Repairing Cracked Mercruiser Exhaust Manifold

This 3.0 L Mercruiser had a cracked manifold. Rather than drop $500 or more on a new manifold, I wanted to see if the cracks could be sealed up without removal and without welding/brazing. This repair only took an hour and required a $20 tube of sealant. When testing, make sure to carefully monitor your engine oil to ensure it is not rising and is not turning milky - those symptoms would indicate water is mixing with the oil. That would signal a problem with the engine or internal manifold water passages. If the manifold damage is only on the outside, then this video may help you get more life out of your old manifold. If the manifold is cracked on the inside where exhaust and water are supposed to be separated, then you have no choice but to replace the manifold. The sealant will flex as the manifold shrinks and expands so I expect the repair to hold up for several seasons. Time will tell. It passed several heat/cooling cycles on river tests with no leaks.
The Right Stuff sealant:
Amazon.com affiliate link (earning me a small commission)
amzn.to/438avFK - I used black but red works just fine.
Amazon.ca affiliate link (earning me a small commission)
amzn.to/3BZ8u2V
0:00 Intro & Disclammer
0:41 Why do manifold crack?
1:03 Method to verify the repair is working and will not cause engine damage
1:22 Review of crack to be re-sealed
1:51 Overview of tools required
2:05 Preparing the surface
2:38 Description of sealant and application method
3:41 Testing the repair with ear muffs
3:52 Verifying the internal manifold exhaust passage is good
4:11 Testing in the river

[@Flymakefish]

I like it! For the older boats that aren’t worth dumping tons of money into this is the way to go.

[@jamesj.litteriojr.433]

Nice Fix! 👍 And Haters are gonna hate!!! Just move on! 😉

[@seanzielinski2007]

Is there any concern with carbon monoxide leaking through that crack, or is that only a water passage?