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Engine Bearings – More than Just a Curved Piece of Metal: Part Two

It’s Doc again,

I’ve returned with the second installment about engine bearing basics. Today I’m going to discuss what the different markings mean on a bearing as well as the causes of bearing failure. Before we get started, I’d like to remind you if you are interested in learning more about bearings go to our eLearning class. Also, if you have any bearing related questions don’t hesitate to ask…think of me as your engine bearing guru!

Bearing Markings

The backs of most bearings have numerous markings and there are a range of things that could be on the shell.

Below are just a few examples (See photos for a better understanding):

  • Production Date (Month/Year)- Example would be 8-10 or 8/10
  • Material Used – Example would be AS
  • Shift – Example would be B-11 or A-10
  • Part Number – Example would be CB663P
  • Position – Example would be Upper or Lower
  • Size – Example would be Std. or 0.25mm

Typical Bearing Markings

More Bearing Markings

Any other stray numbers like 1123221x, etc. are lot and manufacturing codes which are meaningless when it comes to identifying the bearing.

Two things to note: Main bearings will have an MB prefix on the shell, but are mostly sold only in sets with a MS prefix. Go to askmahleclevite.com if you need further help in identifying your bearings.

Finally, many people ask Clevite to identify the size of the bearings, which can be very difficult. The size may or may not be stamped on the bearing and size markings which can be done by color coding isn’t always visible. This is why you should always measure your crankshaft to determine the correct size bearings. Bearing clearance is essential to proper installation and it is much too important to leave size to a guess my friends!

Bearing Failures

Bearing failures do happen… and often, we look for someone or something to blame for the failure. MAHLE Clevite offers a web-based bearing failure guide with 22 different causes of bearing failures. The guide is complete with digital photos, causes, prevention, and remedial actions. I promise you won’t find anything like it, and the best part is there’s no charge to use it. Why, isn’t that nice of MAHLE Clevite?

To explain failures briefly – 40% of engine bearing failures are caused by dirt and the main sources of dirt are #1 – dirt left in during the overhaul #2 – airborne abrasives and #3 – contaminated lubricants.  Myself and the engineers at Clevite can’t stress enough how important it is to clean your crank, rods and pistons before assembly. Scrub oil galleries with a bristle brush, wash parts with hot, soapy water and clean, clean, clean!

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9 Responses to Engine Bearings – More than Just a Curved Piece of Metal: Part Two

  1. Seth says:

    Hey Doc,
    Great addition to article 1! As my automotive books also say, dirt is a major killer of engine parts and just a little bit can ruin your engine. Who knows how many folks have done a great job rebuilding an engine with good parts, proper torque settings, etc only to have it fail because of dirt.

  2. Seth says:

    Hey Doc, Seth here again. Are the connecting rods the most stessed parts of the engine? I’ve read somewhere that the valve train takes the most abuse, but I wasn’t sure. Pistons get thousands of pounds of pressure on them during the power strokes, but the rods have to take all that force over a smaller (and therefore more concentrated) area. Also, in terms of the most stressed parts, is there any difference between standard and high performance engines? Thanks and keep the articles coming!

    • Hi Seth,

      Well I hope you enjoyed your holidays and had a Happy New Year!

      To answer your question: It is hard to make a statement that one part of the engine is the most stressed. The most stressed part of an engine can change depending on engine design and use. For example, valve train design and loads are similar when comparing Inline engines to V engines but Inline engines do not have the cross loads on the crankshaft like a V engine does.

      There are a lot of differences between standard and performance engines. A standard passenger car engine has much less compression and turns at a lower RPM when compared to a race engine with higher compression and RPMs. The valve train and “bottom end” or rotating assembly (Block, Crankshaft, Connecting Rods, and Pistons) of performance engines are made of stronger materials than production engines and sometimes have different designs or manufacturing processes (Forging vs Casting) to withstand the abuse and higher RPMs.

      Let me know if I can be of further assistance and please keep the questions coming!


  3. All parts in the engine of a are machined to within very tight specifications so that there is a minimum of friction.

  4. Seth says:

    Hi Doc, Seth here. Reading through the engine part failure analysis guide and have a couple questions: Can “diesel knock” (too high a cetane value) be just as damaging as spark knock in gasoline engines? I’ve read that diesels detonate normally due to secondary combustion processes happening as the fuel/air mixture slowly ignites. This produces the typical rattle a diesel engine makes. Also, if all the fuel/air mixture goes bang at once, does it produce a chattering sound like detonation or a metallic knock (like rod noise)? Thanks and look forward to any insight you have.

    • Hello Seth,

      Happy Friday the 13th! You are correct my friend! Diesel knock is a side effect of the raised compression and fuel injection/delivery process, and is an acceptable result of the ignition sequence. However as you stated, a fuel higher in cetane value will aid in knock and fuel too low in cetane may not allow your engine to run.

      As for your second question, if I’m understanding correctly you are asking what produces the typical rattle most diesel engines create?

      You and I both know diesel engines make a lot of sounds, but if we are thinking of the same sound, it is caused by the fuel injection process. The fuel delivery system is injecting raw fuel into hot compressed air and the fuel is ignited as the piston is traveling up in the cylinder causing detonation and that famous diesel knock/rattling sound.

      - Doc

  5. Seth says:

    Hey Doc, happy (late) Friday the 13th to you too! Thanks for your insight into acceptable knock for diesels. What I was wondering is this: Is the bad diesel knock, aka “ignition lag”, (caused by the wrong cetane value) as damaging as detonation in a gas engine?

    I’ve read that if you experience the “bad” knock, you’ll need to shut down the engine quickly, flush the fuel system and refill it with the correct blend.


    • Hi Seth,

      To answer your first question: It could be, but it could also be from a bad injector. It’s best to purge your fuel system and let it run for 10-15 minutes. If the abnormal sound still emanates it could be a more serious issue.

      As for your second question: Oh my my…this reminds of the time I had a customer contact me because he accidentally filled up his Duramax with E85! It was quite funny…but not to him of course. If you have isolated your issue to bad diesel, it would be recommended to change your fuel filter, purge your fuel system and then add the appropriate grade of diesel to your vehicle.


  6. Seth says:

    Hi Doc,
    E85 in a diesel would be a disaster for sure. I’ve always gone by the notion that diesel put into a gasoline engine isn’t as bad as gasoline put into a diesel engine. Diesel in a gasoline engine will make a sticky mess and cause the engine to stop running, but I doubt would it cause physical damage since the low compression ratio of the gasoline engine wouldn’t be enough to ignite the diesel fuel. I’m sure you’ve seen customers who have put diesel into gasoline engines as well.
    Anyway, thanks for your input to my last question and for the recent tech tip on cleaning engine parts before assembly. Keep the articles coming!


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