Bolt Length

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A few weeks ago I ran across an article taking about aircraft bolts that answered lots of questions I had. One thing that concerned me on my airplane is that some of the bolts appeared to be too short. Turns out I was right, some of the bolt sizes on the plans are wrong according to general aviation practices. I suspect this was done to save weight.

The article mentioned above suggested that there should be at most one thread inside the hole, the bolt must go all the way through the nut but if there are more than three threads through the nut the bolt might be too long. I also looked up what the FAA says which is in AC 43.13-1B chapter 7.

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.

…………

All bolt installations which involve self-locking or plain nuts should have at least one thread of the bolt protruding through the nut.

FAA AC43.13-1B

Armed with proper knowledge I looked at the bolts holding the aileron brackets. As you can see the AN3-6 the plans call for are too short.

The bolt does not have at least one thread protruding through the nut.
The bolt does not have at least one thread protruding through the nut.

Not only is the overall length of the bolt too short it’s grip length is too short. More than one thread is in the hole, this could allow the bolt too be overtightened crushing the wood.

I also would feel much safer with those bolts safety wired so I ordered up some AN3-7 with drilled heads. With the AN3-7 one thread protrudes through the nut.

One thread is protruding through the nut

With drilled heads I can safety wire the bolts ensuring my aileron will remain attached.

Drilled head on aileron bracket bolt
Drilled head on aileron bracket bolt
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