I noticed some seepage coming from my risers, so I replaced the gasket today. I also noticed corrosion on the exhaust manifolds. It's a 502 and put 400 hours on it now. How much longer do I have before I should replace the exhaust manifolds?

Close up,lower exhaust

, rust is getting soft.

The longer you wait, the more you risk destroying an engine.

Ayuh,.... The failure point is that gasket matin' surface, that you didn't post a picture of,......

The rust ya do show, is irrelevant,....

Ayuh,..... That's Better, but not good enough,..... The gasket is still there, blockin' the view of the top of the manifold,......

What yer lookin' for is the Width of the iron wall 'tween the water passage, 'n the exhaust passage,....
Pull the gasket off, 'n clean the top of the manifold with a big ole flat file, to clean fresh cast iron,....
The flat file helps ya keep the finish surface Flat,....

Once that is done, the iron wall I mentioned needs to be a minimum of 3/16" wide to support the gasket,....
If thinner, failure is just 'round the corner,.....

Thanks for the information, really appreciate the help. I'll just throw in a new one, rather than risk the engine, thanks again

Ayuh,...... Any time,.....

run in salt water or fresh water?

Check with SHUNTER here on OSO (Houston, TX), I know he just removed a set of stock manifolds from his 502 for aftermarkets. His are in pristine shape. I don't know if he is wanting to part with them, just a suggestion. Good luck