Technical Rod Bearing Clearance....what's too lose?

IQraceworks

Corporal
Jul 7, 2020
210
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Missouri
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07' BMW 335i
I'm in the process of putting my N54 together with my new Mahle pistons and Manley H-Beam forged rods. I'm to the point where I'm measuring all of my bearing clearances ......and with my new King standard sized bearing, I'm getting .0025"-.0027" rod bearing clearance. From everything I've found online....you want something between .0015" and .0020". But, the paperwork I got with the Manley connecting rods says something to the affect of "we size the big end bores on our rods for a .002"-.003" rod bearing clearance.

So who's right? Is .0025"-.0027" too lose.....or just right (according to Manley)?

Should I get some +.001 rod bearings to tighten things up, or should I just run it?

Any feedback would be great. Thanks!
 

Bnks334

Lieutenant
Dec 1, 2016
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New York
Sounds odd to me. If you want more clearance, you run an "x" bearing you don't modify the rod. Bearings are designed to be larger than the rod bore. The torque "crushes" the bearing which is what holds it in place and makes it take it's shape. can you link to where they say they modify the rod for more clearance? Seems like something you would really not want to do but not sure how significant of an impact there is on the resulting crush.

The clearance itself would be fine. That's right where you would've ended up with a set of "x" bearings designed for .001" more clearance.
 

IQraceworks

Corporal
Jul 7, 2020
210
121
0
Missouri
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07' BMW 335i
After talking to the tech at Manley.....he pretty much told me that's how they make them, and the .0025-.0027 rod bearing clearance is just fine...even though the same bearings in the stock rods give me a .0018-.0020 clearance. I was doing some research online last night, and I found a few guys saying that they had some Molnar rods that were the same way...slightly larger clearances with stock bearings vs. stock rods. They said that when they called Molnar, they were told the same thing.......the big end of the rod is machine to not crush the bearings as much to add an extra .0005-.001 clearance.

At this point...I guess I'm going to run them.
 
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IQraceworks

Corporal
Jul 7, 2020
210
121
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Missouri
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07' BMW 335i
Here is the paperwork from Manley. The basically say that they design the rods to get a .002-.003 bearing clearance....with standard bearings. The same bearings that should give you a .0015-.0020 clearance in oem rods.

I guess if I'm using Manley Rods.....and they say use .002-.003 for the rod bearing clearance, and not the OEM BMW spec.....that's what I'll do.


MX8jnIE.jpg
 
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desmo11

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Jan 10, 2024
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You want to measure bearing crush. This is the difference between basically the OD of the bearings to the ID of the rods. Try torquing down only one rod bolt with bearings in the rod (out of engine btw) and leave the other bolt out. You should have say ~0.012" clearance between the rod and the cap. I would be VERY concerned if the same bearings in one rod give ~0.0015" clearance, and in another they gave even 0.002"....this suggests that the crush/or interference fit of the rod to the bearings is insufficient. This fit pressure is important as that is what keeps the bearing from spinning in the rod big end. General guideline would be say 12-16MPa contact pressure at this interface.
 

IQraceworks

Corporal
Jul 7, 2020
210
121
0
Missouri
Ride
07' BMW 335i
You want to measure bearing crush. This is the difference between basically the OD of the bearings to the ID of the rods. Try torquing down only one rod bolt with bearings in the rod (out of engine btw) and leave the other bolt out. You should have say ~0.012" clearance between the rod and the cap. I would be VERY concerned if the same bearings in one rod give ~0.0015" clearance, and in another they gave even 0.002"....this suggests that the crush/or interference fit of the rod to the bearings is insufficient. This fit pressure is important as that is what keeps the bearing from spinning in the rod big end. General guideline would be say 12-16MPa contact pressure at this interface.
Thanks for the great info, I will take some measurements.

As far as the bearing clearance goes, am I ok to run .0025-.0027, or is that too lose? Manley says it's just right.
 

Bnks334

Lieutenant
Dec 1, 2016
528
341
0
New York
I built an engine using Molnar rods and ended up with stock-like clearances. That was years ago at this point, though...
 

Bnks334

Lieutenant
Dec 1, 2016
528
341
0
New York
Here is the paperwork from Manley. The basically say that they design the rods to get a .002-.003 bearing clearance....with standard bearings. The same bearings that should give you a .0015-.0020 clearance in oem rods.

I guess if I'm using Manley Rods.....and they say use .002-.003 for the rod bearing clearance, and not the OEM BMW spec.....that's what I'll do.


View attachment 90353
That does not say that they machine the rod to get .002-.003"... that just says that that is the recommended clearance upon final assembly. You would use different bearings, or machine the crank, to achieve that clearance range. You wouldn't do anything to the rod.

That looks like some very generic instructions as well... rod bearing clearance scales with the crank journal diameter and most other manufacturers give a chart so you can calculate the range for your particular crank. Again, where you landed should be fine though.