Drain plug holes rusted...

The drain plug holes including the exhaust manifold plug holes are all rusted out on my '91 SN. Each year when I put the plugs in, I have to wrap them with teflon tape to get any kind of bite on what's left of the threads (barely any) on the drain plug holes to get them to seal. Any idea on how to repair this and get the correctly sized drain plugs ?

I'm still using the brass plugs on both the block and the exhaust...unfortunately both the hole threads and the plug threads are in bad shape. Is there any possible way to fix the ones in the block ? Any when you say replace the ones in the exhaust, are you meaning replace the manifolds completely ?
Thanks for the quick replies.

Brian, The first thing I would try that Chris mentioned is to try and tap the existing holes deeper. Since pipe threads are tapered, this will usually give you new threads. The down side to this is now the plugs will go deeper into the wall of the engine and manifolds. So the plugs can go deeper, you may want to get some brass socket head plugs.



Manifold replacement is not needed at this time. When tapping the threads, only go deep enough to clean up the threads. Do use teflon tape and don't over tighten the plugs. You are not looking at very much pressure in the cooling system.

If this doesn't work, then you would want to re drill and tap to the next size pipe plug if there is enough boss area (the thicker section of casting around the hole) in the casting.

Chris, You really must have had a hard time with the tapping of cast iron at one point. I mentioned it before that it really isn't a problem and is done daily. A small amount of kerosene or a very light oil works best to aid the tapping.

Edit: Brian, Make sure you have enough depth behind the hole for the tap to go into. If shallow, you may have to make a "bottoming" tap by grinding off some of the tip (length).

Brian, Are you running in salt or brackish water? Has the boat ever been in salt? I'm just trying to figure out why you are having this problem! Chris has a point with the pipe thread sealant possibly giving the threads more corrosion resistance. The very best sealant that I have found is Slic-tite. It's more expensive than others but it does contain Teflon which makes dis assembling pipe a lot easier.

Brian, It sounds like you just drain the block and manifolds. Although this was common practice for many years, it is now more common to put the plugs back in after draining and get anti freeze down into the block. Since it does have corrosion inhibitors in it, it would help.

Brian, You'll be fine with the internals. It's just the threads that got eaten up because of the small metal cross section (the tiny peaks of the threads).

Sounds like it has been a rough winter on drains. I have a 1986 2001, getting ready to tow to local dealer and have block drain redrilled. All threads rusted, have tried everything with no luck. Really hope this cures problem. It the future I have been told to pyt grease on the plugs and put them back in for winter. Good luck...

I tried rethreading existing hole, new brass plugs,new steel plugs,tape on old plugs, small hammer......

I installed the brass and plastic ball valves from skidim. My threads were ok but only have to install one time.   Easy to drain and replace with antifreeze