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MG MGA - MGA! Hardware Article

I just sent the following email to the technical editor of MGA! magazine. I am posting it here in the interest of correcting some potentially hazardous and incorrect information that was printed in the last two issues. I will post back here what response, if any, I receive:


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To: "Mike Ash" <mikeash@dmv.com>
From: Del Rawlins <del@rawlinsbrothers.org>
Subject: Hardware article in current MGA! magazine

Dear Mr. Ash;

I just received the May/June issue of MGA! magazine, and feel compelled to comment on some erroneous information in the "Nuts and Bolts" articles by Steve Schultz, in this issue and the previous one. I have addressed this to you, because you are listed as the technical editor, and I could not find an email address for the author listed in the magazine.

In both articles, Mr. Schultz promotes the use of AN standard aircraft bolts as an alternative to SAE grade 8 bolts. Unfortunately, AN bolts do NOT possess equivalent strength to grade 8, and are in actuality closer to SAE grade 5. A quick Google search yielded the following documents listing the approximate strengths of each type of hardware (you may wish to do a more thorough search for the actual material specifications):

http://www.aircraftspruce.com/catalog/hapages/anbolts.php

http://www.rockcrawler.com/techreports/fasteners/index.asp

Note that the tensile strength of a grade 5 bolt is 120,000 psi, while grade 8 bolts are good for 150,000 psi. A standard AN bolt, by comparison, is only required to have a minimum tensile strength of 125,000 psi. There are higher grade aerospace fasteners available in the "MS" series which equal or exceed the strength of grade 8, but if standard AN hardware is substituted for grade 8 in an application where the additional strength of grade 8 is actually required, a failure could be the result, with potentially disastrous consequences.

Del Rawlins
FAA Certificated Airframe & Powerplant Mechanic
Anchorage, Alaska
Del Rawlins

Thanks for the information Del. I don't often see AN fasteners, usually identify them because of the safety wire holes. I'm one to overbuild in some cases, but always use grade 8 fasteners if possible.
Bill Young

I read the same articles in MGA!, but have a different concern. I've seen some opinions that say Grade 8 bolts should not be used, especially on automotive suspensions, because they are too brittle. I'm not taking any sides here, just not sure which is the better choice, Grade 5 or Grade 8.
G Goeppner

That concern is addressed in the second article that I linked.
Del Rawlins

Del, that's a very good article on the subject of Grade 5 vs Grade 8, thanks for sharing it.
George
G Goeppner

I got an email from Mike Ash today, thanking me for the information, and stating that my concern will be addressed in the next issue.
Del Rawlins

Del, I'm fascinated! Please keep us informed.
Barry
BM Gannon

There is also the issue of counterfeit grade 8 bolts. Back when I worked for a major regional hardware distributor here in the Puget Sound area, there were rashes of these reports, and in some cases structural failures traced to counterfeit GR8 fasteners resulted in loss of life. Our company sold only GR8s we purchsed directly from established manufacturers in the US or Canada and we never had any problems. Only buy GR8 fasteners from a source that can verify their source.
David Breneman

A good article. I'd like to add a couple of comments. The amount of tension the bolt (or nut) will carry also depends on how many threads are engaged. This depends on the root diameter and the length along the bolt that there are threads. This cylindrical area x the shear stress gives another limit to the tension.
Another factor is the true state of the threads - if too slack, the cylindrical area referred to above is seriously reduced as it becomes a helical surface, not a continuous cylinder. I think this is how most bolts fail on over tightening (stripping the thread). Another limit is the bolt shearing through the applied torque, as in when removing a rusted fastener. I think failure in the pure tension mode must be very rare.
Dry assembly - usually very difficult unles you are assembling something brand new. Usually there is oil down the hole, and impossible to dry it out. Then it is best to lube the threads and use less torque. Greased fasteners are far less likely to rust. It is much better IMHO to apply thread lubrication, especially on weather-exposed parts such as body bolts.
Art

There have been instances of counterfeit hardware in the aerospace industry as well. Knowingly selling inferior structural hardware which does not meet its specifications ought to be a capital offense.
Del Rawlins

This thread was discussed between 27/06/2007 and 01/07/2007

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