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Captain Sacto

Well Known Member
I'm having a bit of trouble with AN507-6R6 screws twisting off at the head, and leaving the shank stuck in the threads of the K1000-06 nutplate. (This is for the inspection plate/lower main spar flange connection on a 7-A.)

Vans lists the screw as "non-structural", and I would rather call it "just plain weak".

My question? Where can I find a stronger screw of the same size? (I'm thinking that stainless might work best.)

Thanks moocho in advance!

- - Tom in Sacramento


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Hi Tom,
I'm wondering if maybe you've got some smaller platenuts than -6. Or (and I've seen this on some of the platenuts I got from Vans) they were not threaded on the inside of the hole. Either way, you will typically only have problems with the philips head screws stripping, but not usually twisting off. That's an odd occurance if all your hardware is sized correctly.
 
LP writes...That's an odd occurance if all your hardware is sized correctly.

Which is a good observation, and thanks!

Actually, I might have re-sized a few of them (just slightly) with a bucking bar, and I'm re-re-sizing them from within with a screw.

That said, the heads do seem to twist off quite easily, and I would like to find a stronger replacement, if such things exist.

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This is not an uncommon problem with #6 screws and the K1000 nutplates, which implement a locking feature by squeezing the threaded hole until it is oval. Many builders find that simply lubricating the screws resolves the issue - I suggest a waxy lubricant - anything from candlewax to one of the specialized products like boeshield.

Some builders run a tap through the nutplate. Thats ok, as long as you do not remove enough material to disable the locking feature.

I am 99% sure there is nothing wrong with your screws. a #6 screw should be torqued to about 8-10 inch-pounds. An adult using a screwdriver can easily twist the head off of an #6 screw.
 
Put a little boelube on the threads and you shouldn't have any more trouble. However, I would still replace them with stainless myself since the heads don't strip as easily.
 
Yeah, Boelube those suckers up. If I replace a screw in my cowling that's been on and off many many times I still use Boelube to help the new screws out.
 
Stronger screw

Specification AN507 (MS24693) allows for steel, CRES (stainless), brass and aluminum alloy materials. The steel requirement says a minimum ultimate tensile strength of 55,000 psi. The CRES requirement says a minimum ultimate tensile strength of 100,000 psi. So an AN507C followed by the dash number should give you a stronger screw. And, sure enough, the standard does not list them as "structural".

However the AN509 (MS24694) is listed as "structural". However, note that the steel requirement says UTS of 125,000 psi and for CRES a UTS of 85,000 psi. Not sure why the AN507 CRES is higher than this. AN509C followed by the dash number is the CRES part number.

In general, a heat treated alloy steel screw or bolt will always be stronger than the same size CRES bolt because there is less carbon in the chemistry so does not harden to the same strength.

I agree with you about "just plain weak". The AN507 steel specification is a 1010 alloy, which is just barely a steel and certainly not heat treated.
 
Along with the lubrication, I find that using a ratchet driver works better than a regular screw driver. I use a 1/4" ratchet drive which keeps the tension on the screw rather than jerking it like you will with a regular driver. Screws go in easier. A power driver is also smooth but can over-torque the screw.
 
SS-screws

My understanding is that stainless steel screws will be weaker than carbon steel... my $0.02
 
SS Screws

I bought SS torx screws for access panels, wing tips, cowling etc... they're GREAT and do not strip the head life phillips. Oh... and they look good on the plane too! :D
 
509's are "structural" only because they have a grip. 507 screws, no grip length. SS screws are more ductile (I call it smeary) than carbon steel screws. The phillips heads mush out easier than regular steel. They also gall easier. Not as user friendly. I don't know what alloy the common "stainless" machine screws all the vendor's sell are, but its sure not the same as you see in like an AN3C bolt. Doesn't behave the same anyway.
 
Beeswax

Av8torTom said:
My understanding is that stainless steel screws will be weaker than carbon steel... my $0.02
Click to expand...

Agreed.

Buy a wax toilet bowl gasket, the sticky waxy kind. Less than $5.00, and will last a lifetime.

Use a tooth pick to put a small dab of the wax into the nutplate, then run the screw in.

If you use a power driver, run it slowly----real slowly.

Stainless looks good, but is weak, and tends to gall.

http://machinedesign.com/BDE/materials/bdemat6/bdemat6_5.html scroll down to last entry.
 
Search NAS514

Search for NAS514

I have several posts / Threads in this issue of weak vs stronger screws.

There are tech links for some of them.

I am sure this will help.;)
 
When I bought my -8 I had a heck of a time getting screws out of various panels. I took some advice form other threads and:

1) where possible run a tap a little ways into the nut plates. They are still plenty tight to keep the screws in place

2) I use the wax toilet seal ring but just stick the tip into the ring and when ready to put the screw back in

3) I use socket head screws from McMaster Carr that are higher tensile strength. I also bought several different configurations of ball-end and straight-end allen wrenches. Each come in handy in different areas.

After three years and three condition inspections almost every screw in every interior panel has been changed to socket head. One screw sheared off, and that was in the baggage floor in the forward left hand corner - the hardest one to get to! I'm not sure what to do about it but It seems there are plenty of screws in there!
 
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