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2G Main Bearing Clearance (Doubt)

Posted by polverari, Sep 1, 2019

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  1. polverari

    polverari Proven Member

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    São Paulo, South America
    As I often do in my publications, I apologize first for any language errors.
    I did a search here in the forum and didn't find anything that specifically answered my question.
    I have found several articles commenting that the original clearance for crankshaft bearings is from 0.008 inches to 0.020 inches.
    Using plastigage, I identified that my slack is:

    .0010
    .0013
    .0014
    .0010
    .0013

    According to the manual its fine, but...
    I'm making a Stroker (2.3) with plans to reach 470-500 hp (on crank) // 400-450 WHP

    I would like to hear from you. If this clearance is ok or if, despite being consistent with the technical manual, is not enough for a project that wants to reach this horsepower.

    Thank you.
     
  2. bastarddsm

    bastarddsm Proven Member

    4,530
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    Joined Aug 26, 2003
    Mendota, Illinois
    It's a little tight for my liking. But, plastiguage is not infalliable. I just had a go-around like this. Dial bore gauges and micrometers showed that the plastiguage was wacked.
     

    Showcar Build 6K  1

    1991 Eagle Talon N/T
    9.805 @ 144.00 · 1G DSM
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  3. Speedsmith

    Speedsmith Probationary Member

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    Joined Jul 1, 2010
    Gregory, Michigan
    Agree with bastarddsm, plastiguage is not real accurate but if that's all you have, I'd be really concerned with #1 and #4, I run my 2.3 at .0025" @ a 600Hp level. For what it's worth an old rule of thumb for a modified engine is .001" clearance for every 1" of main diameter, since DSM mains are 2.24" that equates to .0022
     
  4. Speedsmith

    Speedsmith Probationary Member

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    Joined Jul 1, 2010
    Gregory, Michigan
    I bore gauge ( set with a calibration ring ) the main bores, cap torqued, no bearings, mic the crank ( 4 places per journal ), mic the bearings ( with the correct style mic ), do the math and mix and match the bearings to get as close to the clearance I want ( note: there is very little variation in bearing thickness, maybe ).00001". That said , I always ( just in case I was drinking ) use plastic gauge during final assembly. In my experience plastic gauge is accurate to hundredths (0.00") so if your observations are on the low side of 0.0015" I'd look into undersized 0.001" bearings for #1 & 4. Or if YOU decide to use the In/.001" rule go with all 0.001" undersized. You are shooting for + X2 times stock HP.

    Good Luck
     
    1990TSIAWDTALON and polverari like this.
  5. polverari

    polverari Proven Member

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    Joined Jan 29, 2018
    São Paulo, South America
    Thank you for this information. You said exactly what I was trying to understand: if there was any common sense (or preparation) rule for this question about clearances. Thank you
     
  6. polverari

    polverari Proven Member

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    Joined Jan 29, 2018
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    I think I understand what you mean and I will follow this tip of .0001
     
  7. polverari

    polverari Proven Member

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    Joined Jan 29, 2018
    São Paulo, South America
    I don't know the correct word but.. I will send the crankshaft for polishing to try to get bigger clearances and reach 0.0020''.
    Im using ACL bearings 0.001'' (0.025mm)
     
  8. Speedsmith

    Speedsmith Probationary Member

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    Joined Jul 1, 2010
    Gregory, Michigan
    You cannot polish that much material off a crank and keep it round. If you think of the process,a belt is pressed against a rotating main, it's not backed up and can flap somewhat, it is no where near as accurate as a grinder at a fixed position. If there is somewhere you can get it polished they may have the equipment ( bore gauge, mics ) to accurately measure the slack and once they do you have to find a crank grinder that will adjust each main individually according to those measurements to get the clearance you want.
     
    1990TSIAWDTALON likes this.
  9. Speedsmith

    Speedsmith Probationary Member

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    Gregory, Michigan
    Hopefully you meant ground not polished
     
  10. polverari

    polverari Proven Member

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    Joined Jan 29, 2018
    São Paulo, South America
    Solved: I change my 0.025mm ACL bearings for STD bearings with a friend.
     

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