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Technical Quick piston question;

Discussion in 'Traditional Hot Rods' started by L79_Acadian, Feb 18, 2014.

  1. L79_Acadian
    Joined: Oct 3, 2006
    Posts: 84

    from canada

    Hi everyone;
    I have a old 392 hemi, that I bought a while ago. The motor was apart, and I never measured the cylinder bore, prior to purchase. Well the motor is already .060” over, and really could use a overbore to .070”. I plan on running a 6-71 on gas, on the street. I would like to know if blower pistons for the 392 that are .070 over, are sold and if so, who sells them and what is the cost on a set of these? I am not sure about a 070 overbore, but I'm not sure about using the pistons that I have with .0095" piston to wall clearance. I have a set of old Jahns pistons that are about 9.5 or 10:1 compression. I also thought of cutting about .125" of the tops of these pistons, but I don't really like that piston clearance. Any ideas, I've never had a Chrysler product before. The motor is going into a 33 Willys, straight axle gasser.
    Thank you,
    Roger Hornby
  2. treb11
    Joined: Jan 21, 2006
    Posts: 3,302


    If you have the $$$ somebody will make you a piston. .070 is a very odd OS.
  3. ROADSTER1927
    Joined: Feb 14, 2009
    Posts: 2,632


    If it is worn at .060 and you want tight piston clearance, .070 probably will not make it.
  4. seb fontana
    Joined: Sep 1, 2005
    Posts: 6,089

    seb fontana
    from ct

    Is the piston clearence because the cylinders are worn [ridge at top of cylinders] or the pistons are an old forged type that required the clearence?
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  5. greybeard360
    Joined: Feb 28, 2008
    Posts: 1,392


    Check the bores for roundness and taper. Some engine builders ran them "loose" like that and .008-.010 clearance isn't unheard of.

    The biggest overbore out of the box I could find were .060, but like mentioned, some of the piston companies will make custom pistons for you.... just make sure the wallet is full when you talk to them.
  6. L79_Acadian
    Joined: Oct 3, 2006
    Posts: 84

    from canada

    Ok let me explain more. The pistons that came with the motor are Jahns forged pistons. The problem is the compression ratio they make. Its to high for a blown application on the street on gas. So i started to look for pistons. After i measured the block it comes out at about 9 to 9 1/2 thou per bore. I measured the piston tops and should be able to cut 100thou off the top thus reducing the compression ratio. But i dont really like that loose a motor for a street car. Also my pockets arnt very deep either. Im at work ,more to follow next break.

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