Lock washers should never be used under the following conditions: might result in damage or danger to the aircraft or personnel

Shake-proof lock washers are round washers designed with tabs or lips that are bent upward across the sides of a hex nut or bolt to lock the nut in place. There are various methods of securing the lock washer to prevent it from turning, such as an external tab bent downward 90° into a small hole in the face of the unit or an internal tab that fits a keyed bolt.

Shake-proof lock washers can withstand higher heat than other methods of safetying and can be used under high vibration conditions safely. They should be used only once, because the tabs tend to break when bent a second time.

The ball socket and seat washers, AC950 and AC955, are special washers used where a bolt is installed at an angle to a surface or where perfect alignment with a surface is required. These washers are used together. [Figure 7-30]

The NAS143 and MS20002 washers are used for internal wrenching bolts of the NAS144 through NAS158 series. This washer is either plain or countersunk. The countersunk washer (designated as NAS143C and MS20002C) is used to seat the bolt head shank radius, and the plain washer is used under the nut.

Slight clearances in bolt holes are permissible wherever bolts are used in tension and are not subject to reversal of load. A few of the applications in which clearance of holes may be permitted are in pulley brackets, conduit boxes, lining trim, and miscellaneous supports and brackets.

Bolt holes are to be normal to the surface involved to provide full bearing surface for the bolt head and nut and must not be oversized or elongated. A bolt in such a hole carries none of its shear load until parts have yielded or deformed enough to allow the bearing surface of the oversized hole to contact the bolt. In this respect, remember that bolts do not become swaged to fill up the holes, as do rivets.

In cases of oversized or elongated holes in critical members, obtain advice from the aircraft or engine manufacturer before drilling or reaming the hole to take the next larger bolt. Usually such factors as edge distance, clearance, or load factor must be considered. Oversized or elongated holes in noncritical members can usually be drilled or reamed to the next larger size.

Many bolt holes, particularly those in primary connecting elements, have close tolerances. Generally, it is permissible to use the first lettered drill size larger than the normal bolt diameter, except where the AN hexagon bolts are used in light-drive fit (reamed) applications and where NAS close tolerance bolts or AN Clevis bolts are used.

Light-drive fits for bolts (specified on the repair drawings as 0.0015 inch maximum clearance between bolt and hole) are required in places where bolts are used in repair, or where they are placed in the original structure.

The fit of holes and bolts cannot be defined in terms of shaft and hole diameters; it is defined in terms of the friction between bolt and hole when sliding the bolt into place. A tight drive fit, for example, is one in which a sharp blow of a 12- or 14-ounce hammer is required to move the bolt. A bolt that requires a hard blow and sounds tight is considered too tight a fit. A light-drive fit is one in which a bolt moves when a hammer handle is held against its head and pressed by the weight of the body.

Examine the markings on the bolt head to determine that each bolt is of the correct material. It is extremely important to use like bolts in replacement. In every case, refer to the applicable Maintenance Instructions Manual and Illustrated Parts Breakdown.

Be sure that washers are used under both the heads of bolts and nuts unless their omission is specified. A washer guards against mechanical damage to the material being bolted and prevents corrosion of the structural members. An aluminum-alloy washer should be used under the head and nut of a steel bolt securing aluminum alloy or magnesium alloy members. Any corrosion that occurs attacks the washer rather than the members. Steel washers should be used when joining steel members with steel bolts.