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Connecting rod bearings.. what do I need to know?


Andre1150

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I can't seem to find any threads discussing this topic, so perhaps this would be good information..

 

How often do connecting rod bearings go out? 20K, 50k, 100k miles? If ever?

 

What are the symptoms? noise all the way through acceleration? at a specific speed? under load? not under load?

 

What are the differences in bearing size = red/blue/ + - .25? etc?

 

Tips on changing the bearings -- can it be done on bike? any special tools??

 

thanks!

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Morning FireKit]I can't seem to find any threads discussing this topic, so perhaps this would be good information..

 

How often do connecting rod bearings go out? 20K, 50k, 100k miles? If ever?---Depends on: lots of variables on this. Run a few miles with no oil in engine & they are junk. As a rule the BMW boxer doesn't have a rod bearing wear issue. More likely to have crankshaft rod journal wear (they wear egg shaped)-- It is difficult to mike the wear but possible. Even Plasti-Gauge can lie as the wear is not at BDC or at TDC so re-torqueing the rod cap at wear point is difficult.

 

What are the symptoms? noise all the way through acceleration? at a specific speed? under load? not under load?---Again difficult to give an exact symptom as noise in the BMW boxer telegraphs to other parts of the engine. Engine temperature & oil thickness also effect rod bearing noise. If they get bad enough you will usually hear something on light throttle up & small throttle-up changes at low speeds high loads. Much more likely to have rear main bearing noise & that is easily confused with rod bearing noise.

 

What are the differences in bearing size = red/blue/ + - .25? etc?---From parts book--BEARING SHELL, COVER SIDE, RED std---BEARING SHELL, ROD SIDE, BLUE std ---BEARING SHELL, COVER SIDE, RED-+0.25mm---BEARING SHELL, ROD SIDE, BLUE - +0.25mm.

 

Tips on changing the bearings -- can it be done on bike? any special tools??-- No real special tools & can be done with engine still in bike. Just make certain that you are dealing with crankshaft journals that are ROUND (not worn egg shaped). Protect the crankshaft journals with bolt protectors so you don't nick crank journals when sliding rods into place.

 

Again ,much more likely to have a rear main bearing worn in the lateral direction.

 

 

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ah, so never having torn down an engine (yet.) are there color codings on the crankshaft or piston shaft that indicate the color?

 

I found a thread ( here ) that shows various colors. However, do you determine the bearing to use by the gap that shows up using plastigage or a micrometer?

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ah, so never having torn down an engine (yet.) are there color codings on the crankshaft or piston shaft that indicate the color?

 

I found a thread ( here ) that shows various colors. However, do you determine the bearing to use by the gap that shows up using plastigage or a micrometer?

 

Afternoon FireKit

 

YES, both if possible but a micrometer is the safest approach.

 

You first need to mike the crankshaft rod journals in a few different places around the journal-- then when the largest OD & smallest OD are found you then also mike across the journals to determine journal taper.

 

You MUST fit the new rod bearing shells to the TIGHTEST place (largest OD) on the crank journals.

 

Boxer engines are notorious for wearing the crankshaft connecting rod journals in an egg shape (out-of-round) so if you don't fit the new bearings to the tight spot you risk burning up a bearing before you go 100 miles.

 

Once you find & identify the tight spot (largest OD) on each crank journal you can plasti-gage in that spot (if you can access the rod nuts in that position). You can NEVER turn the crankshaft with plasti-gage in place or the readings are useless.

 

On the BMW boxer, with an experienced eye, most times you can tell if have loose or worn rod bearing by just looking at the outer layer of babbit that was riding on the crank journal.

 

Obviously if you have a worn out-of-round crank journal or heavily tapered crank journal then fitting new bearing shells is futile.

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Firekit,

 

Do you have an actual issue with your bearings, crankshaft, or are you just assuming that because of high mileage you need to address something?

 

Because 80k on a well maintained Boxer (regular oil changes with filter and name brand oil) is really minimal mileage on that engine. And you should expect no issues.

 

Unless you have time and money to spare, tearing into a good running Boxer based on the assumption that something must be wrong due to high mileage is not the best use of either.

 

IMHO of course.

 

 

RPG

1974 R90s 135k with original crank bearings

2004 R1150RT 72k with original crank bearings. (Don't plan on touching the internals on this motor for a long, long time)

RPG

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That's s good question. Actually, I was told by a non-beemer specific motorcycle repair shop that my bearings were going out, that they could hear noise at 3800 rpms, increased vibration, and that it indicated worn bearings. To me, the bike just vibrates the way boxers do over 4000rpms, and I wanted to see what I might be getting into if I looked into it. I didn't want my question to be the focus of the thread, more so the information that might help anyone facing a teardown.

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