Crankshaft Information | Connecting Rod Information | Technical Information | Rod Bolt Technology
It is nearly impossible to put a power rating on a connecting rod. Power is produced from the expanding gasses in the combustion chamber pushing down on the piston which in turn pushes down on the connecting rods. While there are certain situations that could lead to failed rods due to compressive loads, rods generally don't fail because of power loads. If rods broke due to power, they would experience severe bending to the point of permanent deformation prior to breaking. When you see a broken connecting rod where there is no seizure of the bearing or failure of the piston/pin/cylinder wall, look closely and you will see that the rod was actually pulled in two. This high tension pulling load on the rod takes place at TDC on the exhaust stroke and is caused by the piston trying to continue up the cylinder walls and through the cylinder head and the crankshaft trying to pull it back down. The heavier the piston, longer the stroke and the higher the RPM, the more pulling load is placed on the rod. When you look at a dyno sheet, you will see that as the RPM is taken past peak power the power falls off. However, most of us have seen engines that have had rods break when over revved. If power broke rods, they would never break due to being over revved.
Connecting Rod Bolts
Not only do connecting rod bolts see the same tension loads that try to pull a connecting rod in two, the total weight of the tower portion of the rod is trying to follow the piston up through the cylinder head which adds to this load making it the most highly stressed fastener in the engine. Because of this, it is very important for the bolts to be properly tightened. Setting a torque wrench at a given number and tightening until this set amount of torque is reached is easy, but it can be highly inaccurate. A torque wrench only measures the amount of resistance it takes to turn the bolts and the amount and type of lube that is used will affect the actual clamp load provided by the bolts. Also, each time a bolt is tightened, the mating surfaces of the threads, the spotface on the rod and flange of the bolt get smoother, which changes the amount of torque that is required to properly tighten the fastener. A bolt is simply a very stiff spring and it must be stretched a predetermined amount to keep the rod cap on and the bearing from spinning. Now that you have spent a lot of money to build an engine, why risk damage by not taking a few extra minutes to put it together correctly?