How many washers can I use...

[aarvig]

Anyone have any idea what the rule is on how many washers I can use on AN bolts? The reason I ask is that I had to hammer my lead counterweight flatter so that it would fit without rubbing on my horizontal stabilizer. After doing this I realized that I would have to countersink the bolt holes deeper so the bolts would sit flush with the counterweight. After doing this the shoulder of the bolt protrudes out the other side preventing me from getting a good torque. The callout was for one washer but two makes it so that I can torque it accurately. Is this OK?

[Mel]

Using 2 washers is acceptable. 3 is OK as long as one of them is under the bolt head.
However, washers are 1/16" thick. Two of these will be 1/8". Bolts come in lengths of 1/8" steps.
If you need to use more than 2 washers, consider going to the next size bolt.

[Rick_A]

3 washers is acceptable practice. If it takes more than 3, then you should consider a shorter fastener or make a custom spacer.

[Walt]

Yep thats fine, actually I think 3 is the limit, one under the head and 2 under the nut.

Looks like we all answered at the same time!

[aarvig]

Thanks for the help everyone. I knew there was a rule on this somewhere but I couldn't find it by using the search feature. Thanks again.

[bruceh]

Hmmm. Van's has 3 washers stacked on the plans for the nose gear axle bolt to hold the wheel pant bracket U-713. I guess there are exceptions to every rule.

[flion]

Yes, there are exceptions. The bolt used as an axle is in shear, not tension. I think I ended up with five washers and the rod end between the elevator control horns on my -10. But there was no washer between the bolt head and the horn and only one washer between the other horn and the nut. Even in shear applications, it is generally better to make spacers rather than stacking lots of washers but the need to trap the fairing bracket in your example or center the rod end in my example overrode those concerns. In both cases, too, note that Van's called out the specific washers to use while for other bolts it is expected that we will adhere to the standard practice.

[az_gila]

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....

[daviid]

Originally Posted by az_gila

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....

should does not equal shall

[az_gila]

Originally Posted by daviid

should does not equal shall

OK... but notice there is NO reference to number of washers, and absolutely none to the "one under head and two under nut" that seems to be the "other" reference quoted...

And the "three" quoted is called a "limit" (aka "shall") in some posts above...

Do you have another reference?

NOTE: - if the designer calls out stacked washers, that overides the rules above - designer's specific drawings trump standards.