Strange crankshaft wear pattern

[Kiggly]

I have seen some strange crankshaft rod pin journal wear on my last few tear downs. The wear is predominantly on the tension side of the rod pins, whereas the rod bearing wear is on the compression side. This is a turbo application that sees 60-70psi boost, a bit over 10,000rpm, and makes about 13hp per cubic inch. There are scratches I can get a fingernail caught in tension side, but the comp side is nice and clean.

3E12C4C4-CF4D-4CD3-BE7A-D227CEC9267C.jpeg

166E3E35-B5D5-423E-89FD-7230D5262A22.jpeg

The engine is wet sump, but sorted enough that it doesn’t see oil pressure dips during a pass. I run aluminum rods and about 0.003-0.0035 clearance on 50wt vr1. It is probably pretty foamy by the end of a pass and I’m changing to a dry sump system this year.

Any ideas why it would wear tension side only? I expected compression side on the crank pins.

Thanks,
Kevin

[pcnsd]

Perhaps, particulate carried by the oil and discharged near the location of the scratches is refined or by the time it makes it to the compression side. I have seen such scratches on a bike motor I had the bad sense to clean by abrasive blasting.

[turbo2256b]

what is meant by tension side

[user-23911]

And what's meant by compression side?


I'll guess a stock 4G63 crank, stock size, uncut.
Nitriding is always a good idea with a race crank.


I think it's "scat"? .....a race crank isn't a race crank unless it's been nitrided.

[Roundybout]

The bearing is showing wear and opposite of the crank wear/scratches? If it was foreign matter I'd expect the bearing to show some imbedding on the whole circumference of the bearing. Possibly some distortion going on with the rod big end? You're definitely testing the limits here. Was any thing changed before you started noticing it on tear downs?

[ptuomov]

I have no idea what I’m talking about, but it could be cavitation damage if the bearing clearances, oil, or rpm has changed. Then again, cavitation would also show up on the bearing shell by my guess. In terms of the geometry, are the damaged journal surface and damaged bearing shells aligned at the point where the oil film is likely to cavitate? Total shot in the dark...

[Kiggly]

It is a stock uncut 4g63 crank, they are nitrided from the factory.

By compression side, I’m referring to the part of the journal loaded by compression with the piston at TDC during combustion. By tension side, I’m referring to the part of the journal loaded by tension at TDC during overlap. Sorry, don’t know if there is better terminology here!

Pcnsd- I am interested in the wear you saw. That was only on the one side of the journal like I’m seeint? This engine makes debris from the main bearings touching down a bit and it is aluminim particulates that get caught in the oil filter. It is a bypass oil filter though, so some makes it back through the engine for a while. They don’t really show up much in the rod bearings, but they could act as abrasives in there. What I didn’t follow was why it would be only on one side of the journal.

The rod bearings always show more wear on the rod beam side, not the cap side. They see some minor debris embedded that the engine generates, but not as bad as the crank journal looks by any means.

Cavitation is a possibility I suppose. Wear patterns on the bearings aren’t consistent enough to say for sure. Bearings stay in the engine for about 10 passes before I swap out the upper shells and some lower shells. The crank has probably 40-50 passes and 20 chassis dyno pulls. I’m not bothered by the life at this level, just thought it was weird looking wear.

[rrp]

Let's see the bearing. Cap and rod halves.

[user-23911]

No, they're NOT nitrided from the factory, they're induction hardened.
Nitriding makes them change colour.

Another common internet myth.
Same applies to pretty much every steel crank out of Japan.


So the Leading edge of the journal is worst (bearing wear) and the "outside" of the journal is worst for pin wear.
Wear on the "outside" of the journal is to be expected because that's where the load is when the plug fires.

[4vpc]

+1 for some good pics of the bearings.

Can we refer to it as 'thrust side'? Makes sense to me anyhow, maybe the other side could be 'non thrust side'.
Is it possible you're revving so high that the upward piston forces are now higher than the downward ones?
Also deep scratches on a hardened crank suggest either a soft crank or you've got some hard debris floating round in your oil for some reason.
Soft bearing material shouldn't mark a hardened journal like that.

[Circlotron]

Baseless speculation deleted.

[pdq67]

And just what engine does a, "stock 4G63 crank", come in?

13 hp per cubic inch?? Is this right even with that much boost?

Just asking because I flat don't know so am trying to learn something.

pdq67

[peejay]

4G63 is the 2 liter Mitsubishi dual overhead cam engine. Found in everything from early 90s Eclipses/Talons/Lasers, to some Galants, all the way up to Lancer Evolutions from about ten years ago or so before they finally went to a new engine architecture.

13hp/ci is roughly 1600hp if it's still the stock bore and stroke.

Not bad for a stock crank!

[ptuomov]

The Japanese and German car manufacturers used to make the cranks very strong and stiff to start with and then induction harden the journals. I don’t know why they are so overbuilt, maybe the stiffness was needed to make the desired bearing clearances live and the strength was a by-product?

How much journal hardness does gas nitriding of the whole crankshaft add on top of the factory induction hardening of the journals?

[John Wallace]

When the piston is at TDC, the 'top' of the crank pin throw shows no wear basically, while the 'bottom of the crank pin throw shows wear/problems?
Is this what you mean?

Not sure I understand the question, just trying to understand it more.