RD-17 and RD-18 exhaust leg cracks

Home Forum Ask A Member RD-17 and RD-18 exhaust leg cracks

This topic contains 11 replies, has 8 voices, and was last updated by olcah olcah 1 week, 3 days ago.

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  • olcah
    olcah
    US Member - 1 Year
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    #186408

    Looking at a 1955 25 HP Johnson RD-17 I noticed the cracking that I highlighted with a red pencil. I noticed crack in same location on the outside of the leg. Checked several 1956 RD-18 30 HP Johnsons and found the same crack in a couple of them only not as developed. I see this is a very thin section of the casting and expect these develop from cooling stresses in the casting or when the motor strikes something underwater. My guess is these are “no problem mate” type cracks but I am curious if others with more longer experience have noticed them or had any problems.
    Thank you,

    crack in red

    100_6260

    exterior of leg, same crack

    An RD-18 with similar crack

    • This topic was modified 1 week, 6 days ago by Mumbles Mumbles.
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    olcah
    olcah
    US Member - 1 Year
    Replies: 341
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    #186425

    For comparison I looked at some Ask a Member pages and found 3 images of RD-17s. All three are cracked in same location. 🙂

    Avatar
    garry-in-tampa
    Lifetime Member
    Replies: 3241
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    #186426

    Stress fractures from varying heat expansion rate is a plausible explanation. It never showed up in the factory one thousand hour test engines and I have never heard of any catastrophic failures because of it. There was a company in the mid ’50s that made and promoted a Water ski tow rope attachment that bolted to the narrow section of the Exhaust housing. These quickly developed cracks at that point in the housing which caused OMC to issue a warning that the use of this device would void the factories warrantee.

    outboardnut
    outboardnut
    US Member - 1 Year
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    #186429

    My RD-17 has crack inside but not on outside

    crack

    crack-two

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    olcah
    olcah
    US Member - 1 Year
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    #186445

    Garry,
    Thank you for your experience/take on this. Looks to me like most all of these motors have or will develop this crack and it is no problem. Outboardnut’s photo shows the same location as the others that I have seen, maybe not as developed because the motor has not been run as much. 🙂

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    crosbyman
    Canada Member - 1 Year
    Replies: 1270
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    #186446

    maybe you can bridge the crack on the inside surface with a small piece of aluminium plate by securing with 2 small bolts or have it welded if it is a big concern

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    olcah
    olcah
    US Member - 1 Year
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    #186454

    I believe it is interesting but of no concern. The crack appears to be on nearly all of these motors for at least two model years. If it lead to problems I think people with long experience like Garry would know about it.

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    fleetwin
    US Member - 2 Years
    Replies: 2770
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    #186461

    Just seems like something that develops due to the strain of the fastener that is located right there. I wouldn’t worry about it either unless an exhaust leak develops or the crack spreads

    frankr
    frankr
    US Member - 1 Year
    Replies: 4524
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    #186469

    Once upon a time, many moons ago, there was a service bulletin concerning extra exhaust cooling. Don’t remember if it applied to this or not. It would take me forever to find it, if I even have it.

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    george-emmanuel
    US Member - 1 Year
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    #186524

    Notice how the crack does not extend into the mating surface for the gasket. I’d not worry about it.

    George

    bobw
    bobw
    US Member - 1 Year
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    #186694

    I don’t see any such cracks in my RD-18 but I was just wondering if there would be any benefit in filling the cracks with a low temp aluminum welding rod (Alumaloy or similar).

    Bob

    1954 Johnson CD-11
    1956 Johnson RD-18
    1958 Johnson QD-19
    1959 Johnson QD-20

    olcah
    olcah
    US Member - 1 Year
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    #186701

    I can’t think of any reason to do so. If there is a crack it is not noticeable on the exterior of the leg and If the crack is from thermal stress, I think the crack would just reappear in a slightly different spot after welding. The motor I first noticed it on has more wear than I have ever seen on a Big Twin, really a parts motor. But even so the crack is not large and based on the above discussion does not seem to be any problem at all.

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