AN3C SS Bolts Binding and Broken

[mikeyj350]

Well, here I sit with some broken bolts and a frown...

I thought this morning was going to be a rewarding and quick session before work, exchanging six AN3-5A bolts in the tailcone with six brand new AN3C-5A stainless bolts (and corresponding ss washers/locknuts) that I had just tacked on to my last order from Spruce. Sounds easy enough, right?

The old hardware backed out just fine, but when installing the new bolts, the threads seemed to get stuck while still a few turns short of tight (yes, they are the correct length and still had more thread to go). I said whoop, something's wrong, and went to back it off... it didn't want to go, a little more force and SNAP! The bolt broke. So I said ok, must have been a bad bolt or locknut or something, or maybe a little piece of debris got in there. So for my next bolt, I made absolutely sure it was clean and the threads looked good. SNAP! Same thing.

I climbed out of the tailcone, slithered out the baggage door and took a third bolt and bench-tested it. I got it fully threaded, and then went to back it off. SNAP!



Believe it or not, I've tightened bolts before, typically with unbounded success. I feel like I possess the necessary qualifications to tighten THESE bolts. I'm not going to waste a 4th bolt here until I figure out what's going on. What's different about stainless? Am I supposed to lube these or something? Are they defective? Improper technique?

The learning experience continues...

[krw5927]

Originally Posted by mikeyj350

What's different about stainless? Am I supposed to lube these or something? Are they defective? Improper technique?

What you've experienced is typical stainless galling.

http://www.estainlesssteel.com/gallingofstainless.html

But why are you using stainless bolts in the first place? Do the plans call for them here? They are harder (not necessarily stronger) than cad plated steel bolts, but they are also way further from aluminum on the galvanic series than cadmium, thus more prone to galvanic corrosion when in contact with aluminum.

Generally you're better off with cad plated steel when the plans call for them. Yes, stainless looks neat, but comes with its own set of unique "features" for careful consideration.

Edit: Someone suggested lubricating the threads. Using an arbitrary lubricant can drastically affect the torque-tension relationship, and without some expensive testing should be avoided.

[jabarr]

The biggest question that comes to mind--- Why are you changing from the hardware called out in the plans to stainless hardware? whoops- looks like Kurt beat me to the question!

[BillL]

Just about nothing worse for galling than stainless on stainless. Anti seize is the solution if you just HAVE to go stainless. But - +2 - why?

[mikeyj350]

I guess I should have anticipated the "why" question and answered it first... but here goes.

I'm installing a GMU22 magnetometer in the tailcone, and Garmin seems pretty specific about the thing needing to be miles away from any ferrous metal and/or magnetic sources. There are three bolts on each side of the tailcone holding the F-636 shoulder harness anchors to the longerons that are in fairly close proximity to the GMU22. I guess I'm under the impression that this was a common practice to exchange those bolts for the stainless variety?

(I am aware that some stainless alloys are still somewhat magnetic, but as I understood it, it makes the difference between success and failure... maybe/hopefully someone with this installation can chime in with confirmation on this?)

[Auburntsts]

I have my G3X magnetometer in the tailcone (same bulkhead you're looking at) using the standard seat belt attach hardware and it's not a problem. My ELT antenna (forward) and the pitch trim servo (aft) are also pretty close to the magnetometer. Closer than the install manual stipulates, IIRC, but again no issues passing the calibration test nor in actual flying.

[aturner]

Mike, I installed my GMU 22 on the front side of that bulkhead, just as you are planning. I have the standard CAD hardware for those anchors, and the GMU 22 passed the elaborate self check test with a high score and has been rock solid in operation. Like you, I spent too much time worrying about magnetic interference. if you follow Garmins guidance to the letter, you will be towing the magnetometer on a boom behind the airplane, like the oil and gas exploration guys.

[mikeyj350]

Well shoot, I wasn't expecting to hear two contrary cases in almost as many minutes!!!

To redeem myself slightly and gain a bit more credibility than just "I saw others doing it" I thought I also remembered seeing it in the manual as well... sure enough, straight out of the G3X install manual, page 12-3:

"Use nonmagnetic materials to mount the GMU 22, and replace any magnetic fasteners within 0.5 meter with nonmagnetic equivalents (e.g. replace zinc-plated steel screws used to mount wing covers or wing tips with nonmagnetic stainless steel screws)."

That is definitely good information that there are at least two folks out there with standard hardware and working fine, but I figure since I already have them now, why not? Am I doing something dangerous by exchanging them out? I hadn't considered corrosion, but I did consider tensile/shear loading and determined that the cables are the weak point and not the hardware, with either the standard or the stainless bolts.

BTW I've considered not spilling these beans to potentially save face... but I've also just shelled out $120 on replacement stainless cables to exchange for the steel ones....

Maybe I am overthinking things. It's happened before.

Appreciate all the info so far guys, this is turning into a more exciting discussion than I had anticipated!

[Auburntsts]

Truth in lending, I have the older GMU44, but I doubt that will make a difference.

[9GT]

I read where some folks experienced problems with GMU calibration so I switched my bolts with SS as they did. I did not have the issues you describe though. Sure you have the right thread pitch nuts?