50’s 18hp OMC retrofit question

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This topic contains 25 replies, has 10 voices, and was last updated by Avatar dan-in-tn 1 month, 1 week ago.

Viewing 11 posts - 16 through 26 (of 26 total)
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    wbeaton
    Canada Member - 1 Year
    Replies: 598
    Topics: 65
    #173495

    What I meant was there are no parts to swap either internal or external that will increase the hp of your 18 hp. The only complete powerhead swap that might work is a 1970’s 25 hp because it shares roots with your motor. Personally, I don’t care if you hot rod a 1950-60’s 18 hp. There is nothing rare or special about them. Enjoy the hobby the way you want. I’m just trying to save you some trouble and possible expense.

    Wayne
    Upper Canada Chapter

    uccaomci.com

    Avatar
    garry-in-tampa
    Lifetime Member
    Replies: 3065
    Topics: 29
    #173496

    I believe they went to the 50 to 1 ratio with the light tension “pressure back” piston rings. . .

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    rudefan29070
    US Member - 1 Year
    Replies: 13
    Topics: 4
    #173504

    I believe they went to the 50 to 1 ratio with the light tension “pressure back” piston rings. . .

    And the pressure back was on the 2 ring pistons, right?

    Avatar
    shoestringmariner

    Replies: 241
    Topics: 49
    #173507

    What I meant was there are no parts to swap either internal or external that will increase the hp of your 18 hp. The only complete powerhead swap that might work is a 1970’s 25 hp because it shares roots with your motor. Personally, I don’t care if you hot rod a 1950-60’s 18 hp. There is nothing rare or special about them. Enjoy the hobby the way you want. I’m just trying to save you some trouble and possible expense.

    Avatar
    shoestringmariner

    Replies: 241
    Topics: 49
    #173508

    What I meant was there are no parts to swap either internal or external that will increase the hp of your 18 hp. The only complete powerhead swap that might work is a 1970’s 25 hp because it shares roots with your motor. Personally, I don’t care if you hot rod a 1950-60’s 18 hp. There is nothing rare or special about them. Enjoy the hobby the way you want. I’m just trying to save you some trouble and possible expense.

    Where’s the like button?
    I appreciate your thoughts and advice

    fisherman6
    fisherman6
    US Member - 2 Years
    Replies: 1732
    Topics: 39
    #174295

    I just recently have poured over parts for ALL the years of 18’s looking for pistons for my rebuild. Frank is right there isn’t much difference in them at all. If I had a 1950’s era 18 I would surely run a leaner mix than 24:1. The oils we have available today are MUCH better. I have a 1968 18 that I run basically a 50:1 mix and I throw in 1 more oz for a 6 gallon tank, instead of 16oz of oil per 6 gallons I mix 17. The motor doesn’t complain and it runs like a cat kicked in the butt and doesn’t foul plugs. My best friend has a 1965 18 that I maintain and he too runs the mix I do and doesn’t have any trouble with his either. We both run the Johnson Evinrude TCW3 2 stroke oil that they sell at Walmart by the gallon.

    A ’68 18 is a 50:1 motor anyway. The advertised ratio change was in 1964 to 50:1 from 24:1 if im not mistaken.
    -Ben

    OldJohnnyRude on YouTube

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    foot_doctor
    US Member - 1 Year
    Replies: 48
    Topics: 8
    #174296

    I should jump in here to explain the parts needed to convert your 18 to a 25. The cylinder block/crankcase assembly is the same for either model. You”ll need a 25hp crankshaft, a pair of 25hp pistons, a 25hp head and a 25hp carb. Everything else can be used. The crank has the same stroke as the 18, but it has weights riveted onto the balance weights. The pistons have their piston pin holes located closer to the top. That advances the port timing. The 25hp head has combustion chambers with lower ccs. That restores the compression. lost with the piston change. The carb has a slightly larger venturi, and I beleive a larger main jet. That’s it. Install that stuff in your block, and you will seriouslly increase you performance. I built a lot of thos powerheads when we raced them in the early versions of 25SS. My engine was unbeatable, and I could start last and pass everybody on the course. It was 1 mph slower than my MK20H. Have fun boating. R.T.

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    shoestringmariner

    Replies: 241
    Topics: 49
    #174300

    Excellent, thanks again guys

    Avatar
    Seahawk
    US Member - 1 Year
    Replies: 3
    Topics: 1
    #174797

    @ shoestringmariner .Going back to your main concern where you have an oily mess on your transom after an outing. These vintage motors up until the 70’s discharge there unburned fuel which lucubrates the bears in the crank case right out into the exhaust output and into the water. This is more noticeable at lower speeds. On your motor right behind the intake housing and on the lower left-hand side of the crank case is a drain valve which controls this action and is activated on the downward stroke of the piton. If you block the hole which this raw fuel is flowing and drill a hole on the side of the leaf plate by the lower lower cavity cut out for this fuel to flow. You could redirect the fuel back into the boat and into a container and reuse it. The drilling of the hole in the leaf plate would have to be precise due ti its thickness. Then you would have to insert a thin wall tube and epoxy it into place. This is where a hose could be connected and back to your onboard tank..

    Just a thought!

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    chinewalker
    US Member - 1 Year
    Replies: 445
    Topics: 17
    #174822

    You’re forgetting the other big thing that OMC went with on all but their smallest motors – a thermostat. Stats made the ability to improve tolerances possible, and for the motor to run within those tolerances in varying conditions. Better oils, better bearings, and more consistent operating temperatures all helped make 50:1 work.

    Avatar
    dan-in-tn
    US Member - 1 Year
    Replies: 993
    Topics: 104
    #174825

    I’m with chinewalker and will the thermostat head fit under the old metal cowling if you want to maintain the old look? I think not!

    Dan in TN

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