Dennis writes:
< snip >
>The first question is why do I need a dummy pinion to determine the spacer
>needed under the pinion head bearing? Why can't the pinion in there be used?
>I guess I'm asking for information that explains the Bentley steps 18-28,
>page346. Thanks for any help.
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This is not a simple question to answer. Rear end or differentials require
VERY exacting setup. The gears in these are typically hypoid gears that
are lapped as an assy. A hypoid gear mesh is a highly complex gear mesh,
but it provides the highest loading possible (because multiple teeth are in
mesh at any given time) in the smallest space with very little noise. The
relationship of this gear mesh MUST be maintained or pre mature gear wear,
noise, and possibly breakage from overloading the teeth in extreme cases
will result. This mesh is adjusted in two planes. One area is by the
bearings on either side of the carrier. The other is by shimming the
pinion gear head. The purpose of the "dummy pinion" is to establish the
difference from the ideal or design specification due to tolerance
differences in the bearings and the gear set itself. The collapsible or
noncollapsible shimming is to set the pre-load of the bearings that the
pinion gear rides in. Changing one thing has an affect on the relationship
of all the others. If you change the bearings in any of these areas, then
you have changed those settings unless the bearings had EXACTLY the same
dimensions as the ones you removed. You probably have just about as good a
chance of winning the lottery as finding a bearing to exactly match
dimensionally, the one you are replacing due to tolerance variations.
Therefore you need to re-establish the proper gear mesh again. The way
most of us without the 'dummy' pinion, especially with used sets, is to
establish a gear mesh pattern BEFORE you remove anything. You do this by
painting several of the teeth with paint or grease and mesh them together.
The result is the wear/mesh pattern. I like to use white artists oil
paint. Once this is done then you have a pattern to re-create by shimming
all the aforementioned areas until it matches the old pattern. If you have
already dissembled the diff, then you have only a best guess used pattern
to go by which is, as mentioned only a best guess. New gears have a
slightly different pattern than a used set will have.
Barry Schwartz (San Diego) bschwart@pacbell.net
72 V6 Spitfire (daily driver)
70 GT6+ (when I don't drive the Spit)
70 Spitfire (project)
73 Ford Courier (parts hauler, rain vehicle, getting a V6 soon!)
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