Interesting that everyone quotes three max., but AC 43.13 says something slightly different -
7-37. GRIP LENGTH. In general, bolt grip
lengths of a fastener is the thickness of the
material the fastener is designed to hold when
two or more parts are being assembled. Bolts
of slightly greater grip length may be used,
provided washers are placed under the nut or
bolthead. The maximum combined height of
washers that should be used is 1/8 inch. This
limits the use of washers necessary to compensate
for grip, up to the next standard grip size.
Over the years, some fasteners specifications
have been changed. For this reason, it is recommended
when making repairs to an aircraft,
whose original hardware is being replaced, that
you must first measure the bolt before ordering,
rather than relying on the parts manual for
identification. In the case of plate nuts, if
proper bolt grip length is not available, add
shims under the plate. All bolt installations
which involve self-locking or plain nuts should
have at least one thread of the bolt protruding
through the nut.
Two AN960's are 1/8 inch thick... I could also read four AN960L washers (0.032 thick) as being acceptable.
Note that the FAA says they can be under
either the head or the nut.
Those are the FAA words - waiting for other sources....
True AN960 washers are +/- 0.004 in. for L versions and +/- 0.006 in. for std. versions. I have measured much larger variations on Vans and Spruce washers....