What Can Make a Motor Throw a Rod?by Gus Stephens
Back in the day, "throwing a rod" in an automobile engine was far more common than now. In modern engines that are properly maintained and not abused, a thrown piston rod in normal driving conditions has become extremely uncommon. Nevertheless, when it happens, it's a catastrophic event that usually requires a major overhaul, if not an entirely new engine. A piston rod is "thrown" when it separates from its connection point, whether at the piston pin at the top of the rod or at the bottom of the rod where the rod bearing attaches to the crankshaft. Conditions leading to a thrown rod aren't subtle or multifaceted. In most cases, the causes of a thrown rod are limited to a few major failures.
There are only two reasons why an engine would have oil pressure low enough to throw a rod. Excessive wear in very high-mileage vehicles can systemically lower oil pressure enough to cause a rod bearing to spin out of its bearing cap on the crankshaft due to friction from lack of lubrication. When this happens, the rod detaches from the crankshaft and, depending on whether it occurs on the upstroke or downstroke, either smashes the liberated piston into the cylinder head or knocks a hole in the engine block. The other reason for low oil pressure is, well, no oil in the crankcase. A warning light on the dashboard should let you know when things get that desperate. An oil pressure gauge to give you a continuous readout is a better early warning device. Checking your oil regularly--even better.
Over-revving an engine beyond its engineered limits can cause the rod, bearing or wrist pin to fail from excessive stress and heat. This is difficult to do in the average vehicle with standard factory setup because most have "rev limiters" to prevent it. But engines modified for competition or even just high-performance street operation are capable of putting the rpm's far enough into the red for long enough to spontaneously shed internal components.
A broken intake or exhaust valve may drop into the piston cylinder. If the engine continues to run, the loose valve wreaks mechanical havoc in the cylinder, pulverizing the head of the piston and resulting in a thrown rod from breakage at the wrist pin.
Nuts and Bolts
Connecting rod bearings that attach the piston rod to the crankcase are enclosed in caps secured with highly-torqued nuts and bolts. Conceivably, any nut and bolt screwed together can loosen with age or if they aren't properly tightened to begin with. It's definitely not the first thing to suspect, but once in a while, those nuts work loose and release the bearing which then throws the rod. It can also happen if someone rebuilding an engine opts to reuse the previous bearing cap bolts and nuts which have already been overstressed by torquing and age. New rod bearings should always be installed with new hardware.
Gus Stephens has written about aviation, automotive and home technology for 15 years. His articles have appeared in major print outlets such as "Popular Mechanics" and "Invention & Technology." Along the way, Gus earned a Bachelor of Arts in communications. If it flies, drives or just sits on your desk and blinks, he's probably fixed it.