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Strange main bearing wear. Any ideas WHY?

Okay so I'm doing a rebuild on this 2.0L race motor for a good friend/customer. The engine was over-revved to somewhere past 9300 RPM (Not typo, 9300+). A number of rockers were broken but it appears there was no piston to valve contact.

When I split the case I found that some of the main bearings are showing copper along the edge, maybe 2mm worth on the #4, #5, and #6 bearing. THe picture shows two of them. The wear is towards the #8 bearing end of the case.

Any ideas why?

My only theory so far is that when the crank was ground (it's .25mm undersize) the edges near the counterweights were left a little thick.

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Old 05-25-2005, 09:43 PM
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My 3.6 that I am rebuilding that was also over revved showed similar wear. The crank was just checked today at the machine shop and shows basically zero bend is a standard/standard. I only saw this on the #5 bearing but wondered how it hapenned. In my case, all of the exhuast valves made contact with the pistons and were bent, no bent rockers but I had a couple of bent adjustment screws in the rockers.
Strange.......

Interested to see what others have to say.

Jeff
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Old 05-25-2005, 10:17 PM
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When I see wear like this it usually means the fillet radius of the crank did not match the radius or bevel on the edge of the bearing.
Whenever I assemble a high performance engine, I cut the edges of the bearing shells to give a little more clearance in this area.
If the crank had a very defined edge where the bearing surface meets the counterweight, it would develop cracks at that point.
The radius ground into the crank prevents the crack from starting (hopefully).
Bob
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Old 05-26-2005, 06:30 AM
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Bob,

That's sort of what I was thinking as well. Either that or there's some end-play in the crank. I'm going to measure today.

My concern about this is that if the crank is "sitting up" on that chamfered edge untill it wears that radius down a bit there won't be enough oil pressure to that bearing. However there is no evidence of significant bearing wear from startup on these.
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Old 05-26-2005, 07:34 AM
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Well I just measured the crank on each bearing journal in 3 places.. both edges and the middle. They are all within 1/2 thousandths of each other. 2.2310 to 2.2315 across each surface (67.67mm - 67.66mm) so that doesn't explain much.

Maybe end play? I won't know untill I can test assemble it.
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Old 05-26-2005, 07:59 AM
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I am an EE not an ME.
I was wondering if crankshafts flex or are they way to thick and rigid?
Old 05-26-2005, 08:39 AM
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Well so here's my concern...

Lets say that the radiused edges of the crank were to large causing that bearing wear. It would mean that untill it wears the bearing down at that edge there will be a larger than normal gap between the crank and the bearings. It won't hr the bearing much, but it may reduce the oil pressure on the motor untill it wears down to normal tolerances.

So I'm thinking I need to chamfer the bearings a bit along this edge before installing. Any thoughts?

CHris
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Old 05-26-2005, 05:34 PM
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As I said before...on performance engines I always chamfer the edges on the bearing shells.
I think this increases the oil flow out of the bearing so that you need to have a good oil delivery system in place before proceeding.
I have always used high capacity (not high pressure) pumps when doing the build.
If end play were the case....I think you would see a sort of spiral pattern on the mains....not deep but noticable after only a few runs (old drag racer don't ya know...lol).
Let us know what you find for end clearances.
Bob
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Old 05-26-2005, 07:18 PM
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Check the thrust bearing and make sure there is no unusual wear or excess clearance.
Old 05-26-2005, 07:23 PM
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Thanks guys,

I wonder if the prior builder maybe did the "chamfering" however I'm only seeing it on 4-7.

Back to the measuring equipment.
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Old 05-26-2005, 08:21 PM
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Okay, well I'm getting about 7 thousandths in lateral crank movement. (Case halves not bolted together) The thrust bearing shoulder surfaces measure at the high end of spec. 28.06mm. The thrust bearing that came out of it is measuring 27.80mm. I can't find a spec for allowable lateral clearance there. Anyone know?

The thrust bearing is showing copper on the outside edge (where the Thrust bearing shoulders on the crank would touch it. Makes we wonder if the 7 thou lateral movement isn't causing some accelerated wear during startup when oil pressure is low.
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Last edited by cstreit; 05-26-2005 at 09:01 PM..
Old 05-26-2005, 08:57 PM
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According to wayne's engine book the max allowable runout on the crank is 0.0015". Did you do the measurement dry or with lube?

