mikeyj350

mikeyj350

Well Known Member
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!

:mad:

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...
 
mikeyj350 said:
What's different about stainless? Am I supposed to lube these or something? Are they defective? Improper technique?
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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.
 
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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!
 
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?
 
Rationale

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?)
 
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.
 
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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.
 
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.... :eek:

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!
 
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?
 
Hi David, yes I'm [pretty] sure the hardware is correct. It threads great by hand until it hits the nylon insert, then turning with a ratchet and wrench continues pretty much like normal until all of a sudden it quickly gets harder and harder and... stuck.

The exact hardware, after checking my invoice, is:
AN3C-5A (x6)
AN960C10(x6)
AN365C1032 (x6)

And for the cable anchors:
AN4C-7A(x4)
AN960C416 (x4)
AN365C428(x4)

Kurt's link about galling seems to be exactly what's going on. I'll try some Boelube on the threads for the next attempt. Unless people can convince me to go back to the regular hardware, I guess....

PS- as for GMU22/44 differences, I thought the GMU22 was a newer model and is indeed more sensitive to these kinds of things? Or is that just a rumor?
 
mikeyj350 said:
Kurt's link about galling seems to be exactly what's going on. I'll try some Boelube on the threads for the next attempt. Unless people can convince me to go back to the regular hardware, I guess....
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Caution: introducing a random lubricant can drastically affect the torque-tension relationship, and requires some expensive testing to properly characterize.

Why not try it with the drawing hardware first? If the magnetometer doesn't calibrate, then replace the bolts. Sounds like chances are good that you won't have any problems.
 
Ugggh have you put a magnet to those SS cables and bolts. You might be surprised what you will find. Brass might be a better option if you dont want to use cad plated steel bolts. If you check the shear value I think you will find that you would probably not survive an accident that would those bolts to break in shear.
 
Brass

Buy brass screws, nylock nuts and washers for that application. That's what I did.

Stainless on stainless is never a good combination. I work in pharma, where they like everything to be stainless. But we never do stainless nuts on stainless bolts. It is a question of when, not if, they will gall.
 
mikeyj350 said:
Hi David, yes I'm [pretty] sure the hardware is correct. It threads great by hand until it hits the nylon insert, then turning with a ratchet and wrench continues pretty much like normal until all of a sudden it quickly gets harder and harder and... stuck.

I'll try some Boelube on the threads for the next attempt. Unless people can convince me to go back to the regular hardware, I guess....
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There is no reason in the world for the threads to gall with the conditions you described. I suspect you have 10-32 bolt and a 10-24 nut. That is about how far it will go before it locks up. (experience) Use a thread gage to check the thread pitch. Since you won't be torquing very high, boe lube is a good one to try, but genuine antisieze is really what is called for.
 
BillL said:
There is no reason in the world for the threads to gall with the conditions you described. I suspect you have 10-32 bolt and a 10-24 nut.
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I was thinking the same exact thing, both points you made, however this isn't the case. I am positive these are 10-32 bolts and nuts... I have some standard 10-24 hardware sitting around and tried to mate them as a test:

IMG_0772.JPG

(SS 10-32 hardware on left, galvanized 10-24 hardware on right)

Mixing and matching in either direction does not work at all, and I am now 100% certain I am dealing with the correct hardware.

These DO thread together smoothly, until they decide to seize together. They just seem extremely gall-prone for some reason. As I said in an earlier post, the article Kurt linked described the exact issue better than I could myself. I'm sure that's the problem, now just need to figure out a solution...

I mentioned Boelube because it was the first thing that came to mind, I also have Permatex Aluminum Anti-Seize (#80078) and Loctite C5-A Copper Anti-Seize. I'm sure one of them will work (probably start with the Permatex).
 
Today just isn't my day

Well, I'm back from a lunch break where I attempted to go flying in an Arrow to start my commercial/CFI training, and an issue with the plane kept us on the ground. Sometimes you just have one of those days where nothing goes right...

Anyway, I appreciate all the suggestions to use brass or stick with the original cad plate, but I think the solution that will work for me is the ss hardware, with anti-seize and torqued appropriately. I realize it's maybe pre-emptively trying to solve a problem that isn't there, and that brass may be the "best" solution regarding magnetism but I already have the ss parts and I see no reason to deviate. If the ss parts still interfere with my GMU then I will have to consider brass, but since it sounds like many are using the original cad bolts with success (and many are using ss with success) I'm pretty sure this is going to work just fine. As long as I can thread the darn things. :D

Thanks again for all the information, opinions, and advice.
 
mikeyj350 said:
These DO thread together smoothly, until they decide to seize together. They just seem extremely gall-prone for some reason.
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Just a fact of life you need to accept when using stainless screw/bolts.

I use beeswax whenever I use stainless hardware.
 
Remember, this is still the Interwebs

You might want to check the yield strength of brass before trusting your torso to it......
 
rv7charlie said:
You might want to check the yield strength of brass before trusting your torso to it......
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Yes, noted, thank you. One of the main reasons I'm sticking with ss is the perception I have that brass isn't quite as strong. I haven't investigated those numbers, but I certainly would before swapping them in.
 
I am not speaking here of airplane construction but in other applications I have used many hundreds if not thousands of stainless bolts, nuts and screws and have never had a problem. I would try some bolts from a different supplier, maybe Spruce got a bad batch.
 
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