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Technical Two different sized pistons in a running Flathead. WTF??

Discussion in 'The Hokey Ass Message Board' started by Austin kays, Aug 23, 2020.

  1. Baumi
    Joined: Jan 28, 2003
    Posts: 3,043

    Baumi
    ALLIANCE MEMBER

    I helped rebuild a buddy´s Pontiac 389, tearing it down I noticed three different kinds of pistons, while little pieces of compression rings and oil rings were laying in the pan and around the lifters... It also had three different bore sizes. Std. , 030 and 040 and all of them were so worn that the pistons were slapping around in their bores like they were in the wrong engine.... We finally sleeved it back to std size, because we couldn´t find a better block.
    Edit: The engine actually ran smooth and he had driven it for years, he never change oild because it burnt so much , he just kept refilling... We tore it down because he had oil running out the tailpipes after his last roadtrip...
     
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  2. manyolcars
    Joined: Mar 30, 2001
    Posts: 9,174

    manyolcars

    The first time my 1946 Harley was rebuilt, the front was .010 and .040 on the rear. I ran it many years
     
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  3. Just use a shorter ruler to measure the 2 smaller bores. :p
     
  4. lippy
    Joined: Sep 27, 2006
    Posts: 6,825

    lippy
    Member
    from Ks

    Had a 1950 HD panhead. Front cyl .030, rear .060. Ran great. Lippy
     
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  5. Blues4U
    Joined: Oct 1, 2015
    Posts: 7,589

    Blues4U
    Member
    from So Cal

    Like it's gonna vibrate any more? LOL!
     
  6. scrappybunch
    Joined: Nov 16, 2011
    Posts: 412

    scrappybunch
    Member
    from nj

    I worked at a chevy store, had a 454 truck with a rod knock and 1 bore scored. Regional service manager had a hard on about excesive warranty repair. He had us send it out and bored 1 cylinder. 040 and replace the crankshaft. To everyone's amazement it ran fine, no vibration and left to never return.
     
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  7. oldsman41
    Joined: Jun 25, 2010
    Posts: 1,556

    oldsman41
    Member

    If it ain’t broke don’t fix it. If the engine doesn’t care neither should you. Guys did whatever it took to keep them running back in the day
     
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  8. Hotrodmyk
    Joined: Jan 7, 2011
    Posts: 2,302

    Hotrodmyk
    ALLIANCE MEMBER
    1. Northwest HAMBers

    Meaning no offense but that's called a "farm fix". We disassembled a running Model A engine a while back that had 3 different size pistons and one looked like it was from some kind of compressor or something. It didn't run any rougher than others. Love 'em old Fords.
     
    Last edited: Aug 24, 2020
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  9. brokedownbiker
    Joined: Jun 7, 2016
    Posts: 651

    brokedownbiker
    ALLIANCE MEMBER

    Not enough to notice as long as the pistons were same compression. I did really notice it on a 80" Shovelhead that had one low compression piston and one high-compression piston- that motor rattled and shook a lot more than normal, but they were both the same oversize.
     
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  10. speedshifter
    Joined: Mar 3, 2008
    Posts: 312

    speedshifter
    Member

    I had the heads off about a 62 Pontiac v8 for a warranty top end repair. On one bank the pistons were dished, on the other bank they were flat. Shop foreman said just do the top end work & don't worry about it. It ran just fine. Hmm! Do you suppose it needed a half & half mixture of regular & ethyl gas??? Greg
     
  11. bill gruendeman
    Joined: Jun 18, 2019
    Posts: 807

    bill gruendeman
    Member

    Jim, are you saying size doesn’t matter?lol
     
  12. dan c
    Joined: Jan 30, 2012
    Posts: 2,520

    dan c
    ALLIANCE MEMBER

    legend has it that allstate engines could have 8 different-sized pistons!
     
  13. cfmvw
    Joined: Aug 24, 2015
    Posts: 977

    cfmvw
    Member

    I remember reading a car feature where some guy built an Oldsmobile V8 out of what he had laying around. He used two different heads on it (amongst other various mismatched parts) and put it in a ratty four door Nova. It ran surprisingly well; I think he was getting 11 second quarter mile times out of it if I remember right.
     
  14. treb11
    Joined: Jan 21, 2006
    Posts: 3,947

    treb11
    ALLIANCE MEMBER

  15. If the pistons are in balance, eg weigh the same it'll work OK
     

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