Flying J: see below link.

http://www.offshoreonly.com/forums/s...hlight=525+496

Thanks for the link

I guess that means fiberglass work on the transom but everything else is plug and play

The thing I've noticed aboutt he 525 headers is that they seem to terminate very close to the transom - doesn't leave alot of room to re-route to the existing 496 exhaust holes in the transom, and I wonder if you could ever go with CMI's silent choice system and have any room left at all to reroute...

The 496's don't have as many problems because of the HP/CI ratio.

496 / 425
502 / 525

The displacement difference is negligible (let's not get into too many bore x stroke and compression details) but the power is 100 hp more.

Ream the piss out of a 525 vs. a 496 and chances are the 525 is going to break first even though you'll have a lot more fun doing it

I believe what you are seeing terminating close to the transom is the "tail pipe", not the header. The tail pipe clamps to the header and is considered part of the CMI exhaust system (as in they must go together). Regarding silent choice, Merc. Racing and many others are now saying Do Not run a silent choice with the 525's. They are also recommending to not run an external switchable exhaust. Either straight loud exhaust or a fixed "external" muffler system is recommended.

What is the best way to monitor the header issue....check for milky oil everytime? Are there external hints on the headers themselves that might reveal a problem before it gets to serious?

You will start to see whipped cream/moisture in the valve covers first.

Please excuse this shameless commercial plug but I have a fresh - direct from Mercury - full warranty - brand new exhaust - 525 I can sell for $25,000

is there another exhaust system out there that can be used if these are such a ticking timebomb?

I have a some of the creamy stuff in my lines from the valve cover vent on my 502 mpi, but i took it as just condensation. I run the vents up to my eddie marine flame arrestor. Engine has IMCO headers with the feed through from the manifold to the riser. No external hose fittings. Something to be concerned about, or not?

Any engine will create water vapor in the crankcase, it is a combustion by product. As the vapor cools the water condenses. So a little bit is normal. Any water that makes it to the combustion chamber from a leak or reversion from the exhaust is also going to wind up in the crankcase (at least a portion of it). I don't get nervous until I really see water in the valve covers.