ARP Rod Bolt Stretch Gauge - Tool Of The Month - June 2009
ARP's Engine Assembly Tools
Information is a wonderful thing because it enables us to find better ways of doing things. When building an engine, the traditional method of ensuring that fasteners are doing their job is to tighten them down using a torque wrench. This method works in most cases, but it's not the most accurate method for ensuring that the bolts in critical applications have enough preload and clamping force to overcome the dynamic load of the parts they are attempting to hold together. The best example of this situation is the connecting rod bolts inside of an internal combustion engine. The fasteners holding the big end of the rods together are the highest loaded bolts in the entire engine.
The job of the rod bolt is to keep the two parts of the connecting rod together, while the oscillating motion of the piston and crankshaft tries to rip the assembly apart. The force exerted on the rod bolts is at its greatest when the piston reaches top and bottom dead center of the stroke and has to instantly change direction. In short, it's a lot to ask of a bolt to keep the parts together at high rpm and rod bolt failures are pretty common when they aren't installed correctly.
The best way to ensure the right amount of preload is present in these bolts is to measure the length of the bolts before they are installed and then measure the amount the bolts stretch as they are torqued. ARP's rod bolt stretch gauge makes doing this job pretty easy. The company also offers a bitchin ring compressor that makes installing the piston and ring assemblies into your engine super easy as well. Check it out.