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  1. #1
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    Default mercruiser 3.0 drain plug

    I'm helping a friend on his mercruiser 3.0
    The drain plug is/was on the bottom of the exhaust manifold.
    He says that water pumps through there for cooling.
    If you are looking at the engine from the prop side down on it, it is on the underside on the left towards the rear.
    The hole was completely stripped and partially corroded/rusted.
    I tried to rethread it but the cast iron didn't take the tap very well.

    My questions are:
    Is there supposed to be a plug there?
    Should I try to rethread to larger size?
    Or should I just use a rubber expansion or test type plug like one you would use to plug a live well?

    It's drill and partially threaded to 1/2x20tpi right now.

    Thanks

  2. #2
    Lieutenant Commander fat fanny's Avatar
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    Default Re: mercruiser 3.0 drain plug

    Mine is npt and yes it supposed to be there. There is also one on the block on my 3.0LX right below it on the block to drain the water jackets around the cylinders. If you definately have to go toa bigger size use low rpm on cast.
    1996 Searay 175 Five Series
    3.0LX ser #F553706
    Alpha1Gen2 ser #OF530668
    1998 Chevy1/2 tonW/T
    "faster dad faster" my baby girl in the tube

  3. #3
    Lieutenant Commander superbenk's Avatar
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    Default Re: mercruiser 3.0 drain plug

    Those holes are there with plugs so you can drain the water from the manifold and block during winterization. If you don't, the water will freeze & you'll crack the block and/or manifold leading to a very expensive fix.

    I'd be concerned about the health of the manifold as a whole if that hole is too corroded to take a thread. The rest of the manifold might be corroded inside too in which case you may need to think about replacing it.

  4. #4

    Default Re: mercruiser 3.0 drain plug

    You should also use a flat bottom (blind hole) tap.

  5. #5
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    Default Re: mercruiser 3.0 drain plug

    Thanks. Yeah the hole was badly rusted on the inside, The original plug was NPT thread and was mutilated. I tapped at 1/2 x 28tpi. I'll either helicoil it or just tap a size up. Gotta order a tap now, my set stops at 1/2"
    I'll use my good taps next time instead of my harbor freight set.
    Anyone know how much pressure the water is at in there approx.?

  6. #6
    Lieutenant Commander fat fanny's Avatar
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    Default Re: mercruiser 3.0 drain plug

    If you heli coil it make sure you put just enough of the coil on there to cover the wall thickness if you go too deep the rust and debre will have no where to go when draining also watch for stress cracks after drilling into cast especially something that hold's corrosion like these mani's
    1996 Searay 175 Five Series
    3.0LX ser #F553706
    Alpha1Gen2 ser #OF530668
    1998 Chevy1/2 tonW/T
    "faster dad faster" my baby girl in the tube

  7. #7
    Lieutenant Commander wire2's Avatar
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    Default Re: mercruiser 3.0 drain plug

    You don't really want a machine thread in there, it should be a tapered NPT, so it will tighten and seal. If you have it at ~" now, a " NPT pipe tap will work well.

    To avoid removing a plug from the iron manifold in the future, insert a brass " NPT nipple with sealant, then a brass coupling, then a brass plug.

    The actual water pressure is low in the manifold, probably <1 lb. It's open to the exhaust passage right after the riser.

  8. #8
    Lieutenant Commander fat fanny's Avatar
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    Default Re: mercruiser 3.0 drain plug

    Good tip on the nipple wire.
    1996 Searay 175 Five Series
    3.0LX ser #F553706
    Alpha1Gen2 ser #OF530668
    1998 Chevy1/2 tonW/T
    "faster dad faster" my baby girl in the tube

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