Big end bearing failure! Study for Engineering Degree

[Ryan]

 smashballs said:

There are many reasons why this happens. Starting with fuel, too low an octane rating resulting in detonation, the pressure wave effect when the flame front starts from a point other than the spark plug, In bad cases, the plug sparks, ignites the mixture, a hot spot will also cause pre ignition, the fronts collide and the shockwaves colliding creates the detonating effect. This is at its worst when the rod is hasnt begun its decent on the downward (power) stroke, or is at tdc. The driving force down coupled with the rotating force of the opposing cylinder is what bends/breaks rods and hammers bearings.

Too thin oil is another big culprit, not cushioning having enough viscosity and allowing the sides to touch and wear prematurely. This is the same as old oil, that is contaminated with combustion by products and has degraded, or no oil at all due to a poorly baffled sump and loss of pressure. Then you have design aspects including rod length, rod ratio relative to the size of the main bearing journals. Bearing failures became more frequent when subaru upped the rev limits to 7800-8250 rpm, as each increase multplies the force on the rod journal. The tensile load is therefore increased. Then you have stretched rod bolts, worn bearings, out of round clearance problems and a big end that can move around.

Well put, +1.

[kunchris]

i have had 3 BEB failures after only two years of ownership first was after 162k kms oil pump failed ate all the bearings

2nd was after a badly reaground crank the vibrated eating the bearings, and killing the oil pump

3rd was with my new ej20g swap oil pump failure after two weeks of normal driving using good oil...i come from the nissan camp so are subaru engines that weak???

[dubbedup]

 kunchris said:

i have had 3 BEB failures after only two years of ownership first was after 162k kms oil pump failed ate all the bearings

2nd was after a badly reaground crank the vibrated eating the bearings, and killing the oil pump

3rd was with my new ej20g swap oil pump failure after two weeks of normal driving using good oil...i come from the nissan camp so are subaru engines that weak???

no sounds like an unlucky run, what petrol do you use?

[RAYDEO]

I caught mine just before it actually carked it, had a buggered ring, was letting crap past and comtaminating the oil, this wedged the oil pressure release valve open, on start up i had no oil pressure, I managed to stop the eninge just before it shagged the bearings, but was very close.

the bearings were showing signs of wear when they were removed.