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Are Buick pistons round or elliptical?

Discussion in 'The Bench' started by Yardley, Dec 6, 2003.

  1. Yardley

    Yardley Club Jackass

    I just watched My Classic Car and Dennis and Sam took a trip to Egge Machine and showed how they cast new pistons from original OEM molds. The 392 Hemi piston they showed was ground to an elliptical shape.

    Are all pistons elliptical? Are our 400/430/455 elliptical? They showed a Pierce- Arrow piston that was VERY elliptical, and it surprised me.

    :Do No:
     
    Yardley, Dec 6, 2003
    #1
  2. skylark

    skylark Well-Known Member

    Round, depending on the quality of casting. It is nearly impossible to cast anything perfectly round, but they will be close enough that you can't see with the naked eye. As far as I know elliptical cylinder bores are old technology.:)
     
    skylark, Dec 6, 2003
    #2
  3. Staged70Lark

    Staged70Lark Well-Known Member

    Yardley,

    I know that pistons have a smaller diameter at top than at the bottom.

    Example: My JE 4.350 bore piston

    4.310 at top just above the top ring
    4.345 measure 1/2 up from the bottom on the skirt of the piston.


    Take Care
     
    Staged70Lark, Dec 6, 2003
    #3
  4. GSXMEN

    GSXMEN Got Jesus?

    Jeff - I had always heard that they were slight eliptical....done so to make them as round as possible once fully up to temperature.:Do No:
     
    GSXMEN, Dec 6, 2003
    #4
  5. 73Regal455

    73Regal455 Well-Known Member

    I just looked in my BUICK Chassis Service Manual (1972) and it states that the pistons are cam ground which means they are ground to a slightly eliptical shape.:Smarty:I suspect they are ground that way to help fit more uniformly when heated to operating temperature. Just my humble thoughts. :)

    Ed :beer


    __________________________________________________

    If you can't fly with Eagles, stay in the hangar.
     
    73Regal455, Dec 6, 2003
    #5
  6. Yardley

    Yardley Club Jackass

    Huh.

    Interesting......

    Thanks all.
     
    Yardley, Dec 7, 2003
    #6
  7. pglade

    pglade Well-Known Member

    u guys are talking about two different things here:

    Elliptical --Like Yardley mentioned if referring to what it looks like when you look straight down on the top of the piston

    Pistons also have what's referred to as a barrel shape--this is the shape when looking at the piston from the side----this is why they tell you to check each piston manufacturer on what location on the piston to mike (measure) for diameter when checking for bore clearance.

    Patton
     
    pglade, Dec 8, 2003
    #7
  8. mrgransport

    mrgransport Well-Known Member

    Pistons should be slightly elliptical to allow for differing expansions in pin direction vs. perpendicular to the pin.
     
    mrgransport, Dec 8, 2003
    #8

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