Piston to cylinder clearance?

Small Block Mopar Engine

  1. Cope

    Cope Fusing with fire

    Messages:
    3,529
    Likes Received:
    3919
    Joined:
    Oct 20, 2009
    Location:
    San Jose, Ca.
    Local Time:
    10:12 PM
    A good motor builder buddy told me my pistons are to tight.
    We pulled one, pulled its rings off and slid it up and down the bore upside down. its tight.

    If we go with the 1" to .100 clearance, I should have .429 difference between my piston and wall. with .500runout on the crank we know the pistions can move .500 in the bore (lateral) so would i not want at least that much pistion to wall clearance?

    I get dam near the same read on my pistion and wall numbers. Pistion 4.292 Bore 4.352
    .100-.200 with a cappy mic.

    what do you folks look for in pistion to wall clearance in a race motor?

    Thanks again for the time and advice.

    Merry Christmas!
     
  2. AJ/FormS

    AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s FABO Gold Member

    Messages:
    16,303
    Likes Received:
    5068
    Joined:
    Jan 19, 2014
    Location:
    South-Central Manitoba,Canada, 900ftelevation
    Local Time:
    10:12 PM
    Firstly, um, lose the buddy.
    Secondly, proofread/edit your post before you send it.
    Thirdly,refer to the manufacturer's recommendations, and treat them as the bible.
    Fourthly, pistons and bores need to be measured under strict conditions and with non-"crappy" tools.

    Ok having said that, skirt clearances on a 4 inch piston can be from about .0015 to .007 depending on the materials used, and the intended usage.
    KB hypers for instance; are .0015 to .0045,between street and supercharged.Forged will have different specs, as will cast pistons.

    Merry Christmas back to you, and yours
     
  3. Cope

    Cope Fusing with fire

    Messages:
    3,529
    Likes Received:
    3919
    Joined:
    Oct 20, 2009
    Location:
    San Jose, Ca.
    Local Time:
    10:12 PM
    This is a non factory build so im un sure what specs I need and how loose i can make the rotating assembly before its to loose.

    Thanks again for the time and advice.
     
  4. AJ/FormS

    AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s FABO Gold Member

    Messages:
    16,303
    Likes Received:
    5068
    Joined:
    Jan 19, 2014
    Location:
    South-Central Manitoba,Canada, 900ftelevation
    Local Time:
    10:12 PM
    Loose may not be what you want.
    The clearance is there to compensate for piston growth as the piston comes up to the operating temperature to which it will be subjected to. For performance you want them as tight as possible without scuffing the skirts under the duress of racing. The fully expanded piston will keep the rings square and perpendicular to the cylinder walls.If you cannot figure out whose pistons you have or the type of material they are made of, then you cannot choose a clearance with any guarantee of success, unless success means loose and possibly slow.
    Again Hypers are so stable that a clearance of .0015 to .0045 covers most applications.
    Perhaps your buddy knows more than I was able to ascertain from your first post. If those pistons are forged, and you are going supercharged or Nos, then .001 to .002 will be too tight.
     
  5. RustyRatRod

    RustyRatRod 30 Degrees Outta Whack FABO Gold Member

    Messages:
    53,024
    Likes Received:
    20890
    Joined:
    Jun 7, 2010
    Location:
    Georgia
    Local Time:
    11:12 PM
    Long as they ain't swappin holes, it's all good.
     
  6. Cope

    Cope Fusing with fire

    Messages:
    3,529
    Likes Received:
    3919
    Joined:
    Oct 20, 2009
    Location:
    San Jose, Ca.
    Local Time:
    10:12 PM
    rotating assembly is to tight.

    I want this motor to spin fast and lose.
     
  7. chryslerfat

    chryslerfat Well-Known Member

    Messages:
    11,750
    Likes Received:
    3187
    Joined:
    Aug 23, 2008
    Location:
    Monroe,Michigan
    Local Time:
    11:12 PM
    The last race motor I built was around 7 thou. She revved fast but did use some oil.
     
  8. Fullbore71

    Fullbore71 Well-Known Member

    Messages:
    84
    Likes Received:
    12
    Joined:
    Mar 1, 2015
    Location:
    upstate ny
    Local Time:
    11:12 PM
    The 340 street/strip engine in my car now has 6 thou. clearance and runs fine. Set it up that way for NOS. No issues.
     
  9. moper

    moper FABO Gold Member FABO Gold Member

    Messages:
    15,603
    Likes Received:
    1701
    Joined:
    Dec 27, 2004
    Location:
    eastern CT
    Local Time:
    4:12 AM
    You need to tell us what pistons you have (manufacturer and type, preferably part numbers), and what the rest of the build is. The dimensions you give are all wrong - not sure if they are because of your typing, your measurements, or ignorance. But they're all wrong. .001" is probably too tight. .005" could be fine, or way too loose.
     
  10. RustyRatRod

    RustyRatRod 30 Degrees Outta Whack FABO Gold Member

    Messages:
    53,024
    Likes Received:
    20890
    Joined:
    Jun 7, 2010
    Location:
    Georgia
    Local Time:
    11:12 PM
    Ignorance? Well aren't you just a ray of sunshine? lol
     
  11. krazykuda

    krazykuda Well-Known Member FABO Gold Member How-To Section Editor

    Messages:
    48,350
    Likes Received:
    13358
    Joined:
    Aug 20, 2007
    Location:
    Orland Park, IL
    Local Time:
    10:12 PM

    That's what she said... #-o


    LOL! :D
     
  12. Cope

    Cope Fusing with fire

    Messages:
    3,529
    Likes Received:
    3919
    Joined:
    Oct 20, 2009
    Location:
    San Jose, Ca.
    Local Time:
    10:12 PM
    The pistions are used, Im not sure who makes em.
    I looked on summit to see if i could find them with no luck.

    All i can say for sure is they are not forged.

    my motor building buddy is good, verry good. The motors out of his shop sell in the five figuer price range. (Ive seen him anwser his cell phone and say "Hay Don" like its no thing. Thats big daddy Garlits to the rest of us meer mortals..)

    I cannot afford to pay him what hes worth, He will come by our shop sometimes and look at our cars, hang out and offer his advice.

    He wants to see eight thousands clearance.

    I will take some.carefull picks nextime im at the shop. May hap after work tomarrow if im not to tired.

    Thanks again for all your time and advice.
     
  1. This site uses cookies to help personalize content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.