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Excessive cylinder pressure and main bearing failure

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Hi all.

Can excessive cylinder pressure result in main bearing failure when there is no measurable knock detected, no physical sign of knock on engine components, and no damage to the big end bearings?

If the answer is yes, please explain how.

Thanks in advance

Attached Files

Is it just the one main bearing, and what do the others look like?

Possible causes -

the shells weren't properly seated, usually because of a tiny bit of grit under it. If so, you should see a mark on the back of it, if it isn't too badly damaged.

incorrect clearance.

the cap was fitted the wrong way round. This is usually noticed immediately because the crank is suddenly very hard to turn during assembly.

a lubrication problem - either a lack of it which is unlikely, especially if it fed a big end, or contamination of the oil by debris in the oil feed to the bearing.

something else...

Hey, thanks for the reply.

Mains 2 and 3 failed, 1 and 4 have some light damage (light scoring through the ceramic coat, but not down to copper layer), but I suspect that occurred post failure of 2 and 3.

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It looks like early model 4g63 bearings which is known for crank walking resulting in bearings damage. HP academy released very good webinar on how to mitigate it...

No buddy, VG30 on billet mains

Hopefully some VG30 gurus will chip in soon, as not an engine I'm familiar with other than on paper.

Billet caps would suggest a high level of modification, so some possible additions -

Incorrectly aligned boring/honing when the caps were fitted - if you have access to a straight edge you can check it, but it 'should' have been noticable as the crank getting harder to turn as the caps were fitted.

In the same vein, was the crank checked for straightness - again it 'should' have been felt on dummy build when spinning it, though.

Incorrect main bearing clearances - usually picked up on metrology.

Crank, or more specifically cap, walk is a possibility - were you using OEM or aftermarket fasteners? If the OEM fasteners were a TTY (when a torque is given and then a set number of degrees to turn after that) they MUST! be replaced with new ones. if aftermarket fasteners were used, did yo use the figures supplied?

V6 engines have a complex counterweight and balance requirement, if you're using lighter or heavier 'rods and/or pistons, was it balanced to suit? Oh, on that, what harmonic damp(en)er did you use, and flywheel?

Sorry it's not helping much, if at all.

Hey Gord

I wanted to keep my question intentionally broad and not platform specific, but also focussed on the specifics I mentioned around excessive cylinder pressures and their potential impact on the main bearing.

I am happy that I know why the bearings have actually failed, but the “circumstances” I pose in my original question have been sent to me as another “theory” as to why they have failed.

I’ve spent a fair bit of time researching online and can’t find any other examples, so I was trying to find out if it was even physically possible for the mains to fail by way of “excessive cylinder pressures” when there is no sign of knock, no physical damage to piston (det) or con rod (bend) and no correlating big end failure.

and I know you may be thinking “if it happy with the cause of failure, why am I asking?”…. It’s a complicated situation I find myself in with regard to this engine and I’m trying to establish if this excessive cylinder pressure theory carries any weight at all

Builder blaming tuner? You would think knock or overloading would damage the big end before the mains, unless you live in it's not my problem land.

Michael, If there was a "like" button I would click it!

A little more complex than that, but I won’t get into it

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