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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > N55 Turbo Engine Tuning and Exhaust Modifications - 335i Tuning > Rod Bearing deviation of +/- 5 "g", What is "g"??



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      10-21-2019, 08:42 PM   #1
JonEQuest
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Rod Bearing deviation of +/- 5 "g", What is "g"??

In the TIS info manual, they list the connecting rod tolerances (without bearing) as 53,600 ... 53,616 as "mm" Millimeters. Which I get, but then the "Permissible total deviation of connecting rods in one engine (without bearing shells" is +/- 5 with "g" as the unit of measurement. What is a "g"???
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      10-21-2019, 09:33 PM   #2
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probably rate of change of velocity, aka gravitational force; G Force.
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      10-21-2019, 10:38 PM   #3
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Quote:
Originally Posted by Delta0311 View Post
probably rate of change of velocity, aka gravitational force; G Force.
Yeah, that is the only thing I could think of. Not sure how I would measure that. I just wanted to know if the extra rod I got to replace a bad one is "close enough" to be used without problems. This is from another post I made about the rod sizes.

I already got a replacement rod from a friend but it is just a little out of round for my liking. the sizes I got from measuring the others at 3 places were typically:

Rod #1 was 53.467, 53.441, 53.594 mm
Rod #2 was 53.467, 53.518, 53.569 mm
Rod #3 was 55.885, 53.213, 53.340 mm (this one is obviously bad)
Rod #4 was 53.619, 53.594, 53.594 mm
Rod #5 was 53.518, 53.467, 53.594 mm
Rod #6 was 53.594, 53.341, 53.314 mm

Then the one my friend gave me measured: 53.797, 53.441, 53.645 mm
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      10-21-2019, 10:54 PM   #4
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I thought they came in matched weight sets? How does the weight compare to the other 5?
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      10-21-2019, 11:36 PM   #5
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I don't have a scale but I don't think weight is as critical as people think as long as you are using identical rods and pistons. I recently rebuilt my N54 in another car and swapped in a whole other used piston and rod I bought off eBay to replace my bent #3 rod. I have full bolt-on's and I run the car hard with no problems. I can see that being an issue if you swap in one different variation of an engine that may have a slightly different piston/rod weight. My issue right now is the big end size and diameter on the N55 I am rebuilding now.
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      10-22-2019, 09:46 AM   #6
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Quote:
Originally Posted by JonEQuest View Post
I don't have a scale but I don't think weight is as critical as people think as long as you are using identical rods and pistons. I recently rebuilt my N54 in another car and swapped in a whole other used piston and rod I bought off eBay to replace my bent #3 rod. I have full bolt-on's and I run the car hard with no problems. I can see that being an issue if you swap in one different variation of an engine that may have a slightly different piston/rod weight. My issue right now is the big end size and diameter on the N55 I am rebuilding now.
I posted in your other thread... I am sure it will run just fine, but, there is a science to it. manufacturing tolerances, especially on performance motors, are kept as small as possible. More weight variation = less lifespan at high rpm. More quickly failing DMFW and harmonic damper etc. Too much variation and the engine will tear itself apart. I'm sure you'll be fine with another stock rod. This isn't the 60's anymore. Max deviation might be 5g but from the factory things are probably balanced closer to 1g. Much less deviation in reality than the actual "max deviations" listed on TIS.

It's just important to understand all these things when slapping back together a blown up engine lol.
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      10-22-2019, 12:49 PM   #7
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I definitely understand how important balance and clearances are to something that has hundreds of parts flying around at several thousand RPM. :-)
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