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QuickTalk 8 - TEXTBOOK TRIVIA

A lot of us are happy to simply read the plastic bags from Aircraft Spruce and pull out the appropriate item to be used. However, as time progresses and you become more confident in making modifications, you will want to know HOW those code numbers were assigned.

I'm sure the scenario has happened to several of you from time to time. You're contentedly working on your project and it's time to finally bolt the what's-it to the thingamajig. You suddenly realize that because of the 13 extra layers of BID you added to the mounting plate ("looked a little weak in that area") the bolt is not long enough to secure the pieces. Or perhaps the machine screw called out in the plans will only tighten up with six washers under the head.

The decision is whether to be cajoled by your EAA Designee or order the correct replacement bolt. The latter is probably less humiliating (and much safeer), so you now ask "What do I order?" (You may be Mr. Aviation to the local neighborhood, but most of your engineering experience consists of perfecting the phrase "We'd better call a service man.") Fortunately for composite builders, the only metal parts you have to worry about are the engine, a few pre-machined pieces and a myriad of nuts and bolts.

This issue let's take a look at bolts. You probably noticed that each has the prefex "AN". This stand for "Army-Navy" and is a set of WW-II specifications that were adopted back when the two branches were fighting a common enemy, instead of each other. It was easier for the War Department to stockpile parts if everyone used the same items. The AN specs for strength, size, etc. became common and are still in use today.

Thus, a bolt may be specified as AN10-13. The first figure following AN specifies the diameter in sixteeths of an inch. In the foregoing example, the 10 means 10/16" (or 5/8"). The number following the dash signifies the length of the bolt in steps of 1/8". These steps run 1 to 7, representing 1/8" to 7/8", but 8 and 9 are skipped, and a bolt 1 inch long is represented by 10. Figures over 10 represent more than 1 inch. The 13 of the preceding example means a bolt 1-3/8" long.

A few examples follow:

AN5-7 A bolt of 5/16" diameter, 7/8" long.

AN3-4 A bolt of 3/16" diameter, 4/8" long.

AN11-14 A bolt of 11/16" diameter, 1-4/8" long.

AN8-20 A bolt of 8/16" diameter, 2" long.

Just as -8 and -9 are omitted, so -18 and -19 are omitted. A length of 1-7/8" is -17, and the next step is -20. All of the above are hexagon-headed bolts with holes for cotter pins. To designate a bolt without a cotter-pin hold, the letter "A" is added. Thus, AN10-13A. One more thing - the length of the bolt is measured from the BOTTOM of the head (shank and threads only).

For the next TEXTBOOK TRIVIA, we'll look a little more into an AN system and the material you're using. - RH



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