![]() ![]() The cleanliness of the threads, oil or no oil on the threads, or socket extensions can lead to under- or over-stretching of fasteners. Why torque to angle? Even a properly calibrated torque wrench can be fooled and give misleading indication of achieving proper torque when tightening fasteners to high torque values. “Torque-to-angle” procedures have become the preferred method of achieving the desired bolt stretch on bolts used in highly stressed applications such as cylinder heads, flywheels, crankshafts, and connecting rod. Newer Yamaha four-stroke models specify tightening procedures for some applications not previously used on older models for similar applications. Some fasteners (cylinder head, crankshaft, connecting rod, and crankcase) may specify tightening in steps until final torque is achieved.A tightening pattern may be specified in the manual if multiple fasteners are involved. Tightening to spec using a torque wrench.In some situations the service manuals may specify to apply oil, thread sealant or thread locker to the fasteners. Clean and dry all fastener threads (including the mating threads).Technicians have used some variation of the following procedure for many years to torque bolts: If torque specifications are not listed for a particular procedure, a “general tightening torque” chart is also provided in Section 1. Specifications are also provided in each section of the manual for specific service procedures. Current service manuals include torque specification information tables in Section 1. Torque specifications are determined by the size and strength of each fastener and the component materials the fasteners are securing. To ensure fasteners remain properly tightened, torque specifications are provided in the appropriate service manuals. Yamaha typically does not rely on lock washers or thread locking liquids to secure most fasteners. Mechanical locking is also used when extra security is required for safety and where vibration and other factors could possibly loosen fasteners even when properly stretched. Mechanically locking fasteners with lock washers, nylon insert lock nuts, castle nuts with cotter pins, or thread-locking liquids is typically used when the torque necessary to stretch the fastener sufficiently could result in too much clamping pressure on the components. The right amount of torque is determined by the size and material of the fastener and the components being secured. Over-stretching also weakens a fastener and could eventually cause it to break. Applying too much torque will overstretch the fastener, exceeding its elastic limits also resulting in insufficient tension to keep the fastener tight. Applying insufficient torque will not stretch the fastener adequately to generate the tension necessary to remain tight. Machine screws, nuts, and bolts, when tightened to a specific amount of torque, will stretch to provide the tension necessary to keep a fastener tight. Stretch metal too far and it will remain permanently stretched… or even break. Unlike a rubber band, metal can only be stretched a very small amount and still be able to return to its original shape. Metal, like a rubber band, will stretch under tension and return to its original shape when the tension is removed. Elasticity is the ability of material to deform (stretch) and return to its original shape. “Elastic properties”? That’s not something everyone stops to think about. Using the “elastic properties” of the fastener material to maintain clamping pressure.Typically, there are two means of ensuring fasteners (screws, bolts, and nuts) to remain tight: ![]()
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