Exhaust Manifold Lifespan

[The Dog House]

I boat in fresh water and was wondering what the lifespan of the exhaust manifolds typically is? I had my previous boat for 14 years (bought new) and never changed the exhaust manifolds. The person who bought the boat from me is still running the same exhaust manifolds (18 years total). The exhaust manifolds were new when I purchased my current boat 5 years ago. I know in salt water the manifolds need to be changed regularly, but in fresh water is it still necessary? Is there some way to inspect the exhaust manifolds to determine if they need to be changed? I'm willing to spend the money to change them if necessary, but I'd rather not if I don't need to.

[Big D]

In salt water, 5 years seems to be the benchmark. In fresh water, heck my 69s are original. Short of dismantling and inspecting, pressurising etc., not much else you can do. Even if all that doesn't reveal anything, you can still spring a leak the next day, just no way to be 100% sure. The safest bet is to be proactive and schedule a replaced whether they leak or not. What's the magic number for fresh water? Personally I can't say because I've had very few fail and all were decades old. A lot of variables too, from usage, water conditions, type/make of manifolds, etc.

[prowlersfish]

What Big D said

[jimbo36]

What Big D said

+2 on that.

[Svend]

Here are some tips I have gotten from the many wise people on here...

You can use one of those handheld laser temp devices to check the temp of the manifold and compare with the block tempo.
If the manifold is considerably higher, the theory is the manifold may be clogged, rusted, etc... and need attention...

Svend

[captainmaniac]

I had one fail (cracked) when the boat was 21 years old. The rest are still doing fine after 37 years.

[lawyerdave71]

I had one fail after 37 years!

Mechanic said it was probably defective from new.

By the way, its not fun changing them things.

[Svend]

I have a question on manifolds and boats cooled from lake water...

once the water enters the boat at the bottom seacock, is the route through engine a pretty decent diameter from input to exit...
it looks like intake is around 1/2" - 1".... is this diameter kept throughout it journey to exhaust?
are there any filters in route?

this would be on my 1977 F30 Express with Chrysler 318's

thanks

Svend

[prowlersfish]

No filter unless you have a strainer before the engine . FWIW the trans cooler and( oil cooler if you have one don't think you do ) can get plug up .