Rod Bearing Break-in (hint: there isn't a need)

[jbfrancis3]

There was a lively thread on former M3F about breaking in rod bearings. There are also DIY's, here and elsewhere, with smatterings throughout warning people about the need to break in their bearings. Typically, its covering the rod bearings that someone has just replaced.

With that, you may enjoy this timely video from Andre Simon at HP Academy. Among other myths dispelled, he addresses the false statements about needing to break in your bearings. Fundamentally, it simply makes sense.

The video will jump to 3:22 covering the topic: https://youtu.be/5Ruk6GLJgbA?t=202

[Obioban]

.... maybe.

I'd still do it.

[Tbonem3]

Or at least do an oil change sooner than normal like 1200-1500.

[TexaZ3]

Umm, technically crank is not touching (well, no supposed to) the bearings at anytime, it rides on a thin coat of oil. So no break-in needed. I'd still change oil sooner after the bearing job.

[Dash1]

What's more important than break-in is proper install. Which is what I think individuals nerf when they have rod bearing failure after a replacement.

[terra]

Umm, technically crank is not touching (well, no supposed to) the bearings at anytime, it rides on a thin coat of oil. So no break-in needed. I'd still change oil sooner after the bearing job.

Ideally it shouldn't touch, but if they truly never touched, they wouldn't wear down.

[Arith2]

BMW STATES THERE IS A BREAK IN

My source is BMW so check my attachment.
1200 miles under 5500rpm and under 105mph.

This item has a wear pattern just like cams. That's why if you take bearings out, they must go in exactly how they came out. It doesn't matter what "makes sense" if it contradicts fact. We are about right information and BMW knows better on this one considering the engineers who designed the car said there's a break in. This applies to most vehicles though.

[jbfrancis3]

The engineers are not stating there is break-in. Corporate training materials are stating there is break-in; there's a difference.

Everyone is entitled to protecting their assets and doing what they feel best for their vehicle. I am no saint myself and have done plenty of stupid things.

The purpose of the post to understand the fundamentals and filter out opinions, some which get colorful.

[Obioban]

The engineers are not stating there is break-in. Corporate training materials are stating there is break-in; there's a difference.

Everyone is entitled to protecting their assets and doing what they feel best for their vehicle. I am no saint myself and have done plenty of stupid things.

The purpose of the post to understand the fundamentals and filter out opinions, some which get colorful.

TSBs are written by engineers. The rod bearing TSB says a break in period is required after replacing rod bearings.

[Arith2]

Technical Service Bulletins are written by engineers for technicians. I've worked with and have been mentored by plenty of people who personally helped engineers write TSBs for many manufacturers. Those aren't for the public to see generally. This one happened to be posted but as customer, it's not for you. It's to help technicians do their jobs.

That video has good information but that one part is just simply wrong. It's not an opinion, it's a fact. Ocean is right. If they never touched, then they'd never wear, hence never needing to be replaced. The opposite is true and that's why they are considered an item with wear patterns. He also mentioned proper clearances. Our engines are known to have tight clearances for the weight of oil we use.

Break in your bearings and don't blow off a document written by someone much more qualified to write than literally anyone on this forum. This applies the S54 which our vehicles have. If we blow off good sources of information then what are we? Just another forum based on opinions. That's not M3F.

[ATS///Antone]

Technical Service Bulletins are written by engineers for technicians. I've worked with and have been mentored by plenty of people who personally helped engineers write TSBs for many manufacturers. Those aren't for the public to see generally. This one happened to be posted but as customer, it's not for you. It's to help technicians do their jobs.

That video has good information but that one part is just simply wrong. It's not an opinion, it's a fact. Ocean is right. If they never touched, then they'd never wear, hence never needing to be replaced. The opposite is true and that's why they are considered an item with wear patterns. He also mentioned proper clearances. Our engines are known to have tight clearances for the weight of oil we use.

Break in your bearings and don't blow off a document written by someone much more qualified to write than literally anyone on this forum. This applies the S54 which our vehicles have. If we blow off good sources of information then what are we? Just another forum based on opinions. That's not M3F.

This!!!!

[Jimbo's M]

I had to do it three times, but as I stated on the old forum, whether I thought it was needed or not is irrelevant. If you don't follow their guidelines, blow your motor and they pull the DME (which is guaranteed), guess who's footing the bill?

[terra]

I had to do it three times, but as I stated on the old forum, whether I thought it was needed or not is irrelevant. If you don't follow their guidelines, blow your motor and they pull the DME (which is guaranteed), guess who's footing the bill?

On a 20 year old car you’re paying regardless. But yeah, I think it’s reasonable to just follow the break in procedure. Not like any harm results from doing so.

[Jimbo's M]

Well, it wasn't a 20 year old car 20 years ago when I had the bearings done.

[mrgizmo04]

In the stack of crank->layer of oil->bearing shell->rod/cap there are 2 variables that can change thickness and distance between the crank and the rod/cap. Original tolerances in the engine were built to have fresh bearing shells. Oil layer changes based on temp and oil pressure and acts somewhat like a dampening spring, as does the softer layer of the bearing shell (lead, followed by a harder layer - copper). It's not always just rotational friction, but there is also impact of the bearing shell onto the crank (from bigger boom in the combustion chamber that sends the piston flying a tad bit faster than the crank is spinning away from it, generating contact and hot spots that wear the much weaker bearing shell material instead of the crank when you ask for higher load from the engine). As the bearing wears some of that space is then taken up by more of the oil, but there is also more (tiny talking microns) distance created between the crank and the bearing shell, producing a tiny drop in pressure. Oil viscosity drops with temperature as the engine warms up and you start wringing it out, and thus pressure drops too. You can also get contact/friction/heat/wear between the crank and the bearing shell by running low on oil (burning it, leaking it, etc) dropping the pressure. Once a new bearing shell is replaced, the thickness of the shell goes back to spec what it was designed to be vs the crank via plastigauge, there are no surfaces that need to wear in together (unlike flywheel/clutch disk/pressure plate, or brake disc/pad where they are hard surfaces meeting each other), you get a slight bump up in pressure from a fresh layer of lead (assuming it were off on the old bearing), you have a film of oil coating it and things are back to hunky dory. The "boom" impact (per above description) is lessened (think of dropping a hammer on a nail head from 3 feet high vs now only from few inches high). If you had to wear them in, how far would you need to do it? What is the new equilibrium? Maybe you should just put the old "worn in" bearings back in?