Bent Rod Z27AG 4G15 - More to Come

--- Basics ---
 - bent due to detonation or due to poor lubrication
 - hydro lock the motor at high RPM (blow a head gasket and suck coolant into a cylinder, then when it tries to compress, well liqued won't compress, air/fuel will)
 - due to the piston contacting something solid, either a valve or a piece of the piston itself
 - fatigue

















The long term plan is to bore out 1mm oversized.
To remove the gash.
The bent rod was in contact with the cyclinder wall.






The piston skirt had some contact on the counter weight as well. There were weird score marks on the bearing side of the bearing cap as well. Which we believe the bearing may have been slipping due to under spec rod bolts.
We wont be able to tell until we remove some more rod big end caps

There is couple theories i am working on.

- 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.
 - ignition cross-firing
 - Detonation can cause the bearing to spin and seize then break the rod

 


--- Hydrolock (http://en.wikipedia.org/wiki/Hydrolock) ---
Hydrolock (a shorthand notation for hydrostatic lock) is an abnormal condition of any device which is designed to compress a gas by mechanically restraining it; most commonly the reciprocating internal combustion engine, the case this article refers to unless otherwise noted. Hydrolock occurs when a volume of liquid greater than the volume of the cylinder at its minimum (end of the piston's stroke) enters the cylinder. Since most common liquids are incompressible the piston cannot complete its travel; either the engine must stop rotating or a mechanical failure must occur.


--- Failed Rods Statistics ---
Rod Bearing Failure from Clevite Bearings source
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 ---
Tension and Compression in Connecting Rods , A Failure Analysis by Luke Schreier , EM 325H , April 26, 1999 see here

Jim McFarland write a lot of stuff on engines see here

--- Forged Conrods --- www.erd.com.au
99% of the time, when an engine fails, is pulled down and found to have mangled or twisted/bent a connecting rod/s, the cause of conrod failure is an after effect of the initial cause. When you see a conrod hanging out of an engine block, 9 times out of 10 it is not the fault of the conrod.

Extreme pressure is required to bend any metal component and the conrod is the link between the piston and crank that absorbs all of that energy. All it takes is one hard bout of detonation to bring it all to its knees. Exerting extreme pressure on to the rod, thus forcing the oil film out from between the crank and conrod bearing. This causes metal to metal contact and bruises the surface of the bearing. This is the moment where engine failure begins.

Not all engine failures can be attributed to detonation, though, but it is one of the biggest killers. Oil starvation or contamination, metal fatigue or incorrect assembly are other possibilities which occur.

--- EVO X Motor Mishaps --- we aren't the only ones