Hello All, I have a 99 stock mercruiser 502 mag ,the engine has 50 hours. I noticed some rust around from the stock exhaust and went to remove the manifolds I noticed most of the heads on the bolts are rotted away,no water entered the engine but what would cause this to happen. any suggestions and is this normal thing to happen.Thanks

If you are running in salt water and have steel, and not stainless steel fasteners... it's pretty normal for the stuff to dissolve in a few months.

Mercruiser still uses steel bolts on exhaust since stainless will not hold torque correctly. Best bet is to switch to stainless and give them a cinch about once a year. They are painted by the factory but it gives up fairly shortly from the heat.

In this case heat is your friend, you need a "heat wrench".... If you have time, spray it with some PB blast and let it sit overnight. I've also hammered 12 point metric sockets onto rusted bolts to get them off.

Gator Grip Works Like A Charm

this might help

http://www.offshoreonly.com/forums/s...ght=head+bolts

If you can get to them, just keep pounding on the next size smaller socket. I've found if you can get an impact wrench to them, the jarring effect helps bust them loose. A fire wrench works as a last resort. I wouldn't worry abouth ruining the manifolds, they are probably shot anyway. Once you get twisted off small enough, the manifolds should slide over the bolts, if not, get an angle cut grinder and a metal cutting blade and cut the damn heads off. Watch where the sparks go.

Once you get the manifolds off, get a GOOD pipe wrench and back the bolts out of the heads. You should be able to get PB Blaster to the threads with the manifolds off.

Don't ask how I know.

Weld a nut to them and remove them while their still hot.

I've seen motors have to come out because of this.
I don't entirely believe the SS vs steel thing being totally due to torque holding differences. It just plainly costs way more to make them with the right grade of stainless bolts . I use the THE stainless bolts from the hardware store and they just plain work.
I even suggested to a local marina to change the bolts on a new boat to save from headaches later.
I have seen hard stainless studs, and then some dope puts steel nuts on them and were in the same boat.
Also seen grade 8 hardware, and although better, still rusts, and is harder to remove and really breaks sockets, and resists being persuaded by chistles and drills and easy outs...
Had good results with carbide die grinder in the past. BBB

you may have had a leaker at the gasket on the riser to the manifold. Some will leak to outside before inside goes.

Yes it is a royal pain in the azz. As everyone else posted, some will come out easily, some need smaller socket hammered on,others a little heat, and probably a few will need the die grinder.

I ended up yanking both motors when I did my Stainless Marine manifolds. Still a pain, but it was worth it for access Plud wasn't wild about trying to get a torch in the tight bilge.