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Producing Internal Threads

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The entire racing powertrain, from the engine, through the clutch to the transmission, is littered with threaded fasteners. Studs and bolts are responsible for holding together everything from the most mundane unstressed cover plate to the most highly stressed cyclically loaded fasteners such as those found on con rods, crankcase main bearing studs and cylinder head studs. For every bolt there is a corresponding female thread, and for every stud there are two female threads. Greater...

Nut dilation

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When we tighten a fastener, the aim is to impose an axial load. The method of doing this is normally either to turn a nut on a stationary fastener, or to turn the fastener within a fixed female thread. This is something with which we are all quite familiar. What is less obvious though is what is happening to the female threaded component, and how this might affect the strength of the joint and the relationship between torque and tension in the joint. There is a phenomenon called nut...

Forging ahead, or not?

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When an engine builder picks up a con rod bolt, examines it and lubricates it before fitting, he perhaps doesn't consider the manufacturing processes involved. However, it is very likely that the head of the bolt will have been formed by forging. Con rod bolts are generally equipped with bi-hexagon (12-point) heads. Conventional sockets tighten bolts through contact close to the corners of a hexagon because of large clearances between socket and fastener. More corners equals more...

Tightening to yield

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Where a fastener's dimensions are tightly controlled, and where there is access to both ends of the fastener, it is common practice to measure bolt elongation as a measure of fastener pre-load; the preferred method of tightening con rod bolts uses this method. However, there are many occasions where access to both ends of the bolt is impossible, and we have to choose one of the other methods at our disposal. In previous articles we have looked at some of the other methods, such as...

Tightening using torque and angle

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In the previous two articles on the subject of fastener tightening, we looked at two methods of pre-loading threaded fasteners - torque control and the turn-of-nut method. The method of torque tensioning remains a favourite; anyone with a torque wrench can use this method, but a lot of engineers are wary of it because of the variability of factors that influence the load. Friction is the main variable; it changes with materials, surface finish, lubricant, and is often worryingly...