My bet is detonation ..but here are some things to think over..
The connecting rod connects the pistons to the crankshaft. 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 number one cause of failure is fasteners followed by rod breakage. Fasteners are the weakest link of the con rod. There are two types of fasteners. Through bolts and cap screws. The through bolt requires a nut. a flat must be machined into both the rod and end cap and this breeds stress risers. A capscrew threads into the big end and helps strengthen the whole cap/rod scene.
As I said above , Fatigue is the main cause of broken connecting rods. 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 at the inclusions or stress risers on the rod surface, These are caused by the forging process..its like putting a nick or notch on a coat hanger and bending it until it breaks.If the oil is low or dirty it can speed up this process. Running the engine hot can also speed up the process.
The pin that connects the connecting rod to the piston (called the piston pin, wrist pin or gudgeon pin) gets a lot of wear. If this pin snaps the connecting rod is no longer connected to the engine. For some engines this results in catastrophic engine failure--the connecting rod goes through the engine block or the crankshaft is bent. Piston pin seizure is a common cause of connecting rod failure. It the rod bolts/nuts are still intact the pin probably seized in the piston bore.
Over revving is the main cause of connecting rod failures in new and high performance engines. If the tachometer hits the red--even briefly--the connection rods are in danger of breaking. This is because the forces acting on a connecting rod increase dramatically at high revolutions. It does not matter if the tachometer is going into the red because the race car is traveling at a high speed, is going too fast in a low gear or is simply going too fast because the accelerator is pressed too far while the car is in neutral--the stress is simply too high at extremely high RPMs. One thing that may happen when you over rev is valve float to the point the head of the valve gets smacked and bent and breaks off, gets eaten by the piston and there goes the top of the piston. Then you have rod slapping around the bore. Not good. Generally the rod will be in one mangled piece more or less.
Hydrolock is a deformation of the connecting rod caused when water gets into the piston chamber. This usually happens when the head gasket blows and water enters the combustion chamber. Even a momentary hydro lock will bend the rod and cause it's failure.
One sure way to ruin an engine is DETONATION which would distort the rod bearing and cause it to spin and seize the Big end . Detonation will cause piston damage, bent rods, broken blocks, and bearing failure. I have never seen a case of detonation where there was not evidence in the combustion chamber.
House keeping or lack of proper maintenance will ruin an engine as the table below ( from Clevite Bearings) shows. You plug or restrict oil and the bearing fails . This usually results in a lot of noise but no catastrophic damage..usually..
Lets look at the rod bearing and Major causes of premature Bearing failure
Dirt.........45.4%
Misassembly......12.8%
Misalignment ......12.6%
Insufficient Lubrication....11.4%
Overloading ........8.1%
Corrosion........3.7%
Improper Journal Finish...3.2%Other .......2.8%
Further reading = see
Tension and Compression in Connecting Rods , A Failure Analysis by Luke Schreier , EM 325H , April 26, 1999
