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Why do the plans call for this bolt to be upside down?

IowaRV9Dreamer

Well Known Member
This is the bolt that holds the Exit Air Attach Contraption, er Brackets to the firewall. It is supposed to be installed head down.

That seems odd to me, so I have mine with the head up.

Does anyone know why the plans show it upside down? All I can think is that it keep the AN365 nyloc nut out of the hot air flow, but I can't believe that would matter.
 

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I would have thought because it is difficult or impossible to insert the bolt from the top if the engine mount is in place, but obviously you have done that so must be some other reason.
 
I would have thought because it is difficult or impossible to insert the bolt from the top if the engine mount is in place, but obviously you have done that so must be some other reason.
Yeah its super easy to install the aft bolt from the top... unlike the forward bolts which are darn near impossible to get the nuts on!
 
The head creates less drag than the nut plus threaded portion. Not a flight safety issue, so no need to account for failure modes.
 
Every time

I think every time someone crawls under there to wipe it down or do work, they are going to be thankful that bolt is upside down. If it is put it in with the head on the top side, one might consider putting a cap on the threads that are sticking down. JMHO
 
Best practice is to install bolt heads forward, up, or out. If you deviate from best practice there should be a reason for it.

Designers have discretion for which side the head is located for many reasons including clearance to get bolt installed, ability to get nut on, and room for a torque wrench on the nut and turn the wrench. In my day job, this locates bolt head side most of the time. Clearance to moving parts can also determine head location and happens frequently in tight locations like servo linkages. Also, if fastener is on outer moldline (it sees air flow) the manufactured head (bolt head if a bolt) almost exclusively goes to the external flow side. You don’t see a lot of rivet tails on outer moldline.
A reason that it makes no difference the direction of head is in a structural joint, if the nut falls off, the bolt is no longer carrying much, if any, structural load without the clamp up of the nut torqued properly and may fail as soon as any load is applied. Plus it most likely is in a location that never gets looked at in a preflight.
Where it may matter is in joints that only is loaded in pure sheer like truss structure or control system tubes and cables. Then if nut falls off maybe the bolt will stay in place long enough to be found before something bad happens. Big maybe, but if none of the above is a factor, put the bolt head up.
 
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