There are tens of thousands of blocks in use whose sheaves are made of ultraviolet sensitive plastic. Over time, the sheaves deteriorate, become brittle and may eventually revert to powder. The deteriorated block is usually scrapped. With the ready availability of replacement sheaves, rebuilding may be an attractive alternative to buying a new block.
Any decision to rebuild a block must be done with due regard for the loads imposed on the block and the degree to which the original components can be duplicated or perhaps improved upon. Where normal use of the block imposes loads near the maximum safe limit, rebuilding may not be a viable option because available replacement sheaves may not offer the needed strength. With these caveats in mind, you can nevertheless safely rebuild many of the blocks used on the typical cruising sailboat-blocks which never operate at loads remotely approaching their safe working loads.
Before proceeding, be sure its worth the time and trouble. For a single-sheave block it may not pay to devote the time and effort required to buy a new sheave and axle and do the work. For a fiddle block, however, rebuilding is quite attractive. A replacement sheave for the block shown costs about $2.50. The total cost for replacement sheaves and stainless steel hardware for new axles, washers and self-locking nuts was less than $12. Cost of a new fiddle block with cam and becket will cost $50 to well over $100, depending on size and brand.
The first step in the rebuild is to examine the block and determine how the sheave axle is held in place. In many blocks, the axle is a headed stainless steel pin with one end peened over to lock the axle in place. In this case, it will be necessary to carefully drill out the peened-over portion of the axle in order to remove the axle and sheave. Alternatively, a hand grinder can be used to remove the peened-over material, freeing the axle. The goal is to do as little damage as possible to the cheeks of the block. Any enlargement of the hole in the cheeks will reduce the edge distance from the hole to adjacent holes or the edge of the cheek, thereby reducing the load-carrying capability of the block. For most blocks, an increase in hole diameter of about 1/16″ will do no harm.
With the axle and sheave removed, accurately measure the deteriorated sheave. Manufacturers such as Ronstan list numerous sizes and types of sheaves in their catalogs. The replacement axle can be made from a hex head, stainless steel carriage bolt threaded only on the last 1/4″-1/2″ of its length. It may be necessary to use a few flat washers under the head of the bolt in order to properly position the threaded portion of the bolt. A flat washer is used beneath the self-locking nut, which secures the rebuilt blocks axle.
Because the bolt used as the replacement axle will rarely be the exact length needed, it may be necessary to cut off the protruding end beyond the locking nut. This is easily done after the rebuild is complete. A sharp hacksaw will do the job in a couple of minutes. Dress the rough end with a fine file, then sit back to contemplate the money you saved.
-Chuck Husick
