Monday, March 10, 2008

Repair Guide: Fasteners (Part 2 of 2)


There is a variety of nuts used on vehicles. Slotted and castle (castellated) nuts are designed for use with a cotter pin. These are mainly used for various suspension and wheel bearing fasteners, where it is extremely important that the nuts do not work loose.

Other nuts have a self - locking feature. A soft metal or plastic collar inside the nut is slightly smaller than the bolt threads. When the nut is turned down, the bolt cuts a thread in the collar and the collar material jams in the bolt threads to keep the nut from loosening.

Still other varieties of nuts include jam nuts and speed nuts. A jam nut is merely a second nut which is tightened against the first nut in order to hold the first nut in place. Jam nuts are widely used where an adjustment is involved. A speed nut is a rectangular piece of sheet metal that is pushed down over a screw or stud.


A lockwasher is a split or toothed washer. It is usually installed between a nut or screw head, and a flat washer or the actual part and is used to help keep a nut or screw from loosening in service. The split washer is crushed flat and locks the nut in place by spring tension, while the toothed lockwasher, usually used for smaller bolts, provides many edges to improve the locking effect.

Cotter Pins

Cotter pins are used with slotted or castle nuts to lock the nut in position (preventing it from loosening or coming off in service). When used, the stud or bolt has a hole in it. When the nut is tightened, you align the slots with the hole so that a pin can be inserted. After the cotter pin is inserted through the nut and bolt, the legs of the cotter pin are bent over to lock the pin in place.

Loosening Seized Nuts and Bolts

See Figure 15
Occasionally, nuts and bolts that are rusted resist the ministrations of mere mortals and refuse to budge. Most of the time, penetrating oil or a sharp rap with a hammer will loosen stubborn nuts.

Another method, used in extreme cases, is to saw away two sides of the nut with a hacksaw. The idea is to weaken the nut as much as possible by sawing away two sides as close to the bolt as possible without actually damaging the bolt threads. A wrench will usually remove the remaining portion of the nut. Another option to this method is a special tool called a nutcracker. This tool often resembles a "C" - clamp with a chisel tip (other versions of this tool may be completely round with a tip at the opposite end of the threaded portion). Tightening this tool against the nut splits the nut and it then can be easily removed with a wrench. Figure 15 "C"-clamp type nut cracker (top) and impact driver (bottom) can be used to remove stubborn nuts and bolts

Removing Broken Bolts
See Figure 16

Unfortunately for the do - it - yourselfer learning the feel for how tight is too tight is an acquired skill. Breaking bolts is an unfortunate learning experience for most new mechanics. Most often, the original threads are still in satisfactory condition, however there is no longer any means to turn the bolt. When this occurs you can try to drill the bolt out and rethread the hole using a tap. However, this would probably cause you to go to the next size bolt.

The more common method to remove a broken bolt is a tool called a bolt extractor, often referred to as an Easy - Out®. Bolt extractors are available in various shapes and sizes, and are often sold in kits. You will need to know the original bolt size to select the correct tool. Once selected you will drill a small hole in the center of the bolt. Then you will insert and lightly tap the tool into the hole until it is snug. Finally, you can turn the tool and hopefully the remains of the bolt removing them from the hole. Figure 16 Bolt or screw extractors come in a variety of shapes and sizes

Repairing Damaged Threads
See Figures 17, 18, 19, 20 and 21

Several methods of repairing damaged threads are available. Heli - Coil®(shown here), Keenserts® and Microdot® are among the most widely used. All involve the same principle - drilling out stripped threads, tapping the hole and installing a pre - wound insert - making welding, plugging and oversize fasteners unnecessary.

Two types of thread repair inserts are usually supplied - a standard type for most inch - coarse, inch - fine, metric - coarse and metric - fine thread sizes and a spark plug type to fit most spark plug port sizes. Consult the individual manufacturer's catalog to determine exact applications. Typical thread repair kits will contain a selection of pre - wound threaded inserts, a tap (corresponding to the outside diameter threads of the insert) and an installation tool. Spark plug inserts usually differ because they require a tap equipped with pilot threads and a combined reamer/tap section. Most manufacturers also supply blister - packed thread repair inserts separately plus a master kit containing a variety of taps and inserts plus installation tools.

Before effecting a repair to a threaded hole, remove any snapped, broken or damaged bolts or studs. Penetrating oil can be used to free frozen threads; the offending item can be removed with locking pliers or with a screw or stud extractor. After the hole is clear, the thread can be repaired. Figure 17 Damaged bolt holes can be repaired with thread with thread repair inserts

Figure 18 Drill out the damaged threads with the specified bit. Drill completely through the hole or to the bottom of the blind hole.

Figure 19 With the tap supplied, rethread the hole to receive the threaded insert. Keep the tap well oiled and back the tap out frequently to avoid clogging the threads.

Figure 20 Screw the thread insert onto the thread installation tool until the tang engages the slot. Screw the insert into the tapped hole until it is ¼-½ turn below the top surface. After installation break the tang off with a hammer and punch.

Figure 21 In some cases threads can be restored by running a tap in the hole, or a die on the bolt

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