WHICH "AN" BOLT DASH NUMBER? But seek no longer, Table I is a reproduction of the AN specification which is used to determine bolt dash numbers. The first number following AN in a bolt designation specifies the diameter of the bolt shank in sixteenths of

By Luther D. Sunderland (EAA 5477) 5 Griffin Drive Apalachin, N. Y. 13732

an inch. For example, an AN 7 is 7/16 inch in diameter.

Diameters can be specified between 3 and 20. (Don't be surprised if there is a rather long delivery time on a -20.)

E

Next there comes either a dash or a letter. Following the

(VERY HOMEBUILDER KNOWS that to meet aircraft quality standards, it is necessary to use aircraft

dash or letter is the length designation number, commonly called the dash number. Note that length changes in

quality hardware and avoid the temptation to use bolts

1/8" increments as does grip length so data on dash numbers from 55 to 80 can be generated by continuing each column in 1/8" increments. Bolts can have safetying holes in either or both the head and shank. Holes are specified by letters before and after the dash number as shown in the following example:

from the local hardware store. In the U. S., that means using hardware which complies with military specifications. This article explains the military bolt designation system and provides reference tables which are needed to determine proper designations for ordering bolts.

Originally, the applicable hardware specifications were designated AN (for joint Army and Navy) but these are being replaced with a new set of Military Standard specifications abbreviated MS. Although the AN specification system is used almost universally throughout the civil aviation community, it has been almost completely replaced in military applications. Unfortunately, it is not possible to convert from AN hardware designations to MS numbers without reference tables. Parts suppliers, however, have the reference tables, so they can fill your orders if you specify requirements with either the AN

or MS designations. The problem is that most builders have trouble ordering bolts, even under the old AN system. Along with diameter, the dimension which it is essential to specify when ordering a bolt is grip length, or the distance from the head to the beginning of the threads, see Figure 1. Except for non-structural applications where screws can be used (like for attaching thin sheet

metal), it is absolutely necessary that the threads do not bear against the parts being secured by the bolt. This is the first good aircraft practice which every mechanic should learn. But, once you have learned it, just try to find out how to determine the proper dash number for a

given grip length. You will soon discover that it can't be done without a reference table. And where do you find that? Some parts suppliers interestingly provide in their catalogs a conversion chart to get from bolt length to dash number, but that is no help since the length of the thread-

ed portion is different for each diameter bolt. Other suppliers properly include a scale in their catalogs which can be used to obtain dash numbers from grip length. It is strange that two mechanic's pocket reference manuals in the author's library do not contain this essential bit

AN5-10A AN5H10

Undrilled Both head and shank drilled

AN5H10A

Head only drilled

AN5-10

Drilled shank only

Tolerances on the various bolt dimensions are important to' the homebuilder for it is often necessary to obtain tight fits between bolts and various parts. Table II lists various bolt dimensions and tolerances. Old timers in the aircraft industry use interesting tricks to get holes which give tight fits with standard AN bolts. Notice that an AN 4 bolt can have a diameter between .246 and .249. Many AN 4 bolts run at the low end of the tolerance band so a hole reamed with a standard .249 (.248/.250) reamer could have a .004 slop. In a wing fitting, this could be excessive. There is no standard drill or reamer size between .246 and .249 but there are two ways to get a smaller hole. With a standard .249 reamer spinning in a drill, lightly rub it with a piece of 400 wet-ordry paper. This dulls it a bit, but it will still cut and at an undersize diameter. Another way is to use an undersize drill. Drills usually cut slightly oversize, so a D drill (.246) will usually make a hole large enough for a tight fit on an AN 4 bolt. Special low tolerance bolts can be purchased. They have tolerances for 1/4" bolts of .2487 to .2492 but are usually not readily available. As a Designee, I am often confronted with the question of how many washers are permitted on a too long bolt. Since washers are 1/16 inch thick and bolts increase in 1/8 inch increments, if more than two washers are needed, the next longer bolt should be used. Thus, the FAA limits you to two washers per bolt. This is just a

of information. AN6H10

AN6-10A ,_ 1

" un

— UNDRILLED

I r

AN6H10A

r^vs n I -- U

BOTH HEAD AND SHANK DRILLED

I S -of


UNF-3A 5/16-21, UNF-3A

AN6

AK 7

AN8 AN9

ANIO AN12

TABLE II — AN BOLT TOLERANCES

39/6L .265 .328 .?96 21/U .360 .3^ L7/61 .122 • 390 7/fl .185 .1,5) 63/61 .51,7 .515 l-3/)2 ;

RATED STRENGTH (POUNDS) JLT1KAIE lENalLE YIELD TENSILE SINGLE SHEAR A: ROOT DIA AT FULL DIA AT HOOT DIA AL AL AL STES: ALLOY STKEL ALLOY Si EEL ALLOY 2 210 L O'JO 6 500

3/3 -21j UNF-3A 10 100 ' 1 16-^0 UNK-3A 13 600 1/c -
2 12 c, 710 9VO 1 100 1 690 2 030 3 130 1 310 3 t-*0 1 '15 5 750 2 6S5 3 220 L 9?^ 2 OdO

3 2UO 9 290 3 9'0 l» 350 11 250 5 2?0 9 190 lu 190 5 920 lij 700 * 850

5 02^ V IUO 6 750 10 130

?/ 16-19 UNF-3A 23 600 11 700 ie 100 7 550 5/^ -16 UNF-3A 30 100 lit 9JO 23 080 9 610 3/ii -16 'JNf-3A •ill 000 21 800 33 730 lb 100

18 700 H 700 23 000 10 V50

33 lt-0 J 5 500

This beautiful dark green and white Pitts Special is the

work of John R. E. Day (EAA 78327), 13 Sidwell Ave., East St. Kilda 3183, Victoria, Australia. Powered by a

180 Lycoming, VH-AOY was started October 1971

made its first flight on November 29, 1974.

and

SPORT AVIATION 65