Replace intake manifold?

Hello, I have a good running 340 in my old 74' century resorter. I have done all of the exhaust manifolds within my ownership, they had a couple hairline cracks so I did it to avoid having more serious issues.

I am wondering if I should be thinking the same way about the intake manifold. It is original. Looking inside the thermo housing, it's very rusty and rough looking inside. Is this a point that will eventually rot out and cause engine damage? Anyone have a good source for a marine manifold? Is it the exact same manifold as a 318?

Thanks,
Steve

Not sure, but I believe the ports on a 273/318 may be smaller than the 340/360.

The cast iron 4 bbl intake manifolds on these engines are heavy and not prone to rusting out. The bottom side is bathed in oil so rust free (picture 1). What you are seeing is the heat damage to the paint caused by the choke pot cavity exhaust passage (picture 2). You could clean it up a bit with a wire brush and repaint it if you like, but the heat will likely bake the paint off again.
The intake manifold will have a casting number on it just forward of the distributor (picture 3) which should be used when and if a replacement becomes necessary.
In short you are quite likely OK, unless your engine has been exposed to a the weather for some time.

Thanks for the replies. I am not concerned with surface rust on the outside of the manifold but the corrosion from the water inside the manifold. When I open the thermostat housing there is rust inside the manifold and the surface is quite pitted. Has this been an issue for anyone?

My engines are closed loop and antifreeze runs through those passages. The inside of mine still looks good.

Toonl said:

My engines are closed loop and antifreeze runs through those passages. The inside of mine still looks good. View attachment 20962

I'm guessing yours must be an open loop system exposed to

raw water or even salt water perhaps?​

​

Click to expand...

Yes, mine is running raw water. No salt ever. Mine looks a lot worse than yours.

I'm afraid that is about as far as I can go then. Surface rust is not an issue, and should the condition be worse it is impossible to us know the condition of your centerhead without actually inspecting it. If you are uncertain, you could have a local mechanic give it a quick look. I hope it turns out to be OK.

It's pretty hard to pressure an intake manifold if you don't have blocking plates, so you may want to consider this.

Find a local shop that will "boil" it in a cleaning tank. Then once it's clean, give it a good inspection (NDT mag-particle if you have capability).

If you can't mag-particle inspect, give it the screwdriver and hammer test. Use screwdriver to try and poke a hole in clean cast iron manifold-pretty hard to do if it is in good shape. If it has been compromised, it'll likely give way.

Then, hang manifold up in garage somewhere from a string tied through a bolt hole. Lightly tap areas with ball-peen hammer and make note of sound (resonance). Like any object that isn't cracked, it should have a certain "ring" to it. If you hear a thud, investigate that area and make a determination of integrity.

Or, send it to a local NDT inspection shop.

Good luck, Noel.