exhaust plug leaking

I have a Ford 351 PCM engine in my ski boat. There is a drip from the plug on the backside (the coolant hose is on the frontside)... I'm thinking I should be able to seal it with some high temp RTV of some sort, but figured I'd see if there is a specific product anyone recommends from having this issue themselves. ....go!

I'd say it depends on what has caused the leak. If your manifold has rusted bad enough it is leaking through the threads of the plug you may have bigger problems than just plugging a leak.   But first, what is the plug made from - Steel, brass or plastic?   if it is steel you may have a heck of a time getting it out of there anyway. If it's brass I'd expect internal rust issues that you may want to address first. Then again, it may be as simple as the plug never got tightened enough when drained last for winterization. But as a general rule, if the plug and manifold are both in good shape all that I've ever used is a few wraps with teflon tape. Since the plug is tapered, with the teflon you should be able to tighten it enough to stop the leak. If not, clean the threads out with the correct size tap, wash out the area to eliminate debris and install a NEW brass plug with teflon.
Still leaking? Might have an issue other than the plug ... such as a crack in your manifold that just makes it appear to be leaking from the plug.

bthorsen wrote: Why do you say no on the RTV? They make high temp stuff for exhaust leaks and head gaskets on a car... and I'm pretty sure the temp on my boat exhaust are lower than a car.

Ben,
RTV is great on flat surfaces but I've found to be difficult to get completely into threads such as pipe threads. What's the problem with Teflon tape? The normal tape you find is rated for 260 F. and you are no where near that on a water cooled manifold. Even pipe thread compound will handle the temp. There are time when I actually use both tape and compound. I prefer Blue Monster compound. RectorSeal Tru-Blu is a close second.

Did you lightly chase the threads with a tap yet? Have you taken a real close look at the manifold around the threaded hole to make sure it's not cracked?

No room for a tap with the pcm riser in place I suspect

However you can make a makeshift tap from an iron plug by cutting some grooves in it with a cutting wheel

GottaSki wrote: No room for a tap with the pcm riser in place I suspect However you can make a makeshift tap from an iron plug by cutting some grooves in it with a cutting wheel

Tom,
Great idea. I've used it several times in a pinch and it does work.

The alternate for tight spaces:

I installed a new 3/4 plug this weekend and that appears to have resolved the issue. After looking at the other side (I was going to swap the plugs to see if the problem moved to the other side) I realized how much material had degraded over the nearly 25 year (1996 Tige). I installed a galvanized 3/4 plug, not "black steel" from HD.

Pipe threads seal on the points of the threads. They are tapered so tightening can fix a leak but sometimes tightening a taper can crack the housing.
A new plug gave you new threads with sharp points on those threads which will help seal your leak.
Old used plugs have the points flattened out a little and take more torque to make them seal. In a strong base that works fine, in a cast steel manifold you made a great choice to install a new plug. Brass would be better as already mentioned.
How many old galvanized pipes have you seen rotted out in your lifetime. I have seen plenty.
Brass, not so much, it holds up.

gt40KS wrote: Galvanizing is a very hard coating as well. If you are having sealing problems anyway the hard coating won't help much if the threads are worn or rough.

Joe,
I hate to tell you but pipe and pipe fittings are galvanized and THEN the threads are cut so, there's no zinc on the threads. Also, you may what to look up the Rockwell hardness of zinc.