It could be that the extra several thousandths of runout is what is allowing the crank chamfer to contact the bearing.
Old 05-26-2005, 09:11 PM
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Wow. This one isway off then. I'm going to have to bolt it together and recheck for sure then, because I'm 0.0055" to much. I measured it assembly lube on the old bearings. If indeed the lateral runout is off that much, then it would explain a few things. In particular this would probably allow the crank to move back and forth enough to cause the crank journal radius to contact the bearing. I couldn't see this happening visually, but I'm betting that spec is there for a reason.

Chris
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Old 05-26-2005, 09:53 PM
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I agree with Bob Hutson above. I don't think 0.005" is a big deal, I had 0.011" end play on my race engine with no bad effects, although I would never recommend this kind of clearence normally. The radius on the crank is probably over spec but thats a good thing, just compensate by modifying the bearings to accomadate it. The main reason not to have excess clearence is to prevent the crank SLAMMING back and forth when shifting, usually not a problem on a race car because its torn down frequently.

Have you used plastiguage to check the crank. Sometimes this shows up things that you can miss using a mic. Its not as accurate but will easily show any taper.

Another thought, since the crank was reground maybe they messed up on the journal where the thrust bearing is located. somehow offseting the crank. It might bear a closer look at that location. In fact it could be at any location, one or more radius thats to large, forcing the crank to be offset all the time. In the extreem the radius is just to large for the bearing to fit without being chamfered.

I don't think the problem is serious as long as the engine and the main bearing dosen't get over heated during breakin. Once broken in, the remainder of the bearing, having the correct clearence, works as it should. The edge simply wears away because of insufficient oil clearence. Probably a very very bad thing to do on purpose, as its a un predictable, uncontrolled process.

Last edited by snowman; 05-27-2005 at 11:28 AM..
Old 05-27-2005, 11:05 AM
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Jack,

I'm with you. I'm thinking about just radiusing the new bearings as much as the old ones are now. What I don't want is that reduced oil pressure if there is too much clearance on the first few runs. Yes I'll lose potentially 5% of bearing material, but I doubt this is significant.

I'm then going to tell the owner that he needs to start looking into a new crank. This is a 90 hour motor, so it'll give him some time to find a new one, and there won't be a ton of hours on the main bearings before a refresh is needed.


...and as I might have mentioned, the thrust bearing is showing copper on the shoulder surfaces, further strengthening the lateral end-play theory.
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Old 05-27-2005, 11:42 AM
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Quote:
According to wayne's engine book the max allowable runout on the crank is 0.0015". Did you do the measurement dry or with lube?
Runout is different than endplay. The .0015" sounds like the right spec for bearing journal runout. The end play should be no more than .010"
If the clearance between the radius and the bearing is too tight, the oil flow out of the bearing will be choked. This can cause higher pressure and overheated bearings. There is no doubt that a bigger radius in the fillet makes that crankshaft stronger.
CCR takes the cranks out to the Porsche specified max 2mm radius. Even though the bearings are supposed to clear, some engine builders have him chamfer the bearings anyways. He has a fixture and charges $60 to do all the bearings.
Armondo from CCR is at 650-342-0568
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Old 05-27-2005, 12:15 PM
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Can take a picture of the cranshaft radius? I would like to see it. Some crank grinders grind the center portion of the journal and leave the radius alone. Very bad..
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Old 05-27-2005, 12:18 PM
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Yeah, I confused runout and endplay in his message. There is no runout. Enplay is 7 thou.
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Old 05-27-2005, 12:19 PM
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Yeah, i thought about that, but there is no sign of a shoulder from the crank surface to the radius. Got in there with a magnifier last night.
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Old 05-27-2005, 12:25 PM
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Chris, I think it's from crankshaft flex at high rpm.

I had a crank shaft flex and crack from a piston circlip that broke, and the piston basically was hammering away on the gouges on the cylinder wall. The hammering flexed the crank and hammered away at only one main bearing journal.

I suspect that at that high rpm, combined with the valve/piston contact, that the crank was flexed enough, and in many directions to cause that wear.
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Old 05-27-2005, 01:26 PM
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