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There are good reasons to use caution when replacing steel washers with
aluminum.

Dissimilar metal corrosion is one. The correct part number would be
something on the order of an AN960PD416. This is a standard "AN" washer
which is anodized aluminum (the "PD") followed by the size (in this case
1/4"). If I remember correctly a straight "D" washer is UN-anodized plain
aluminum.

Trivia - The "D" stands for duralumin. The trade name for one of the
original alloys of aluminum, magnesium, manganese, and copper. Duralumin,
which is the same as modern 2017 aluminum alloy, was first used by the
Germans to build the Zeppelins of World War I. The term dural, which was at
one time used for almost all the high-strength aluminum alloys, was taken
from the name duralumin.

Another caution comes with the issue of compression. Aluminum is softer
than steel and fasteners can lose torque (or more accurately proper
compressive preload) over time as the washer is stressed in service.

I've seen steel washers in service on Beech Baron and Bonanza nose landing
gear actuator parts become hammered in service resulting in loss of
compressive preload and consequent looseness of parts. This looseness
causes loss of proper rigging and accelerated wear. It can ultimately
result in fracture of attaching bolts and rod ends.

Using aluminum washers under say propeller attaching bolt heads or nuts for
example would be a VERY bad idea.

Ted Stanley A&P-IA
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