ARP Rod Bolt Stretch Gauge How-to
Most engine builders concede that torquing rod bolts during final assembly is no longer an accepted practice. While no doubt adequate for general engine rebuilding where loading rarely approaches that of competition levels, basic rod bolt torquing simply cannot account for the many variables that affect the ultimate integrity of the fastener and rod cap relationship. Closer examination is necessary to fully understand the factors involved and the reasons why any particular method offers superior service and durability. Two methods are commonly used to tighten rod bolts and rod bolt nuts, but only one excels for application to racing and high performance engine assembly.
Common Methods of Tightening Rod Bolts
- Torquing a bolt or nut to recommended specs with a torque wrench
- Tightening a bolt or nut to the manufacturer's recommended stretch factor
Among these, tightening a bolt or nut to the recommended rod bolt stretch factor is the preferred method for all racing and high performance engines. It has long been recognized that improperly tightened fasteners will not apply the required preload to the application. If rod bolt toque is insufficient for the application it will not apply the proper clamping force and the assembly is subject to failure due to excessive movement. If the bolt is over-torqued it exceeds the design stretch limit and is subject to failure by surpassing its maximum yield point. All fasteners require a specified amount of preload to achieve the optimum design clamping force. Correct preload is only achieved by stretching the bolt the proper amount as determined by its metallurgical properties and extensive testing. In the practice of race engine assembly few fasteners can be properly checked for stretch because it is not possible to measure the overall length of blind hole fasteners as they are tightened. Rod bolts are a fortunate exception. They provide access to both ends of the fastener for easy measurement.
Factors Affecting Fastener Performance
Manufacturers determine a bolt’s ideal stretch factor through known metallurgical characteristics and extensive testing to establish a “Yield” point or dimensional range where the bolt becomes permanently elongated and will not return to its original length. The “Yield” point is primarily affected by the parent material of the bolt, bolt length and shank diameter, ductility, heat treatment and other factors that contribute to the bolt’s performance characteristics. Once the yield point of a bolt has been identified manufacturers typically peg the recommended stretch at 75-80% of the known yield point. This provides the maximum clamping force known to exceed the known or anticipated load factor of the engine’s rotating assembly. According to ARP a bolt stretched to .001-inch or more of its original static length has been compromised and must be replaced. This minimal amount of excessive stretch serves to underscore the importance of stretch and the loss of clamping force that occurs beyond the design limit. In the absence of proper stretch every cycle of the rod and piston assembly will witness a slight separation between the rod and the rod cap. The separation reverses as the reciprocating forces alternate causing an ongoing cycle of stretching and relaxing that ultimately fatigues the bolt. To preventing these separation cycles the bolt’s preload or clamping force must exceed the forces exerted by the engine’s rotating assembly.
Cap screw type rod bolts that come in your connecting rods are chosen by the manufacturer to meet the specific requirements of the rod’s intended application. Their selection is no accident as there are at least six or more primary rod bolt materials all with different specifications according to use. Their tensile strength ranges from about 210,000 psi up to 280,000 psi. Each will have its own recommended stretch factor. As recommended by leading manufacturer ARP (Automotive Racing Products), each rod bolt must be stretched to a recommended spec when the rods are installed in the engine. ARP’s rod bolt stretch gauge allows you to measure each bolt’s static (free) length and monitor its stretch as you tighten it. As seen in the accompanying photos an ARP stretch gauge is particularly convenient because it incorporates an integral finger loop that makes it easy to grasp while inserting it into the bottom of the engine for quick stretch checks as you tighten each rod bolt. Follow the procedure as specified below to adjust rod bolt stretch to the recommended range as specified by the manufacturer.
- Install the rod cap and insert the rod bolts with the recommended lube on the bolt threads and the bolt seating surface.
- Use a nut driver or the appropriate ratchet and socket to snug the bolts and gently seat the rod cap.
- Loosen both bolts to ensure there is no preload.
- Zero the stretch gauge and measure each bolt independently to determine and record its free length.
- Torque both bolts to an indicated 25-30 ft-lbs to ensure a small amount of preload.
- Tighten one bolt to the manufacturers recommended torque and measure the stretch.
- If necessary increase torque until you reach the recommended stretch.
- Once the bolt is stretched to the recommended spec, loosen the opposing bolt and retighten carefully until the recommended stretch is achieved.
- Check and record the torque value required to reach the recommended stretch.
- If you can duplicate the torque value on at least two rods many engine builders will consider that a valid torque figure for the rest of the batch.
- Other builders will measure and record every rod bolt to ensure maximum integrity of the fastener assemblies.
The spindle points on the gauge must center in the dimples on each end of the rod bolt to achieve an accurate reading.
Be certain to re-zero the dial gauge for each bolt that you check. Consistency is critical. Apply the same amount of ARP Ultra-Torque lubricant to the threads and seating surfaces of each rod bolt.
Cleanliness is mandatory. The rod bolt, parting surfaces and threads must be absolutely clean to ensure consistent results.
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