Head gasket diameter

Engine, Transmission & Driveline

  1. Cazman

    Cazman Active Member FCBO Gold Member

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    I am finishing up my 440 stroker build and ready to put the head gaskets on. I am .030 over bore with Cometic 4.350 (.030 over) bore diameter gasket. I have a very slight overhang into the cylinder on the block and into the combustion chamber on the head. Very slight, like maybe .002".

    I have built several motors over the years and have always used standard gaskets that had more diameter than the bore. But I needed the Cometic and figured I would get the exact bore diameter, because I could. Does anyone have experience with this?
     
  2. 65Fury440

    65Fury440 Senior Member

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    My block was also a .030 overbore, had to go with Fel Pro 17059 Marine gasket because of the same issue, the 1009 hung the ring in the bore.
    You would think a company like Cometic would have that handled. Can you just clearance them a little?
     
  3. CBODY67

    CBODY67 Senior Member

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    Just curious, is the 4.35" bore the piston size and then the piston skirt clearance is put on top of that for the total bore size? Or is the 4.35" bore the end size after honing for clearance?

    Were the head gaskets "made to size" or is that one of their normal-spec items?

    Thanks,
    CBODY67
     
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  4. Cazman

    Cazman Active Member FCBO Gold Member

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    The bore size is finished. It is the normal Cometic gasket, but maybe people use the oversized for a normal bore and I should be using larger too.
     
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  5. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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    If it's only hanging over the bore .002" as you said, I think I would use them.

    But to make sure, why not call Cometic? They could give you the authoritative answer. I'm sure it's come up before.
     
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  6. Cazman

    Cazman Active Member FCBO Gold Member

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    I would have called, but I wanted to finish up this weekend. Hoping that someone here had run into the same thing.

    Also, a pet peeve of mine when aftermarket manufacturers / suppliers sell to non-professionals, but assume that those same non-professionals have some institutional knowledge about these things. It's not hard for Summit or Cometic to supply a bit more information on parts like these.
     
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  7. CBODY67

    CBODY67 Senior Member

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    I feel you have a valid concern in this area. The overhang could become a later potential pre-ignition point and to shave it might possibly breach any sealing at the edge of the gasket (which might be there). Used to be that one would have to go back to the manufacturer, not the vendor/seller, for any parts warranty situations.

    Personally, I'd suspect the OD of the cylinder fire ring should be just larger than the bore OD, but it also might be that what you have is within the production tolerance for Cometic. Large enough that when the gasket compresses under torque, that it makes one smooth "front" for the cylinder OD.
    So contacting Cometic directly would probably be the best way to do things. You might also need purchase documentation from the vendor, though.

    Keep us posted,
    CBODY67
     
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  8. Cazman

    Cazman Active Member FCBO Gold Member

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    So, Cometic said yea you should order .030 over your finished bore size. I should have figured that, but thought that they might have worked that into consideration on their specs. Never know until you see the parts sometimes.
     
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  9. twostick

    twostick Senior Member

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    If the overhang is only .002 I doubt there would be a problem but unless the block deck has been cut a bunch I would be astonished if that 4.35 gasket isn't hanging at least .100". My 76 block had a chamfer that looked like it was cut with an axe. Huge. Earlier blocks weren't so bad for this.

    A Felpro 1009 has a 4.410" bore and on my build it hung over the chamfer enough that Ray Charles could see it. I went with a regular 8519 which if memory serves has at least a 4.5" bore.

    Kevin
     
  10. Cazman

    Cazman Active Member FCBO Gold Member

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    I have heard about large chamfers, but mine does not seem to have much of one.