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.
1.A set of calipers is used...
1.A set of calipers is used to accurately record the rod bolt lengths. Knowing these numbers will be helpful the next time the engine is disassembled. By measuring the bolts again, we can determine if they are in good enough shape to re-use.
2.With the rod bolts lubricated...
2.With the rod bolts lubricated with the right lube (in this case we used ARP moly lube) and installed into the rod hand-tight, the stretch gauge is adjusted so that there is about .400 inch travel in the gauge once it's positioned into the head of the bolt. It's important to center the points of the gauge into the dimpled portions of the end of the bolts and then zero the gauge. This requires a bit of feel to get the gauge reading accurately because there's enough slop in the dimples for the gauge to move around a bit.
3.Next, the rod bolts are...
3.Next, the rod bolts are torqued just shy of the recommended torque spec. In our case, ARP recommended the bolts be torqued to 70-75 lb-ft or stretched to .0050-.0055 inch. Most of the bolts did stretch to .0050 inch with 70 lb-ft of torque, but several required up to 80 lb-ft of torque in order to stretch properly. There are two reasons for this: The torque method is sometimes inaccurate due to varying coefficient of friction at the interface between the rod and the bolt. The other reason could simply be an inaccurate torque wrench. This is where the stretch gauge becomes an invaluable tool. Without the gauge, we wouldn't have known that the proper preload on those specific bolts was not achieved by torquing them to the minimum value.
4.ARP also makes this killer...
4.ARP also makes this killer ring compressor. It's a machined aluminum tube with a tapered end that allows you to just slide the piston assembly into the bore without the rings hanging up. It makes putting the short block together quick and easy. These ring compressors are available in many other sizes as well.