Certainly one of the most critical components inside an engine is one of the most stressed. Connecting rods take the power from the combustion process and convert it to crankshaft rotation, which, of course, is what makes things go.
Rods undergo tensile, compression and bending stresses, and getting them to survive all three is a matter of design, materials and manufacturing. When a piston rises to top dead center and the crankshaft begins the second half its rotation, pulling the big end of the rod, the two ends of the rod are essentially being pulled apart from each other.
That puts stress on the rod cap and rod bolts, which also transfer their load to the big end of the rod. Compression load occurs when the piston transfers load from the combustion to the wrist pin and to the rod and crankshaft.
Compression load also occurs on the compression stroke, albeit to a lesser extent. A connecting rod also undergoes bending stresses, which rise commensurately with rpm and power output. The engine will then become stiff as the piston is not aligned with the cylinder bore.
The engine could start at this point and you would not realize you bent the rod. You may also just bend the rod slightly and not even notice. If the engine does start with a bent rod you may be able to run it for awhile but eventually it will either seize or the rod will break.
Torque wrenches are not that accurate for tightening fasteners because the wrench only tells you how much force is being applied to overcome friction. The condition of the threads, the number, length and pitch of the threads and the type of lubricant on the threads all affect the reading. Moly-based lubes act differently than ordinary assembly lubes or motor oil.
Different viscosity motor oils behave differently. Some thread lubes provided by aftermarket bolt suppliers are more consistent than other lubricants.
Even so, many rod suppliers recommend using a dial indicator to measure stretch so you can have the right amount of clamp load regardless of how the threads are lubricated.
Advertisement Also critical to connecting rod longevity is checking and double-checking rod bearing clearances and wrist pin fit. Too tight or too loose can cause major problems at either end of the rod. Also, make sure rod bearings have the proper crush fit.
The backs of the rod bearings should be installed dry, while the face of the bearings are coated with assembly lube. Finally, when everything is together, recheck all rod bolts to make sure they have been properly tightened and none have been overlooked. Features: Justina Reusch, Reusch Diesel. Magazine Current Issue Past Issues. Connect with us. Advertise Subscribe Contact Us. Compound Turbo 5. By Larry Carley. The rods also tie the pistons to the crank so the pistons can reciprocate and complete their intake, compression, power and exhaust strokes.
A more common cause of rod failure due to structural overloading can occur if a rod lacks the tensile strength to handle extreme RPMs. The tensile load on the rods will depend on the mass of the pistons, rings and wrist pins, the stroke of the crankshaft longer strokes create more inertia and the speed of the pistons as they accelerate and decelerate up and down inside the cylinders. The cushioning effect can also be lost during the compression stroke if the throttle suddenly snaps shut and shuts off airflow into the engine.
A broken rod whirling around inside the crankcase can do a tremendous amount of structural damage in a very short period of time. Better Rod Bolts One of the most important upgrades for any performance engine regardless of what type of rods you choose to use is to upgrade the rod bolts to stronger aftermarket bolts. The PM rod is pressed and hot forged into it final shape, then placed in an oven and heated to a temperature where the individual iron flakes and other ingredients bond together.
One new rod that was unveiled at the recent PRI Show in Indianapolis uses a unique three-pocket design near the big end to improve bending strength 60 percent over compared to a common H-beam rod.
Forged and billet aluminum rods are another upgrade option for serious competition. Rod Installation Tips One of the first things you should always do prior to assembling any parts is to carefully inspect each and every connecting rod. Rods including brand new ones should also be checked for dimensional accuracy. For a stock rebuild, reusing the original equipment rod bolts may be okay provided all of the bolts are in good condition and are not stretched.
Also critical to connecting rod longevity is checking and double-checking rod bearing clearances and wrist pin fit. In this article: connecting rods , Rod Failures. Watch Engine Builder's latest discussions and explanations.
Shop the latest Engine Builder merchandise. Stay up to date by signing up for the Engine Builder newsletter. By carley. By Engine Builder Staff. It converts the linear motion of the pistons to the rotary motion of the crankshaft.
On every stroke, the connecting rod is stretched and compressed. This pressure, plus other factors, can cause the connecting rod to break. The broken rod can go through the engine block completely, ruining the engine--a condition known as "throwing a rod. Fatigue is the main cause of broken connecting rods--especially in older engines. The constant compression during the power stroke and stretching during the exhaust stroke, over thousands of times a minute, eventually wears the metal out and it becomes brittle and finally breaks.
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