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Thread: Broke a 6g72 Crank!

  1. #1

    Broke a 6g72 Crank!

    Last night I finally started to tear down the 3L V6 engine from my Spirit.

    Quick background, about a year ago I was driving home from work, turned a corner and the engine started knocking like it had a spun rod bearing... With a lot happening, I set the car aside until this past weekend when I pulled the engine...

    Before I pulled the engine I drained the oil and pulled the pan. The oil showed maybe mild signs of metal flakes, but little else... Almost second guessed myself, but decided I had "caught" it early enough that there wasn't a lot of damage.

    So I dove into the teardown expecting to find a spun bearing on a rod. Pulled the crank bearing cap assembly and everything look fine there. Started to spin the crank to pull the rods and pistons out when I noticed that the crank didn't always turn together...

    Closer inspection and I noticed that the crank was broken between piston 2 &3 bearings...

    Not sure how this happened, but I suspect the crank was damaged a while ago since when the knock happened throttle was mild... A little history on the crank. This is the same crank that I spun two rods on years ago (6-10 years ago?) during some drag racing... The crank journals were ground, oversized bearings were installed. One rod was resized and the other was replaced.

    Since then the car has run two Drag Weeks and much daily driving... I will also note that the rod bearings showed a LOT of wear...

    Anyway, anticipating a spun rod bearing I had ordered a reman'd crank and a reconditioned rod... I won't need the rod...

  2. #2
    turbo addict
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    Re: Broke a 6g72 Crank!

    I would check the line bore of the crank main bores before you slap another crank in , Have seen a out of line mains snap cranks before .

  3. #3
    Moderator Turbo Mopar Staff Vigo's Avatar
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    Re: Broke a 6g72 Crank!

    Interesting! I seem to remember Brent bent a crank by detonating on it, but i dont remember hearing of a snapped one. I'm just glad to hear you are working on that car!

    Dont push the red button.You hear me?

  4. #4

    Re: Broke a 6g72 Crank!

    With a little luck my wife will be racing it at Rocky Mountain Race Week this year!

  5. #5
    Supporting Member Turbo Mopar Contributor Ondonti's Avatar
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    Re: Broke a 6g72 Crank!

    Quote Originally Posted by Vigo View Post
    Interesting! I seem to remember Brent bent a crank by detonating on it, but i dont remember hearing of a snapped one. I'm just glad to hear you are working on that car!
    The crankshaft probably bend from coolant backwashing into the cylinder, hard to tell. I do know it keeps getting more and more bent once it bends.

    I would bet Ed just had a bad spot in the casting that we have always heard about. This one obviously wasn't serious enough to prevent the crank from taking some abuse. Any crack on a cast part will also keep propagating till failure. Warranty/reputation wise it was probably smart of Mitsu to go forged on more highly stresed setups. I take the risk still. Maybe all of us will eventually kill our boosted cast crankshafts. I collected few spares after I lost one. The crank in my 10:1 duster is still "bad" on one journal after spinning and not fixing anything properly (out of spec after polishing).

    Lets see pictures!!!
    Brent
    GREAT DEPRESSION RACING
    1992 Duster 3.0 The Junkyard- Now - Megasquirt II/Extra. OEM 10:1 3.0 + funny business
    - Old - 11.5@125 22psi $90 Stock 3.0 Junk Motor - 516whp 519wtq 20psi 91 oct built 3.0 stock n/a computer
    1994 Spirit 3.0 - 12.3@113 15 psi - Daily Holset He341 280whp 304ft/lb @10psi
    1994 Spirit 3.0 a670 - Wife's - Disassembled
    1990 Spirit 3.0 E.S. 41TE
    1993 Spirit 3.0 E.S. 41TE
    1994 Duster 3.0 A543
    1990 Lebaron Coupe 2.2 TII non IC, T1 31TH
    1981 Starlet KP61 Potential driver, 1981 Starlet KP61 Parts, 1983 Starlet KP61 Drag
    1998 Dodge 12v Cummins 47re 4x4 4dr longbed. No fuel plate. Mack plug, AFC & Spring mods. Boost elbow.

  6. #6

    Re: Broke a 6g72 Crank!

    As requested, pictures of the broken crank.






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    Last edited by c2xejk; 06-09-2016 at 06:47 AM.

  7. #7
    Moderator Turbo Mopar Staff Vigo's Avatar
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    Re: Broke a 6g72 Crank!

    Pics are broken. I'm very curious!

    Dont push the red button.You hear me?

  8. #8

    Re: Broke a 6g72 Crank!

    Quote Originally Posted by Vigo View Post
    Pics are broken. I'm very curious!
    Ok, I think I have fixed them...

  9. #9
    Garrett booster
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    Re: Broke a 6g72 Crank!

    Quote Originally Posted by c2xejk View Post
    Ok, I think I have fixed them...
    I see the pics. Whether the failure was caused by over stress or a bad casting I cannot say. What about the oil hole? Maybe you had one that was a little too close to the edge. Maybe a combination of both. Crazy s*** nonetheless and thanks for posting the pics!

  10. #10
    Supporting Member Turbo Mopar Contributor GLHS60's Avatar
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    Re: Broke a 6g72 Crank!

    I thought this was going to be a James Reeves type post!!

    Sorry for your strange problem.

    Thanks
    Randy


    Randy Hicks
    86 GLHS60
    86 GLHS 373 : SOLD, but never forgotten
    89 Turbo Minivan
    83 Turbo Rampage : SOLD
    Edmonton,Alberta,Canada

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