Replace exterior rusted manifolds?

vector4

Member
Jun 24, 2015
59
Baton Rouge
Boat Info
2000 Sundancer 270
Engines
310hp
First I have a full closed cooling system. This is a 7.4l mpi with 740 hours on it. I yanked my risers and they are not terroble themselves but going to replace them. Now my manifolds have surface rust pretty decent. In fact the screws into the block are rusted and will need to be cut off and drilled out. That's the extent of the surface rust.

Now the inside of the manifolds and riser spacers looks absolutely fine. Barely any surface rust on the interior walls at all. Is the rust on exterior surface enough that I should replace them now? Or should I continue to use them as they are working fine?
 
Were there symptoms leading to pulling the manifolds?
Engine temperature?
Water seepage?
Hot spots?
One side running hotter?

Or was this just routine maintenance and all was fine operationally?
 
Sorry, didn't grab pics when I was there today.

I'm pulling the motor to have the coupler changed. Thought it's a good time to do manifolds if need be. I've known the bolts are rusted badly for a while. No issues with exhaust otherwise. The shop can do it while the engine is out but I'm looking at a decent cost to have them screw around with extracting the studs after they cut them off.
 
Sorry, didn't grab pics when I was there today.

I'm pulling the motor to have the coupler changed. Thought it's a good time to do manifolds if need be. I've known the bolts are rusted badly for a while. No issues with exhaust otherwise. The shop can do it while the engine is out but I'm looking at a decent cost to have them screw around with extracting the studs after they cut them off.

Often you can hammer on a smaller size socket to remove the studs or drill off the heads and vice grips after the manifold is off. Not a big deal if the motors out of the boat. If at all in question..replace them.
 
Given there were no symptoms I would think your passages are fine. The mating surfaces are the concern.

Was water leaking from the joints causing excessive rust? If you have surface rust is that degrading the mating surfaces where the gaskets keep water passages separated? Any problems there then manifolds must go.
 
First I have a full closed cooling system. This is a 7.4l mpi with 740 hours on it. I yanked my risers and they are not terroble themselves but going to replace them. Now my manifolds have surface rust pretty decent. In fact the screws into the block are rusted and will need to be cut off and drilled out. That's the extent of the surface rust.

Now the inside of the manifolds and riser spacers looks absolutely fine. Barely any surface rust on the interior walls at all. Is the rust on exterior surface enough that I should replace them now? Or should I continue to use them as they are working fine?
If you end up having to drill bolts out use a left hand drill bit. Works great and most likely the bolt will back out on its own.
 
If the bolts are rusted and need to be cut off, do it now with the motor out of the boat.
It’ll be much easier than having to do it with the motor in the boat.

Heating them with a torch is much safer to do in a workshop vs. in your bilge.
 
If the bolts are rusted and need to be cut off, do it now with the motor out of the boat.
It’ll be much easier than having to do it with the motor in the boat.

Heating them with a torch is much safer to do in a workshop vs. in your bilge.

Espos is totally right. Don’t delay that type of job - much easier standing upright in a workshop than hanging upside down using a mirror to see anything. Also much less likely to cause more damage while doing it.

My concern is the reason they are so rusted. If your manifolds were leaking so bad, the mating surfaces must be trashed. Check your valves for rust.
 
In the photo, the rust around the cylinder to the far right was caused by a manifold leak above the bolt. I almost had to cut the bolt head off it was rusted so badly. The leak also caused surface rust on the manifold. All the other bolt heads were fine. This motor is 19 years old.

I am curious why your bolt heads are so rusty they have to be cut off?
 

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Big blocks like his attach differently than yours, they're short bolts and hold the manifold on in an area that has no cooling. So they get way hotter than yours and heat promotes rust.
 
Big blocks like his attach differently than yours, they're short bolts and hold the manifold on in an area that has no cooling. So they get way hotter than yours and heat promotes rust.

So on the 5.7 (my other boat) and this 4.3, the manifolds at the head run cooler than big blocks by design?

I have dealt with V6’s, small block V8’s, outboards, and diesels but never a gas big block.

After 20+ plus years, I still learn everyday!
 
Change them now. You will be way ahead of the game. Murphys Law - change them now, you know you are good. Wait . . . . and half way through this season you will wish you had. Just did both of my 496s this past fall. No real overheating problem, just a slight rise in temp on one motor when running hard (3800 rpm). Also had no record from PO of when they were last done. Normally cruise @ 3250 / 3400. Was soooo glad I did. After getting them off found all 4 of them were in danger of inner wall breakdown. Could easily push a screwdriver through in more than one place. Merc parts guy said I was just 1 cruise short of a catastrophe. You really cannot tell their true condition from the outside as the inside is what rusts through first and causes the problem. Mine looked great from the outside.
 
So on the 5.7 (my other boat) and this 4.3, the manifolds at the head run cooler than big blocks by design?

I have dealt with V6’s, small block V8’s, outboards, and diesels but never a gas big block.

After 20+ plus years, I still learn everyday!
The manifolds don't run cooler at the head, the bolt head does. That area on the BB where the bolts are, get so hot they wont hold paint. On the small blocks the bolt heads are much further away from the heat.
 
I don't have any logs from when they were last changed. I've had this boat 4 years and about 300 hours.

I guess from the recommendations here most people are saying go ahead and do it, which is what I was leaning towards. It's mostly on the port side bolts and I think it's from drive fluid leaking from the reservoir and getting on them mixed with the heat.
 
I don't have any logs from when they were last changed. I've had this boat 4 years and about 300 hours.

I guess from the recommendations here most people are saying go ahead and do it, which is what I was leaning towards. It's mostly on the port side bolts and I think it's from drive fluid leaking from the reservoir and getting on them mixed with the heat.
Drive fluid and heat isn't going to make them rust.
 

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