The following is a reprint of Special Service Letter – Dealer No. 179, dated May 22, 1956.

Because of many rear axle differential case and related parts changes that were made up until the present time and changes that will go into effect within a month or so, we have prepared this bulletin to describe these changes and to list the service procedures which vary according to each change. The first portion of this bulletin will, therefore, describe all these differential changes that have been or will be made while the second portion will be devoted to service information and tool changes.



  1. The original differential case, Gr. 5.510 Part #1168702 (without side gear thrust washer) was replaced by a new case package (#1392756) which was machined to accept two (2) side gear thrust washers Gr. 5.543 Part #1174391, which were included in the package. At the same time the side gear thrust washers went into production, side gear hub length was increased from .460″ to .520″. To further improve the operation of case package 1392756, the Parts Department released for service Gr. 5.510 Part #1392855, package differential oil baffles and felt insert washers. This package includes two (2) steel differential oil baffles and two (2) felt insert washers, which are to be installed in casting recess as shown in Figure 35.
    1956 Buick Rear Axle Oil Baffle Installation-First Type

    1956 Buick Rear Axle Oil Baffle Installation-First Type

    (NOTE: One (1) baffle and felt insert washer for each side gear.) . You will note that the side bearing outer race compresses the felt and oil baffle and holds them in position. The addition of the felt and oil baffle will insure lubrication of the side gears where they rotate in the case and help prevent wear which contributes to the clunking noise.

  2. Similar felt washer and oil baffle changes went into production approximately May 3, 1956. This production arrangement is shown in Figure 36.