29 800-826-3045 FASTENER TECH THE IMPORTANCE OF PROPER ROD BOLT STRETCH/TORQUE… A rod bolt stretch gauge is one of the most important tools a serious engine builder can own. It’s valuable in properly setting up a rod for resizing, obtaining the proper torque load when installed in the engine, and monitoring the condition of the bolt while in use. Rod Bolt Stretch Monitoring Chart Rod #1 Rod #2 Rod #3 Rod #4 Inside Bolt Inside Bolt Inside Bolt Inside Bolt In Out In Out In Out In Out Outside Bolt Outside Bolt Outside Bolt` Outside Bolt In Out In Out In Out In Out Rod #5 Rod #6 Rod #7 Rod #8 Inside Bolt Inside Bolt Inside Bolt Inside Bolt In Out In Out In Out In Out Outside Bolt Outside Bolt Outside Bolt Outside Bolt In Out In Out In Out In Out Whether measured by stretch or by torque, properly pre- loading a rod bolt is essential for trouble-free performance. If a bolt is installed without sufficient preload (or pre-stretch), every revolution of the crankshaft will cause a separation between the connecting rod and rod cap. This imposes additional stretch in the bolt. The stretch disappears when the load is removed on each revolution, or cycle. Over time, this cycle stretching and relaxing can cause the bolt to fail due to fatigue, just like a paper clip that is bent back and forth by hand. To prevent this condition, the bolt’s pre-load must be greater than the load caused by engine operation. A properly installed bolt remains stretched by its preload and isn’t exercised by the cyclic loads imposed on the connecting rod. A quality bolt will stay stretched this way for years without fail- ing. The important thing is to prevent the bolt from failing due to fatigue by tightening it to a load greater than the demand of the engine. Protect your bolts – tighten them as recommended. You can easily monitor the condition of the rod bolts through use of a stretch gauge, or a micrometer for that matter. Prior to installing the rod, measure the length of the bolt in a “relaxed” (untorqued) state. Write this down. You can make up a chart similar to the one shown on this page to properly keep track of the data. When you tear the engine down for maintenance, again measure the length of each rod bolt – being careful to keep everything in the proper order. If any of the rod bolts have taken a permanent set and have stretched by .001˝ or longer you should replace the fastener IMMEDIATELY! The stretching is a sure indicator that the bolt has been compromised and taken past its yield point. In other types of bolted joints, this careful attention to tightening is not as important. For example, flywheel bolts need only be tightened enough to prevent them from working loose. Flywheel loads are carried either by shear pins or by side loads in the bolts; they don’t cause cyclic tension loads in the bolts. Connecting rod bolts, on the other hand, support the primary tension loads caused by engine operation and must be protected from cyclic stretching. That’s why proper tightening of connect- ing rod bolts is so important. See pages 25-26 for recommended stretch and torque. Friction is a challenging problem because it varies so much, and is extremely difficult to control with most commonly known lubricants. The best way to avoid the pitfalls of friction and the known variables associated with different lubricants is by using the stretch method. This way preload is independent of friction, and can be controlled by measuring the exact amount of bolt stretch. Each time a new bolt is torqued and loosened, the friction factor gets smaller. Eventually the friction levels out and becomes constant for all following repetitions, making it necessary to tighten and loosen a new bolt several times before final instal- lation, when the stretch method can not be used. The number of cycles depends on the lubricant. Most lubricants require, 5-7 tightening and loosening cycles to level out the friction before final installation. However, with the introduction of ARP’s new Ultra-Torque fastener assembly lubricant, cycling a new fastener before final installation becomes a “thing of the past.” S ee page 121 for more information on ARP Ultra-Torque ® fastener assembly lubricant.