GENE RAL I NFORM ATI ON 5 1 (2) metric fasteners. The threads are cut differently than those of American fasteners (Figure 7). Most threads are cut so that the fastener must be turned clockwise to tighten it. These arc called right-hand threads. Some fasteners have left-hand threads: they must be turned counterc lockwise to be tightened. Left-hand threads are used in locations where no nna l rotation of the equipment would tend to loosen a right-hand threaded fas- tener. Metric ~ ~ 60' 60' ~ -: ISO I\le tric Scr ew T hrea ds ISO metric threads come in three standard thread sizes: coarse, fine and constant pitch. The ISO coarse piteh is used for most common fastener applications.
The fine pitch thread is used on certa in precision tools and instru- ments. The constant pitch thread is used mainly on ma- chine parts and not for fasteners. The constant pitch thread. however. is used on all metric thread spark plugs. ISO metric threads are specified by the capital letter M followed by the diameter in millimeters and the pitch (or the distance between each thread) in millimeters sepa- rated by the sign - . For example. a M8 - 1.25 bolt has a diameter of8 millimeters with a distance of 1.25 millime- ters between each thread. The measurement across two flats on the head of the bolt indicates the wrench size to be used. Figure 5 shows how to determine bolt diameter.
8 NOTE If purchasing a bolt from a dealer or parts store, it is important to know how to specify bolt length. The correct way to measure holt length is to measure from underneath the holt head to the end of the holt (Figure 8). Al ways measure holt length in this manner tener design determin es the type of tool required to work the fastener. Fastener material is carefully selected to de- crease the possibility offai lurc. to avoid purchasing or ins/ailing bolts that are too long. Nuts, bolts and screws arc manufactured in a wide nmgc of thread patterns. To jo in a nut and bolt. the diamc- ter of the bolt and the diameter of the hole in the nul must be the same.
It is also important that the threads on both be properly matched. Ma chin e Screws There are many differen t types of machine screws. f- "i2- ur e 9 shows a number of screw heads requiring different typesoftuming tools. Heads are also designed to protrude above the metal (round) or to be slightly recessed in the metal (flat). See Fi2ure 10. The best way to tell if the threads on two fasteners match is to tum the nut on the bolt (or the bolt into the threaded hole) by hand. Make sure both pieces are clean; remove Loctite or other scaler residue from threads if present. If excessive force is required. check the thread condition on each fastener.
If the thread condition is good but the fasteners jam, the threads are not compatible. A thread pitch gauge (Ffgure 6) can also be used to deter- mine pitch. Yanmar marine engi nes arc manufactured with ISO (International Organization for Standardization) Bolls Commonly called bolts. the technical name for these fasteners is cap screw. Metric bolts are described by the diameter and pitch (or the distance between each thread).