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Thread: Dual-Plug Heads?

  1. #1
    Join Date
    Jul 2008
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    Default Dual-Plug Heads?

    Are there any minor or major differences in the 74 and 80 heads for Shovels? A friend is looking for 80 dual-plug heads for his 83 FLHS 80 shovel. He can find several 74 dual-plug heads on ebay; but, none noting 80. We assumed the combustion chamber might be slightly different and maybe slight valve train differences; but, essentially the same head. His shovel is basically stock except for the new 80 barrels and pistons we installed this past summer. Has anyone run 74 dual-plug heads on an 80 model? His alternate would be for a machine shop to drill/tap his heads.

    Thanks,
    William
    William Edwards, AMCA #10035

  2. #2
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    Jun 2005
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    Default

    IIRC, there was no functional difference between 74 and 80 heads. The 80/81 heads had the short valve guides, the mid 78 and later heads had a boss on the pushrod side to mount the bigger air cleaner. I don't remember if there was a difference after the pushrod corks were phased out.
    VPH-D

  3. #3
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    Thanks VPH-D, I do recall the early heads having tall fins between the rocker covers. William
    William Edwards, AMCA #10035

  4. #4
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    Default

    What is his reason for wanting the dual plug heads? Just curious as they are a fine addition on performance engines but of little value on a stock one. The entire concept is flame front and burn time. Hemi heads and high crown pistons (high compression) affect flame travel, so a second plug helps with complete combustion. Is he after performance or bragging rights? The former has some real value, the latter none.
    Robbie Knight Amca #2736

  5. #5
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    Thanks Robbie, I get what you are saying. No, he’s definitely not a bragger. He is hoping to get a little more performance out of his mill (appears not so on a stock mill). And he hasn’t mentioned any plans stroking his mill either. I’m wondering if there are there any gotcha’s to be concerned about (fit, internal clearances, etc.) using 74” dual plug heads on an 80” stock motor?
    William Edwards, AMCA #10035

  6. #6
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    Default

    No difference in internal dimensions. The later heads just had easier to replace guides than the earlier ones. Cylinder to head connections are identical. Bear in mind that dual plugged engines generally work best with the timing slightly retarded.
    Robbie Knight Amca #2736

  7. #7
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    Thanks Robbie
    William Edwards, AMCA #10035

  8. #8
    Join Date
    Mar 2010
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    Neshanic Station, NJ
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    Default

    One thing to keep in mind is that some of the earlier heads, not sure of years, used short reach plugs and others used long reach. Make sure of all your clearances before stomping the kicker or hitting the starter button.

  9. #9
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    Mar 2009
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    Princeville, IL
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    Default

    I have dual plugs on a 75 FXE and the heads are original to the bike. When I machined down thru the fins to get enough clean up for the plug port, The long reach plugs had a thread or 2 sticking down into the combustion area. After awhile they get a little carbon on them and would tighten up when removing them. So, I use a set of long reach in the left side and short reach on the right side. I can tell you this, I don't think they make any real difference in performance, The engine has 8.25 compression and an older Sifton Cobra solid lifter cam and a 2 into 1 Supertrapp header and an S&S super B carb. It runs real nice for a 74 inch but no screamer for sure.
    John Underwood
    AMCA#14400
    Central Illinois

  10. #10
    Join Date
    Mar 2017
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    141

    Default

    Quote Originally Posted by Underdoggie View Post
    I have dual plugs on a 75 FXE and the heads are original to the bike. When I machined down thru the fins to get enough clean up for the plug port, The long reach plugs had a thread or 2 sticking down into the combustion area. After awhile they get a little carbon on them and would tighten up when removing them. So, I use a set of long reach in the left side and short reach on the right side. I can tell you this, I don't think they make any real difference in performance, The engine has 8.25 compression and an older Sifton Cobra solid lifter cam and a 2 into 1 Supertrapp header and an S&S super B carb. It runs real nice for a 74 inch but no screamer for sure.
    For the short reach plug, do you use a 3/8" or 1/2" reach plug? Also you can buy sparkplug indexing washers. Why not kill 2 birds with one stone and index your long reach plugs while spacing them out a thread or two at the same time. Just my 2 cents.

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