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40hp mercury

Probably just a water jacket cover gasket. Mercs dont have a head just take the cover off and get a new gasket clean all the surfaces good and use a light coat of aviation permatex on both sides of the gasket. Thats the brown stuff you brush on get the bolts good and snug and let it sit overnight then you should be set to go.
http://www.ebay.com/itm/Mercury-40-..._Accessories_Gear&hash=item4ac14ec70d&vxp=mtr
Depending on your HP and serial number it should look similar to this it`s just leaking out the water jacket cover around the spark plugs. Real common. Be real careful loosening the bolts heating it up around the bolts prior to loosening helps. The bolts tend to break real easy the older the motor. Take your time!

took water jacket plate off have a small hole in cylinder wall above top spark plug looks to be old but not sure if the motor is toast looks small enough to patch and what should be used
 
Seen that many times, caused be overheating due to a bad impeller.

The cover has to come off. The bolts will most likely snap off if you simply unscrew them, so you'll need an acetyylene torch to heat the nuts red hot BEFORE trying to remove them. With the cover off, you should flat plate sand it flat, then install a new gasket with lots of silicone. And use all new grade 8 bolts!

Jeff
 
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