well. I went up the mast last summer (used a bosuns chair with my wife at the winch) and checked out the sheaves and the problem. The haulyard sheave seemed to eb split. I ordered the new sheaves from Z-Spar and got a Ascender climbing setup from Topclimber. Using a static line and a safety line I went to the top of the mast.
The masthead has a crane that the back stay is attached to and a slotted hole that i put a heavy duty 3/8 Eyebolt and 5/4 flat washer on the nut side. This gave a very secure point to attach my climbing harness. I was able to stand up on the stirrups and remove the insturment plate from the top of the mast. The Lines were loosened and tension was removed from the haulyard and topping lift line by looping them below the mast head crane assembly. The sheaves wee relitively easy to remove. I removed the axle pin and reused it on the new sheaves. The pin fits into a slot and the sheaves have a seperator between them (this is part of the mast head assembly). The sheave and akle pin assembly dropped in place and the actual replacement job was completed in less than 10 minutes. The top insturment plat was restored and the nuts tightened. The main haulyard was used to take the tension off the Eyebolt. The eyebolt was removed and I was dropped don with the main haulyard with the topping lift used as a safety.
Tools used - #2 phillips long handle screwdriver, medium flat blade screwdrive (3/8 blade width), deep well socket (11/16 I think) and a 3/8 drive socket handle. I tethered all tools with a 5 lenght of 1/16 cord to allow for accidental dropping.
The complete job took 3 hours - mostly because I am 225 pounds and 55 years old. I am also recovering from a broken leg that the metal hardward (screws and plate) was just removed last winter.
The job is quite easy and done correctly is quite safe. It is important to use some type of safety line when moving the harness to the eyebolt on the crane.