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  1. #1
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    Did OMC have "pressure backed piston rings"?

    I think I remember pressure backed piston rings in old Johnson and Evinrude brochures, but of course I could be wrong.

    There is an article about piston rings in this months Hemmings Muscle Machines magazine. It discusses horizontal and vertical gas ports that allow the combustion pressure to help seal the ring to the piston wall. The article reminded me of what I may remember about old advertising.

    GAS PORTED RINGS (totalseal.com)

    5 Reasons Why Gas Ported Piston Rings Make a Massive Difference (lsxmag.com)

    Hemmings | Hemmings

  2. #2
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    yes

  3. #3
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    Did the pressure backed piston rings work well? Were they cost effective?

  4. #4
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    I think they worked well until the piston to wall clearance wore to the state that the ring could slip out of the groove. These were on the old Crossflow motors. They started out with lots of clearance and only got more. I do think they worked well, Chris.

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  6. #5
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    A Word About Tolerances

    Quote Originally Posted by ChrisCarsonMarine View Post
    I think they worked well until the piston to wall clearance wore to the state that the ring could slip out of the groove. These were on the old Crossflow motors. They started out with lots of clearance and only got more. I do think they worked well, Chris.
    Tight tolerances and fit do not come cheap. Lots of "clearance" comes from the economic decision to accept component tolerances at some boundary but also the labor and cost effort associated with component assembly. Want tighter tolerances? Prepare to pay for it.

    Honda used to make engines with custom crankshaft bearing shell sets to absorb the machining variability of each crankshaft during block assembly. Outboard engine companies are no different in their design objectives.

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  8. #6
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    yes they did use them

  9. #7
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    OMC slugggs of the era were big an bulkie..... the pressure back were triangle in cross section.......the wedge shape put some /tooo much load on the piston land.... the early ones were high location..... not enughf ring land.....which broke...... later ones had the rings lowered.......from what I remmmmbure..........

  10. #8
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    I think you’re talking about “semi dykes” rings.

    Tons of companies did this, including bikes.

    As a fan of ring sealing, I like the theory.

    But, as stated, increased sealing = increased friction.
    I'd rather be competitive w/junk I built in my garage than win w/stuff I bought.


    I refuse to allow common sense to interfere w/my boat buying decisions.


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  11. #9
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    The High ring pressure back were only in 1978/79 crossflows, carbon built up on the back side also causing ring jacking.

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  13. #10
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    If I remember correctly it was low octane fuel that beat the high/pressure back ring lands out...didn't know about the carbon buildup doing damage. Was told to use premium and good oil.
    Hustler 15' Wildcat / Merc 80
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  14. #11
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    yes bad gas omc was giving pistons rings gasket to most anyone who had trouble i know i rebuild a lot of motors for them free parts sometimes even a new block but the new pistons were heavy and on a 150 v6 you would lose about 500 rpm but some of the motors i bilt were still running 15 to 20 years later

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  16. #12
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    KNew I'd posted these homemade pressure backed piston
    https://www.screamandfly.com/showthr...hrysler+Racing
    Last edited by FMP; 01-26-2023 at 04:51 PM. Reason: K

  17. #13
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    Anyone have pictures? All of my old crossflows have high ring pistons. The rings are different from loopers as they have a bevel to collect compression to increase ring tension.

    That is the way I saw it anyway. I collected every high ring piston I could find for my 2.6L Xflows. And had them all machined with the XP skirt cut. I smoked a couple pistons and learned... you need spares!

  18. #14
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    78-79
    Attached Thumbnails Attached Thumbnails s-l500.jpg  

  19. #15
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    It's called gas porting, ok on a race engine, but on a 2 cycle it tends to carbon up fast...whether it's top, side or ring ported. I imagine it was an economical alternative to more precise piston to wall clearance.
    Last edited by ksfastman; 01-31-2023 at 05:03 PM.

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