Factory overbore

Small Block Mopar Engine

  1. Stock1659

    Stock1659 Active Member

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    I got a 273 66' and it has original bearings. Rods are 001 oversized. Pistons have C stamped on 6 Pistons, a B on one and a D on the last one. The oil pan rail on the block has C stamped in two locations. Is this a code from the factory for overbore?
     
  2. inertia

    inertia Well-Known Member

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    I'm not sure about the Letters,, but it was not uncommon to find brgs, lifters, and I believe pistons 1 and 2 thou over/under..

    IIRC ma mopar had a shortage of cranks, and had to use some undersize.

    As I remember those motors had a "maltese cross" stamped on them..

    That was sooo long ago..

    I believe you may still be able to get those 1 and 2 under brgs..

    hope it helps
     
  3. pishta

    pishta I know I'm right....

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    Maltese cross indicates one or more bearing journals are .001 under. Cross with an X means all are under .010 or some, look on crank counterweight #3 for M1-10 or R3meaning main 1 or rod 3 is .010 under, etc. An A after the serial number on pad means a cylinder is .020, (didnt say pistons) a diamond means a lifter(s) are .008 OS and a O.S on the head means a valve guide is .002 over. This is from the 66 FSM.
     
  4. toolmanmike

    toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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    I had the block bored .030 and the crank turned and polished .010/.010. I also had the decks checked all just to be safe. The late model machines at the machine shop are far more accurate than the machines at the factory were after machine thousands of blocks. If a few blocks sneak out of spec they didn't junk out the whole block. They just made it work with undersized and oversized parts. tmm
     
  5. JBurch

    JBurch FABO Gold Member FABO Gold Member

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    The letter stampings on the pan rail indicate manufacturing tolerance. The cylinders get bored and honed, measured to find where they fall in the tolerance zone. The pan rail was stamped to let the engine assembler know what pistons to use, cuz just like the cylinders, the pistons had a range of sizes due to manufacturing tolerances, it was the easiest way to do select fit.
     
  6. mderoy340

    mderoy340 Well-Known Member

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    This X2
     
  7. Stock1659

    Stock1659 Active Member

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    Thank you!

    Merry Christmas

    Mike Bolton
     
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