28 replies to this topic

[mniguy_nz]

Hi Guys,

I recently replaced the primary gear bushes to only find out the thrust washers were stuffed and the crank was floating in the block. The pictures are what I found upon dismantling
the engine. There aren't any of the bore or pistons but they're rather scored as well. That ridge on the centre crank journal is 0.010"larger than the surrounding which is from the groove in the bearing. The engine was built by a world champ jet sprint builder so he knows his stuff and the crank was reground with new bearings and pistons/rebore

I'm thinking it was oil failure to destroy so many things at once. Just wanted to hear what you think.

Cheers


Tim

[MRA]

Is the centre main worse than the two end bearings by any chance ?

If so then it is caused by cavitation normally due to high sustained revs, but can also be caused by a weak oil pressure and normal revs........

Is that an ACL bearing by any chance ??

[Sprocket]

There looks to be some forign debris contamination

Scoring of the pistons and bores would also go with forign debris.

What does the bottom center main bearing look like, as that is the bearing that takes the most abuse

[Stevie W]

I had an engnie ruined like this, also built by someone who should know better.

On a local trip to the chip shop (round trip of 8 miles) the engine began to sound very harsh then the oil pressure dropped quickly.

Drove the last mile home with banging big ends, rumbling mains and no oil pressure.

My crank and pistons where totally wiped out.

Investigation found swarf in the oil galleries , which, I know I should have checked during building the engine.

Your damage definately looks like sharp debris rather than oil starvation.

Steve.

[Cooperman]

I had something very similar years ago after doing a rally and using the new Shell syntheti oil plus pulling a sustained 7400 rpm down the same long straight 4 times during the event. In addition to bottom-end damage, the cam bearings also disintegrated and the bits went through the gearbox - very expensive. always used 'proper 20w50 oil after that, not synthetic 'weasel *yellow human water*'.
Check the cam bearings as well.

[mniguy_nz]

Is the centre main worse than the two end bearings by any chance ?

If so then it is caused by cavitation normally due to high sustained revs, but can also be caused by a weak oil pressure and normal revs........

Is that an ACL bearing by any chance ??

Yeah the centre bearing is substantially worse than the outers but they're quite bad. I'm not sure but I think they may be ACL's, the reconditioner bought them.

[Sprocket]

Not sure how the bearing manufacturer has anything to do with forign debris damage or oil starvation for that matter. I am also not sure what is meant by cavitation, as there are engines running 8krpm without problem running the same design pumps

[MRA]

Bearing manufacture was a simple comment.......

This engine did not fail due to foreign object damage, cavitation is about supply and demand, 8000rpm engines don't last long without measures being taken, the main one is cross drilling the crankshaft

Cavitation is where the flow required cannot be sustained by the pump, this occurs when the crankshaft is spinning at or higher than a certain speed, the bigend bearings literaly suck the oil out of the crank, the pump can't keep up with supply and the resultant is damaged / destroed mains etc.....

Similar design of pump..... there are quite a few different pumps out there in the market place some flow better than others.

[mniguy_nz]

Not sure how the bearing manufacturer has anything to do with forign debris damage or oil starvation for that matter. I am also not sure what is meant by cavitation, as there are engines running 8krpm without problem running the same design pumps

Yeah I was just wondering what brand the bearing is, I'm sure it had no part to play in failure. I had low pressure issues with a compressed relief spring but thought it was fixed then one problem uncovered but I'd say oil pressure was the original cause which lead to debris and the scoring . As for cavitation I'm not sure , I wasn't aware it could occur in oil systems

[MRA]

ACL bearings make very good bearings, it has nothing to do with the failure.

Cavitation is a pressure related issue, as your engine has oil pressure......... then it can play a major (although detrimental) role in the life of your engine...

There we go then, low oil pressure, causing cavitation of the bearing due to reduced oil supply. The "supply & demand" was not maintained, cavitation occurs when the demand outstrips the supply, this causes a vacuum on the bearing surface and can / does strip the surface away, this in turn causes more "low" oil issues and sudden engine failure

Edited by mra-minis.co.uk, 26 March 2010 - 10:34 AM.

[Sprocket]

Come on Martin, you really should know better

http://en.wikipedia....wiki/Cavitation

[MRA]

ANd your point is ?

[GraemeC]

If flow cannot keep up with demand it is simply starvation, not cavitation, however complex the mechanism, for creating that starvation.

As the wiki link explains cavitation would be produced by the pump and its rotational speed, not by the demand for oil and the inability of the pump to supply that demand.

[MRA]

You need to consider the crankshaft as the pump in this case, then you will see.........

Although cavitation can also occur in pipe work, just after a restriction and works in a similar way to a venturi on a carburettor.

Edited by mra-minis.co.uk, 26 March 2010 - 08:28 PM.

[Paul Wiginton]

Oil has nothing to do with this failure, its down to the thrusts failing, either due to incorrect fitting, debris, or incorrectly adjusted clutch.
How old is this engine?
Was the crank hardened?

Paul