View Full Version : Rusted exhaust manifolds

11-05-2010, 10:19 PM
I was poking around my engine today and noticed that the exhaust manifolds have really bad rust/flaking on the bottoms. There are some thick chunks coming off them. It is a 98 and I'm sure that they have never been touched but should I worry about this and spend the money to replace them? Something is obviously leaking but I'm new to the boat so I don't know how bad or if some leaking is normal. Any thoughts?

11-05-2010, 10:52 PM
It's normal for rust to develop on inboard manifolds. It does get worse if not maintained each year. Unless they are leaking I wouldn't worry about. What does interest me is the fact it's doing that in such a dry climate as Arizona. Anyway, get a stiff wire brush and scour them clean then paint them up. Quicksilver makes a great paint made for Mecruiser that's super heavy when dry and protects well. Or just use a good exhaust manifold paint.

11-05-2010, 11:10 PM
Rust is oxidation, which is a heating process (exothermic). The exhaust manifold at the bottom as it bends upward is one of the hottest places on the engine/exhaust. This, along with moisture, promotes rust specifically in that area. It is 'normal' and every inboard will do it.

I use a wire brush and knock off as much as I can and then hit is with some black paint from a spray can. Do it each year when you winterize and your manifold will last a long time and look good too. Those things are quite thick, so you should not need to replace it. However, it's tough to say without seeing.