Question for the nuts and bolts experts

[Captain Sacto]

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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[lostpilot28]

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.

[Captain Sacto]

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.

.

[bharral]

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.

[WingsOnWheels]

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.

[Sid Lambert]

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.

[terrye]

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.

[flion]

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.

[Av8torTom]

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

[LifeofReiley]

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!