V4 Crossflow Cooling System Air Bubble?

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    US Member - 2 Years
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    I’ve been working on much more of the V4’s this season and have noticed a trend on these motors where the #2 piston seems to be blown on several, or chewed up. In several of the cases the cylinder has survived any major damage and compression is within 15lbs of 1, 3, &4, but the #2 piston is chewed up along with the cylinder head. Is there some sort of cooling system bubble (similar to the mid-to-late 80’s 3-cylinder 60/70hp) with these motors, and any work around to it?

    It looks as if this winter I will have 3-5 of these powerheads to rebuild by fixing the #2 piston as the motors still run normally up and down the power curve but clang away at idle due to slop from the damaged piston/cylinder head.

    T2Stroke….are you out there?


    US Member - 2 Years
    Replies: 892
    Topics: 142

    The workaround is the same as with the three cylinders: Have a telltale vent at the top of the cylinder block (one for each bank?) that serves the dual purpose of venting air and verifying water flow. My suspicion is (but I am no expert on these) that if the cooling system is in good order there is enough flow to push the air pockets down and out. But if the pump is marginal (impeller) or the thermostat/pressure regulator is not in good order there is potential for air lock.

    On my V4 the telltale is plumbed into an existing 1/8″ pipe thread port that was originally used for a water heated choke, but this is inferior to a port at the top of the water jacket(s).

    For what it’s worth, the same problem exists in many modern cars where the radiator cap is not at the high point of the system. It works until the head gasket begins to leak gasses into the water jackets, or…. A robust cooling system should always be able to vent non-condensible gases from the highest point.

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