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Thread: General Question about Cylinder Heads casting process

  1. #21
    Moderator Turbo Mopar Staff Vigo's Avatar
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    Re: General Question about Cylinder Heads casting process

    If you drill 1000 heads with the same boring bit, then all the holes should be the same size, and the same camshaft should fit in all of them.

    The differences are where EXACTLY that whole was drilled in relation to the rest of the head. There are small differences there. That's why you can't always mix and match caps. The half-moon cut in the cap might end up being slightly offset from the half-moon cut in the head, and all of a sudden you don't have a perfectly round hole anymore.

    Dont push the red button.You hear me?

  2. #22
    Super Moderator Turbo Mopar Staff 135sohc's Avatar
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    Re: General Question about Cylinder Heads casting process

    Camshafts are one piece and can be made with enough repeat precision that they don't need to be custom fitted for every cylinder head.
    A mass produced casting has core shift and when it is multiple different pieces coming together no two are going to be identical, hence they are a 'machined matched pair'

  3. #23
    boostaholic
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    Re: General Question about Cylinder Heads casting process

    Quote Originally Posted by Vigo View Post
    The half-moon cut in the cap might end up being slightly offset from the half-moon cut in the head, and all of a sudden you don't have a perfectly round hole anymore.
    Right, looking back I'm pretty sure the motor I did the cap swap on had peanut shaped holes for the cams. But back then I had no idea. The head on the motor was bored slightly lower than the cap donor head.
    MinivanRider

  4. #24
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    Re: General Question about Cylinder Heads casting process

    you need to forget about the camshaft, all the valvetrain hardware- those parts are not the concern. they will be machined to a specific specified size at the factory- for all intents and purposes you need to consider them exact copies of an original part.

    cylinder heads along with all other parts that are cast will have production variances due to the casting process. those parts are cast in a mold made of sand. a worker takes a "plug" and then assembles that by hand and packs sand around it in the casting box. things like core shift, which worker was assembling molds that day etc will all affect the final dimensions of the part. things like lifter bores, spring pockets, cam towers will all have variances in dimensions due to those factors during the casting process. you need to consider a cast part a copy of a copy of a copy.
    the longer the production run goes on the more variances there will be you have to consider each cylinder head as unique as a fingerprint. none of them are dimensionally the same the cam journals on the head may be in the same places, but the cast part of the head that they are machined out of will have subtle variances. one cam tower might be a couple thousandths taller than the one next to it, or slightly wider or slightly shorter etc. the line honing process is the only way to ensure those journal surfaces are perfectly in line with each other.

    if those cam caps from a different head do happen to work then you messed up by not buying a lottery ticket instead because you got god damn lucky.

  5. #25
    Garrett booster
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    Re: General Question about Cylinder Heads casting process

    Quote Originally Posted by crusty shadow View Post
    you need to forget about the camshaft, all the valvetrain hardware- those parts are not the concern. they will be machined to a specific specified size at the factory- for all intents and purposes you need to consider them exact copies of an original part.

    cylinder heads along with all other parts that are cast will have production variances due to the casting process. those parts are cast in a mold made of sand. a worker takes a "plug" and then assembles that by hand and packs sand around it in the casting box. things like core shift, which worker was assembling molds that day etc will all affect the final dimensions of the part. things like lifter bores, spring pockets, cam towers will all have variances in dimensions due to those factors during the casting process. you need to consider a cast part a copy of a copy of a copy.
    the longer the production run goes on the more variances there will be you have to consider each cylinder head as unique as a fingerprint. none of them are dimensionally the same the cam journals on the head may be in the same places, but the cast part of the head that they are machined out of will have subtle variances. one cam tower might be a couple thousandths taller than the one next to it, or slightly wider or slightly shorter etc. the line honing process is the only way to ensure those journal surfaces are perfectly in line with each other.

    if those cam caps from a different head do happen to work then you messed up by not buying a lottery ticket instead because you got god damn lucky.
    That is exactly what the engine machinist said!

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