Engine drain plug hole rusted over threads

[andrewterri]

I have a 1989 sea ray with a mercruiser 3.0 with an alpha one outdrive. I recently got the boat and after replacing the impeller I went to start the engine with the muffs on and saw water spraying out of the side of the engine under the exhaust manifold. After getting that sinking feeling that I have a cracked block and I just wasted my money I realized that the water was coming out of a screw hole. After some reasrch I found it was the drain plug hole. My problem lays in that the threads are rusted solid on the bottom side of the hole. I found that the plug was a 1/4 NPT and bought a couple of them along with a stiff wire brush thinking that I could clean the threads and force thread one plug one there and then use the second one with the good threads, however, this is not working. I tried to call machine shops to see if they can die and tap the hole and they wont because the engine is in the boat. I am worried about doing it myself because I have never done it and don't want to go to deep and puncture the water jacket. Any ideas would be a great help.

TIA Andrew

 

[Bt Doctur]

It`s still a do-it-yourself project. Measure the depth of the hole and mark the tap with tape. The tap used is called 1/4 inch pipe thread. Get 2 of them and grind the second one down taking off about a 1/4 inch from the tip.Clean the hole andf install a new plug.
when it comes to winterizing, fog and drain everything first, then fill with A/F .

 

[andrewterri]

Thank you for you quick reply. Am I grinding the second one down so it will have a flat tip? When I'm using the tap do I need to worry about hitting the threads right?

 

[Bt Doctur]

the pipe tap is a tapered thread tap. Taking a 1/4 inch off the end allows you to start tapping threads sooner, start it with the factory tap and finish with the ground down one. Grind off the 1/4 inch ,hitting the threads dosent matter, your just making the tap shorter.. Just make sure you mark the depth with masking tape.

 

[bspeth]

Also soak the crap out of it while you are looking for a tap.

 

[andrewterri]

Thank you for you quick reply. Am I grinding the second one down so it will have a flat tip? When I'm using the tap do I need to worry about hitting the threads right?

 

[andrewterri]

Sorry for the double post. What do I need to soak it with?

 

[Grub54891]

Soak it with pb blaster,or just oil, anything to lube the tap,or threads. When starting the taps,turn them counterclockwise by hand and try to feel where the threads start. Then turn em clockwise and attach the handle,or crescent just go carefully.It'll be fine. If there is no " bite " it may require tapping to the next bigger size pipe thread. Remember pipe thread is tapered,tap to deep, plug will not seat till it's to deep.

 

[Scott Danforth]

welcome aboard

once you get the pipe tap started to chase the threads, it normally only takes half a turn to clean the threads because it is tappered. Good luck

 

[andrewterri]

Well I went to the hardware store and got a 1/4-18 npt tap and I'm about to give it a shot. Thank you all for your help. Hopefully I won't be asking how to replace an engine because I tapped into the water jacket. I will post back and let you know how it went. Thanks again

 

[andrewterri]

Thanks again for all of your help. I got it tapped and it worked perfectly.