41 Fastener Surface Finish and Condition of Threads In addition to the lubricant used, friction is affected by the surface finish of the fastener itself and the condition of the threads. For example, black oxide behaves differently than a polished fastener so it’s important to follow the torque recommendations with each fastener kit. Then there’s the very real problem of burrs and debris in the bolt holes that can significantly affect the amount of torque required to achieve the recommended clamp load. All bolt holes should be thoroughly cleaned using thread chasers to clean the threads before installation. ARP offers these special cleaning chaser taps on page 172. Torque Wrench Accuracy It is possible for even the most expensive torque wrenches to lose accuracy over time. Rough use or repeated loosening of fasteners using your torque wrench as a “breaker bar” will exacerbate the loss of accuracy. In fact, ARP field technicians have seen a wide range of torque wrench reading errors as much as 15-30%. This just emphasizes the importance of treating torque wrenches with the utmost of respect and having them checked periodically for accuracy. The Torque Angle Method Since the amount that a bolt or nut advances on the thread per degree of rotation is determined by the thread pitch, it would appear that any amount of stretch in a given bolt or stud can be accurately predicted by measuring the degrees of turn from the point where the underside of the bolt head or nut face contacts the work surface. Termed the “torque angle” method, this procedure has long been the standard of civil engineering. It has been suggested that torque angle is a relatively simple and valid procedure to use in blind hole installations—where it is not possible to physically measure the actual bolt stretch. ARP has conducted extensive evaluations of the torque angle method, and concluded that – for high performance engine applications – it is suitable only when calibrated for each installation. Our investigation has proven that installed stretch is dependent not only on the pitch of the thread and the degree of rotation, but also on the amount of compression of the clamped components, the type of lubrication, the length of the male fastener, and the amount of engaged thread. It’s important to note that for the same degree of rotation, the amount of bolt stretch will differ from aluminum to cast iron cylinder heads, or when installing a steel main cap on a cast iron or aluminum block. Furthermore, each length fastener requires a unique torque-angle to obtain the correct stretch for that fastener. The torque angle method can be accurate – but only if each individual application has been calibrated by direct measurement of bolt stretch. If you do employ the torque angle method, it’s best to begin calibrating rotation from some small measured torque rather than the first point of contact with the work face. To achieve optimum accuracy, always use ARP Ultra-Torque® fastener assembly lubricant whenever possible. Proper Fastener Retention FASTENER TECH
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