Cometic Head Gaskets

Discussion in 'Touring Models' started by bigdog905, Oct 23, 2009.

  1. bigdog905

    bigdog905 New Member

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    I'm doing a 103 kit with HD 211 cams, HD race tuner and had the heads cleaned up with a 3 angle valve job. I stayed with HD stock flat top pistons and am considering the Cometic thinner head gaskets. The stock HD gasket is .40 and Cometic said I could go with a MLS .18 thick gasket. What would this raise my compresson to and is it safe and dependable ? Anyone done or heard of anyone doing this ? I don't want a race bike, just want to dust my Uncle.
    bigdog
     
  2. billnapabill

    billnapabill Active Member

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    You might want to call Cometic and find out what your compression is going to be. Anything over 10:1 is going to cause all the problems Smitty mentioned + there will be timing issues, heat issues, pre ignition, pinging, lots o'cool things to worry bout. How come you don't just go with the Harley stuff. A 103" w/headwork, cams and a Race tuner ot'ta run out pretty good. My 103" made bout 85hp and 98ft#'s o' torque without cams and headwork, darn thing would stretch your elbows if ya hit it too hard.
     
  3. Jack Klarich

    Jack Klarich Expert Member

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    I agree with Smitty why would u want the problems with changing the compression with the rotten gas and heat problems we already have JMO
     
  4. Drumrguy

    Drumrguy Account Removed

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    The problem with doing that, is that if you adjust the pushrods out the correct number of turns, you need to double check valve to piston clearance, trying to raise compression with thinner gaskets = bad idea. In fact most people who build stroker motors build low compression stroker motors, if you want to raise compression, you also need to figure out how much more you can go before you need compression relief.
     
  5. glens

    glens Banned

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    Let's see. I edited your quote a bit, but I only condensed it; did not alter anything. See what I see?

    Stock 103 compresses 9.6:1. Depending on cams, engine speed, etc., if the stock gasket is 0.40" and you substitute it with 0.18", your compression ratio would jump to around 18.6:1

    How's about 0.040" and 0.018" instead? That'd bump 9.6:1 up to 10.1:1, or so.

    Or maybe you were talking millimeters? That'd bump 9.6 to around 9.8

    Since I'm new 'round here, this was meant in fun, so take it that way.
     
  6. glider

    glider Veteran Member

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    Aw heck what's a couple of zero's and a decimal point amongst friends :D

    Nice catch glen.
     
  7. glens

    glens Banned

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    Shucks. I'd grabbed a sheet of paper and a pencil. Jotted down some figures and punched some numbers. I'd used mils in my cipherin' and came up with 9.6:1 from 9.2:1. Then re-read the post just far enough to see it was 103. I'd figured at the 96 c.r. Looked up '09 and '10 trikes and '10 cop bikes. All said 103 @ 9.6:1 so figured that was a good figure to figure with and did it again.

    Then I went to post the results (the answer to the question was 10:1) and saw the figures were hundreds of thousandths and not tens of 'em. I knew that was going to be fun...

    To get 18.6:1 with the stock 103 pistons and combustion chamber volume would take a stroke of 8.48" :)

    Actually, none of my results are correct. I didn't use (displacement+chamber_volume)/chamber_volume but just displacement/chamber_volume and my play time's about over.
     
  8. bigdog905

    bigdog905 New Member

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    Thanks for all the opinions and info. After thinking about it all day and reading all the comments, I'll just stay with what I have. Sometimes it's easy to out think yourself.
    thanks, bigdog

    Okay, you got me, I'm found out. I did sleep through 7th grade math, but I graduated anyway.
    bigdog
     
    Last edited by a moderator: Oct 24, 2009