Rusted Drain Plug Hole

[firemandave]

I made the mistake of leaving my engine drain plug out over the winter and now the threads are rusted. I am nervous about ruining the threads if I try to put the plug back in. Is there a good way of cleaning the threads before I replace the plug? I have learned my lesson and will replace the plug after draining the block next year. The boat is a 91 SN closed bow with the 351 engine.

Thanks

[Mahoo]

I would try a small wire brush with some WD-40 to start. That should get the rust out. If that doesn't do it, you may need to chase the threads with a pipe tap. My threads are 1/2" npt in my boat. Just don't thread the tap in too far as the pipe tap is tapered and will make the hole bigger as it threads in.

[firemandave]

Upon further exam, it looks like the threads are pretty much gone. Should I tap it 1 size larger?

[DanielC]

If it is the plug on the sides of the block, below the cylinder heads, I believe it is actually a 1/4-18 tap. Note, this tap is going to look much bigger than you would think for a nominal dimension of 1/4, but pipe sized are measured by the ID (inside diameter) of the pipe. So, with the thick walls used on pipes, a 1/4 pipe tap is almost a half inch in diameter.

Get a 1/4-18 pipe tap. Carefully thread it into the bad hole, by hand, making sure you hold it straight. It will be very easy to start it crooked, and if you start it crooked, it will just keep screwing itself in crooked.
Turn it only a turn at a time, remove the tap, and try the pipe plug. if the pipe plug does not go in, put the pipe tap in another turn, and try again.
You may notice the tap "feeling" different, or sounding different as you screw it in farther. This is normal, as you go deeper, you start to cut clean metal, instead of rust out of the threads of the hole. If you go too deep with the pipe tap, you will make the hole too big for the plug.

The worst case possibility, you have to go to the next bigger size pipe plug. That is 3/8-18.

Do not go to the bigger size unless you have to.

[Mahoo]

Sorry guys, I misread and thought it was the boat drain and 1/2 NPT. Engine plugs are 1/8 and 1/4" NPT. Before I would try a larger tap, I would try the correct size tap and just run it in a little deeper. That should clean the threads up. If that doesn't work, you can go to a larger tap, but I would drill the hole out to the correct size prior to starting the tap. You will need to remove the old threads or it will cause the tap to go crooked.

[firemandave]

Thank you for all your help. I used the 1/4 tap and cleaned out the threads. Then installed a new plug with some anti-sieze on it. It screwed in fairly easily and isn't leaking. Problem solved! Next year I will drain the block and then replace the plug for the winter.