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  1. #1
    Seaman
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    Default Help with Mercruiser 4.3 drain plug

    Hey all. Need a little help here. I've got a 1995 Mercruiser 4.3 LX. Getting a late start with the boating season. Changed the crank case oil and a few other little projects. Here's where I went wrong.

    When I pulled the cylinder block drain plugs when I parked it, I had issues with the port side plug. Namely, the brass sleeve that threads in to the block and the actual plug threads in to came out of the block. I wasn't too worried about it at the time, since either way the water drained out. Of course, I didn't have the forethought to reinstall the plugs after draining the water.

    After hours and hours of trying, I couldn't get the sleeve to thread back in to the block today. I think there must be some rust build up in the there. I couldn't find any good way to clean the threads (no wire brushes small enough, no thread brushes available anywhere). I think I may have to chase the threads with a tap, but it seems to be a weird size. I can probably find a properly sized tap to "borrow" from work. Just wanted to hear everyone's take on it here before I jump in to it. I'd hate to run a tap into the block and majorly screw something up.

    It also seems that the hole seems further back and higher up on the block than I remember. It's towards the aft end of the block, directly above the oil filter location.



    Here's a closeup of the threads:



    There's some rust staining coming from a hole in the block directly below the drain plug on the exhaust manifolds. The hole seems to be plugged, too small, and not threaded, so I'm fairly sure it's not the correct location.




  2. #2
    Senior Chief Petty Officer cooter2506's Avatar
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    Default Re: Help with Mercruiser 4.3 drain plug

    Thats the drain for sure. The threads are actually pipe thread. Im not sure what size though. Sorry didnt realize we were looking at two differnt locations here. Last two pics seem to be of the block drain, first two are of just a threaded hole. It doesnt go thru block. It is just there. I just went out and looked at my block and it doesnt go thru and doesnt have anything atached to it. Hope this helps.
    1994 Crownline 182BR w/4.3LX 4bbl Alpha 1 gen 2
    2007 Dodge Ram 1500 SLT w/5.7 HEMI
    2004 Chevy Tahoe w/5.3
    1970 Chevelle SS 396-OK its the 18 year old daughters car but hell its fun to drive!!

  3. #3
    Lieutenant Commander Rocky_Road's Avatar
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    Default Re: Help with Mercruiser 4.3 drain plug

    1/8 NPT is pretty universal...I'd try that. If that is too small, then the next size would probably be 1/4 NPT.

    Or...take the plug to your local hardware store (one that actually has knowledgeable clerks), and have them match it to the proper tap. When you use the tap, coat it with some grease...to catch the slivers!

  4. #4
    Seaman
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    Default Re: Help with Mercruiser 4.3 drain plug

    Damn, I was worried someone would say that. The block hole is definitely clogged with a fair amount of rust then. Couldn't even push a screw driver up through it. Ugh...

    I'll definitely need to find a tap the right size. Or maybe I should just break down and bring it in to a shop. Assuming it is indeed a pipe thread, I'm guessing 1/2-14 NPT. Can anyone confirm this?

  5. #5
    Senior Chief Petty Officer cooter2506's Avatar
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    Default Re: Help with Mercruiser 4.3 drain plug

    Get a new block adapter for the plug and take it in so they can match you a tap for the threads. If I read your thread right you need a new adapter
    1994 Crownline 182BR w/4.3LX 4bbl Alpha 1 gen 2
    2007 Dodge Ram 1500 SLT w/5.7 HEMI
    2004 Chevy Tahoe w/5.3
    1970 Chevelle SS 396-OK its the 18 year old daughters car but hell its fun to drive!!

  6. #6
    Seaman
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    Default Re: Help with Mercruiser 4.3 drain plug

    Actually I have the adapter and it seems to be in good shape. It's just the friggin hole in the block that's all hosed up.

    If I can't find the right style of tap at work, I guess I'll have to hunt around for a decent hardware store here. Pretty sure Home Depot and Lowes are going to come up short.

  7. #7
    Senior Chief Petty Officer cooter2506's Avatar
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    Default Re: Help with Mercruiser 4.3 drain plug

    oh ok sorry I read it wrong. Actually I bought my pipe tap set at Home Depot.
    1994 Crownline 182BR w/4.3LX 4bbl Alpha 1 gen 2
    2007 Dodge Ram 1500 SLT w/5.7 HEMI
    2004 Chevy Tahoe w/5.3
    1970 Chevelle SS 396-OK its the 18 year old daughters car but hell its fun to drive!!

  8. #8
    Lieutenant Commander Rocky_Road's Avatar
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    Default Re: Help with Mercruiser 4.3 drain plug

    Quote Originally Posted by lombard View Post
    Actually I have the adapter and it seems to be in good shape. It's just the friggin hole in the block that's all hosed up.

    If I can't find the right style of tap at work, I guess I'll have to hunt around for a decent hardware store here. Pretty sure Home Depot and Lowes are going to come up short.
    They will have the tap you need...but you may be on your own, as to help!

    Good luck!

  9. #9
    Lieutenant Commander Rocky_Road's Avatar
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    Default Re: Help with Mercruiser 4.3 drain plug

    Should have added:

    If the hole is crudded up, you will probably do better with a bottoming tap...it doesn't come to a point, and will catch the existing threads without any interference. A bottoming tap is used to tap threads to the 'bottom' of a blind hole....

  10. #10
    Lieutenant Commander Rocky_Road's Avatar
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    Default Re: Help with Mercruiser 4.3 drain plug

    Bottoming tap:


  11. #11
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    Default Re: Help with Mercruiser 4.3 drain plug

    You need a 1/4 in. pipe tap.Use it to clean the threads only, it`s a tapered thread ,DO NOT THREAD IN FULLY to clean the rust,and DO NOT really crank in the plug, it`s a tapered thread too.

  12. #12
    Seaman
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    Default Re: Help with Mercruiser 4.3 drain plug

    Success!

    As you guys have mentioned, it was indeed a 1/4-18 NPT tap. Chased the threads, reassembled everything, and started it up this evening after work. After letting it run up to temp, I went back in and pulled to plugs just to make sure I had water flow through the block. Everything looked good. Hoping to hit the water tomorrow.

    Oh yeah, didn't even check home depot for the tap. Stopped at my local Ace. Dude there didn't really know what pipe threads were, but he went above and beyond to help me find the tap. Customer service is a good thing.

  13. #13
    Lieutenant Commander Rocky_Road's Avatar
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    Default Re: Help with Mercruiser 4.3 drain plug

    I'm a 'sucker' for happy endings...!


  14. #14
    Fleet Admiral Gary H NC's Avatar
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    Default Re: Help with Mercruiser 4.3 drain plug

    I have always had better luck with stuff like that at Ace hardware..
    I try to avoid the big box stores..
    Glad it all worked out..


    1981 ChrisCraft 210 Scorpion K,175 Johnson SeaHorse

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