Rod bearing failure

[13fx]

I am working on a Farmall Cub I just bought and am having to pull pistons. Every piston has a top rod bearing that is delaminating. The bearing appears to be flaking apart, but only on the road side and not the cap side. The rod journals show no signs of wear or damage and look to have either been turned recently in its life or a new crank put in. Someone has overhauled this motor pretty recently (hrs wise anyway) and there is very little wear going on anywhere else. I have never seen a top rod bearing so this and am wondering if this is a manufacturer defect or if someone else can tell me what has happened here.

Thanks
Steven

 

[T.R.K.]

I'm thinking it had water in the crankcase at some point. Was there any evidence of a leaking head gasket? Like a piston with a cleaner top than the rest.

 

[GermantownGreen]

Not mine, saw at the truck shop.
Late model Cummins. 20,000 on o/h.
Former shop did not get something right,

yep piston is upside down! GG Wes

 

[13fx]

I haven't seen any evidence of coolant or water in the crankcase. Every piston actually had excessive carbon buildup for the amount of use this engine seems to show. This Cub seems to have been run pig rich after a rebuild. I have contemplated fuel contaminated oil leading to this flaking, but the cap side of each rod bearing shows none of the flaking and almost no wear.

 

[Stan Coryell]

The water theory is possible. It doesn't look like a lack of lubrication. I'd be surprised if you find a smoking gun.

Is there an oil hole in the rod?

How do the journals measure?

How does the big end measure?

Is the big end round?

Are the bearings marked manufacturer or for size?

What was the oil pressure?

Did the big ends seem to be torqued correctly when disassembled?

 

[joeysmith]

To me it looks like it was put together dry

 

[Stan Coryell]

To me it looks like it was put together dry

Maybe. Why isn't the cap side ripped up?

I'm not familiar with that engine. Why no oil hole in the top of the bearing?

My money is on the big end being out of round.

 

[joeysmith]

Maybe. Why isn't the cap side ripped up?

I'm not familiar with that engine. Why no oil hole in the top of the bearing?

My money is on the big end being out of round.

My thoughts (which are just that nothing to really back it up) the top takes all the abuse

After the first few power stokes oil pressure hit before causing damage to the lower shells

 

[Stan Coryell]

My thoughts (which are just that nothing to really back it up) the top takes all the abuse

After the first few power stokes oil pressure hit before causing damage to the lower shells

I can see that. But, I think the babbit would have wiped down to the cap.

I'm thinking the big end was put together wrong and the top is tighter to the centerline. Nothing to back that up of course...

I doubt we'll find an answer. Lol!

Good thing we aren't building jet planes!

 

[used red MN]

Why no oil hole in the top of the bearing?

That’s the upper rod bearing, the wrist pin is splash oiled as it is in many applications not pressure oiled.

 

[showcrop]

googling "photos of rod bearing failure" it gave me one that was very similar that was blamed on overload.

 

[tom upton]

I am working on a Farmall Cub I just bought and am having to pull pistons. Every piston has a top rod bearing that is delaminating. The bearing appears to be flaking apart, but only on the road side and not the cap side. The rod journals show no signs of wear or damage and look to have either been turned recently in its life or a new crank put in. Someone has overhauled this motor pretty recently (hrs wise anyway) and there is very little wear going on anywhere else. I have never seen a top rod bearing so this and am wondering if this is a manufacturer defect or if someone else can tell me what has happened here.

Thanks
Steven

The rod side is where load is concentrated, so any excessive clearance / out of round will fatique the rod side much worse than the cap. I would get your rods checked out and probably resized. Maybe former operator had a habit of lugging engine hard at lower rpm's and with excessive clearance it did the damage?

 

[miangus]

It's hard to imagine in a cub engine but it looks like pre-detonation damage. Was there enough carbon present to cause the piston to strike the head. Maybe the oil pump was slow to prime during start-up. Carefully inspect crank pins and turn the engine over without plugs on start-up to ensure a primed oil pump. Cross fingers.

 

[13fx]

Its a road day for me so I'm away from home, but it seems I've asked the right place I think. I've been wondering if the rod bearing may have been the wrong size when put in. The motor appears to have a bunch of machining done to it in the past and the crank doesn't show hardly anything for being 70+ years old. I was thinking that the crank had either been cut down or it's new and the back of the bearing says it's standard. While someone has stuck a ton of new parts in this in the past someone was most likely a first timer. The honing in the bores look like someone just let the hone sit in one place while they turned it, the rod caps seemed like they were torqued too tight, but that might be the locking nut, and I think the idle tube in the carburetor was snapped off and wedged back into place causing someone to be unable to adjust the carb properly(but that's just speculation). As to the carbon question, yes there was enough carbon buildup where it could have been hitting the piston, however I don't see any impressions from that after cleaning the piston tops. Tomorrow I'm going to have to see if I can get a micrometer up in the engine and determine what's going on with the rod journals.

 

[kencombs]

I've seen similar on an engine that had not been run in a long while. I blamed it on acidic oil etching the bearing. Probably wrong, but I thought so. But don't recall only 1/2 the bearing being affected.

 

[Jim in LA]

googling "photos of rod bearing failure" it gave me one that was very similar that was blamed on overload.

King Engine Builders has photos and likely causes on their web site.

 

[Mule Meat]

Overloaded before it got oil when first started.

Repeated high reving a splash oil system engine to high on startup will cook the bearings in no time.

 

[da.bees]

WAG but I'm leaning toward pre-ignition for which cause isn't obvious from conventional observation. That's a mouthful but what I'm trying to say is that pistons are being slammed backward just before TDC. If that were true and reason isn't obvious, could crank have been ground off-center? Could slot in timing gear,crank or cam been mis-aligned due to malfunctioning manufacturing equipment? Indications that engine had been into recently coupled with # of new parts is why I envision something near unheard being root problem. This might be one of those times only a real guru with eyes on can figure it out.

 

[MM-TX]

I have no real experience but this PDF might support @Jim in LA and @da.bees

Fatigue of Aluminum Lining

https://www.kingbearings.com/wp-content/uploads/2025/07/king-bearings-failure-guide-download.pdf

 

[Hobo,NC]

I'm going with Surface Fatigue pg 22

POSSIBLE CAUSES1. Overloading (lugging, detonation, or overfueling).
2. Uneven loading (see sections on misalignment).
3. Bearing material of inadequate fatigue strength for application.
4. Bearing failure due to surface fatigue can be the result of the normal life span of the bearing being exceeded.

4 would be a good guess.