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Coolant leak from riser bolts.

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  • Coolant leak from riser bolts.

    Looking to see if anyone has had experience with the exhaust riser on a Mercruiser 170 leaking form the bolts and gasket.
    I know this is the last incarnation of the "loved" 470 but my riser is different from most of the ones I see in service manuals.
    When I first got the boat there was a significant leak on the riser into the exhaust manifold which caused the motor to overheat and the head gasket to blow.
    After rebuilding the top end of the motor and reassembly I continued to have issues with the leak. I removed the exhaust manifold at the request of the machine shop
    and they made all the gasket mating surfaces flat. This seemed to do the trick and all was good until the outdrive had issues of its own which are now finally fixed.
    Now that the boat is back in the water the riser is starting to leak again but not in the exhaust manifold as far as I can tell.
    The leaks are not significant as after 2 days of running the boat and fishing on the water I refilled the riser with about a 1/3rd of a cup of coolant.
    The one thing I remember seeing is that there were 2 riser designs and one design had multiple gaskets including a steel gasket.
    The other just used 1 fiber style gasket. Mine is the latter best I can tell.
    Thunderstruck, 19 ft 1979 cuddy. 86 170 merc. A1OD.

  • #2
    It might be time to pull the riser the replace the gasket again. Take a file and go across the mating services in several different directions to get it flat and clean.

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    • #3
      New gasket with a light coat of sealant on both sides.
      1981 Hardtop Cuddy Alaskan Bulkhead Rebuild Thread
      1972 HT Cuddy-Rebuild Thread
      8' 650cc Kawasaki powered dinghy

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      • #4
        Is yours raw-water cooled? Or does it run engine coolant through a heat exchanger? You mentioned adding coolant, so I am assuming it is the latter.

        At least on the 470, the exhaust manifold has water jackets around the exhaust port through which coolant runs to keep the manifold cool and to begin to cool the exhaust gases so they can exit the boat cool enough not to fry the rubber boot to your exhaust tube running down to your outdrive. There are water jackets in the bolt-on riser as well, just the same, but in an application where coolant is used, there is a stainless plate that blocks the passages, separating those in the exhaust manifold from those in the exhaust riser. The raw water, once it departs the heat exchanger, flows into the exhaust riser, travels through the internal water jackets there, and then exits into the exhaust stream at the far back.

        So you would need two gaskets plus the stainless plate. It's a good idea to pull that and inspect it, making sure the water jacket passages are clear and functioning so the raw water can flow through it properly. Then clean up the gasket surfaces, shiny-clean, and install a new gasket, the cleaned-up stainless plate, then another gasket, then install the riser. I have never used sealant on it, but also never saw any harm in using it either, other than making cleaning it up next time a real chore. I've always put them together dry.
        -- Tim Taylor

        1979 Glasply 21-foot I/O Overnighter w/ 5.0L Volvo-Penta duoprop (Cardinal)
        1982 Glasply 16-foot runabout w/ 50hp Mercury OB (JV Junior)
        1978 Glasply 19-1/2 foot runabout, project boat (Beer Goggles)


        I didn't say it was your fault, I said I was blaming you.

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