I'm looking at a boat that has Merc 525s' in it with about 210 hours. The original CMI headers were replaced because the owner said they discovered that they had a small leak. He still has the old ones. How or what can I check to see if any permanent damage was done to these motors with the old leaky headers? I don't know yet how many hours they were run that way.

Have a certified Merc Mechanic look at them! Then you u know for sure!

If the engine had water damage, it would be obvious. If it runs, there is no damage. Hydro-lock is catastrophic.

Ok...Thanks for the input.

Not necessarily.

I had leaky CMI headers that damaged the valve seats and block, but they still ran.

Ultimately required complete rebuilds, block welding, new heads, new headers etc Very expensive.

Get them checked. Remove the headers and look.

+2
Had a valve stuck due to the CMI leaks, motor ran ok in idle, pretty rough above 1500- 2000. Ne valves, machining, the works. AND I WAS CAREFULL with those damn CMI's. My issue was due to reversion not leaks in the headers.
Get the engines checked and have them do a complete leak down. THAN you know.

Couldnt be farther from the truth.

Its not catastphic,,,,,they get pin hole leaks that eat at the valves

Turns out the old headers had an external leak only. They were replaced in 2010 and the new ones have about 60 hours on them. Judging by when they were replaced, does anyone think that the new headers have the new "cool collar" design? How can I tell the difference?

If they are 2010, they should have the cool colllar. The outer tube doesn't go all the way to the head flange.

And since there is a gap at the collar (1/2" or so)of outer tube with cooling water, this is where they crack and leak. The small length of inner tube is not cooled, pretty much the same temp as the flange and then hotter then the rest of the tubes with the "wrap" around. Easier to manufacture "Cool collar" my A$$.