Hi guys, I haven't been here for a while, but I still love you all!

My 95 Sport, 351 is leaking on the starboard side. I replaced the gasket. Replaced it again with RTV. still leaking...pretty badly. I'm running in brackish water, so this is especially not helpful to my engine block.

What are next steps? I thought I'd prepared both surfaces very well with block sanding, making sure to keep everything straight/even, but it's apparently not good enough. Do I need to take it somewhere to have the manifold ground down a bit and leveled? 

Skied this morning anyway. 

If it's leaking externally, it may be leaking internally too, but you can't see that happening.

You could pull the plugs on that side and turn the engine over to see if any water comes out and also look for water on the plugs.

A  machine shop planing job on the manifold and riser would be the second best solution.

First best but pricey  Not knowing how many hours on run time you've had in brackish water, you might at least think a little bit about maybe getting  new manifolds and risers depending on how ugly the internal water passages look

There isn't any pitting on surfaces that meet. I thought I'd sanded surfaces evenly. I tightened manifold bolts from middle out in both directions.

At least I hope you're not leaking water there 

If it's coming from one of the flanges with the red lines pointing at it, you've got problems.

The only thing that's supposed to go through those joints is exhaust gases

https://ibb.co/21xCQXsq" rel="nofollow">

Having a lot of water dripping could be a few things.

Your manifolds are only 4 years old, so they're probably good and not leaking internally, but one could be bad

That would leave some other choices like maybe a blown head gasket letting water into a cylinder or maybe a cracked head or block letting water into a cylinder..

I'd start with pulling the plugs like mentioned earlier in the thread and look for water on a plug or plugs, then crank the engine over with the plugs out and see if water comes shooting out of any spark plug holes.

That should tell you something

While the plugs are out, a compression test would give you some info also

Pressurizing the engine cooling system with the hoses to the manifolds blocked off,  to let you know if the engine has a leak or not is also relatively easy

 you have 3.5" risers? are they as new as your manifolds? PCM had a rash of 3.5" risers with internal leaking. and or units with thin inner castings that could potentially leak sooner than later.

keep in mind that the 3.5 PCM risers are on back order right now and it does not look like they will be available earlier than June. if you suspect the Risers causing water intrusion you might want to make an inquiry here on CCF for a clean set of used ones.

here is the part number for real deal PCM riser gaskets. www.nautiqueparts.com

RM0002

I haven't done anything yet except examine the manifold to riser gasket and all sides of the manifold. The water is almost certainly coming from the flanges as pictured above by Ken. I haven't pulled plugs yet, but am going to be really surprised if there is water in a cylinder. Boat runs great, doesn't get hot, and no excessive steam in exhaust. I'm guessing (hoping now) at this point that there is an inner casting that is has failed like you suggested and the water is escaping from the manifold gasket (connection to block). I realize it could be worse than this. 

My question is--knowing the history you do about the risers, is it more likely the failure is in the riser than in the manifold itself? 

You say the boat runs great, and to get water out the exhaust manifold to head gasket, you'd need to have 2 separate issues. That would be a leak into the exhaust from either the engine or the manifold/riser plus your gasketed joint would have to leak.

As I think about it, it's probably not to likely to be the case.

You didn't say what version of the 351 you have, but your diary entry says 310 or so HP, so I igure you have gt40 heads.

One thing about those gt40 heads is that they tend to crack the water jacket right near the exhaust ports and leak externally which would make a leak like you're seeing.

You should lay on the floor  and get a good closeup look for cracks in the head.

People  have had this problem and you might say that JB Weld or whatever epoxy you might likejust might be your best friend for a while maybe longer 

It might buy you years of extra time or at least this season so you can go skiing instead of making repairs now.

Here's a link to some reading about the head cracks

http://www.google.com/search?q=correctcraftfan+gt40+head+cracks&oq=correctcraftfan+gt40+head+cracks&gs_lcrp=EgZjaHJvbWUyBggAEEUYOTIGCAEQRRhAMgcIAhAAGO8FMgcIAxAAGO8FMgoIBBAAGKIEGIkFMgoIBRAAGIAEGKIEMgoIBhAAGKIEGIkF0gEJMjAxNDBqMGo0qAIAsAIB&sourceid=chrome&ie=UTF-8" rel="nofollow - link

and another link to one thread in particular

http://www.correctcraftfan.com/forum/forum_posts.asp?TID=31923&title=cracked-cylinder-head-92-snob" rel="nofollow - link

Not the best thing, but not the worst either.

PS......I'm looking for a picture but not having any luck