25 hp Merc - Leaking from water jacket?

[jakeboomer]

I have a '88 25 horse mercury (ob246376). There is a small water leak from the gasket on the bottom cylinder. Does this motor have a water jacket and a cylinder head unit. The diagrams in the manual show a single head unit and a gasket, but doesn't show the head having locations for the spark plugs. Is there two pieces to the head unit, i.e. water jacket and head? What needs to be removed to fix the small leak from the gasket. Can I just use permatex or some type of sealant. <br /><br />The engine runs great with compression about 120 on both jugs.<br /><br />Thanks for you help. I have been looking all over this site, but still confused.<br /><br />jboomer

 

[Laddies]

Re: 25 hp Merc - Leaking from water jacket?

it's only a block cover, change the gasket

 

[rabidfish]

Re: 25 hp Merc - Leaking from water jacket?

Ditto.. There is no head, it's cast into the cylinder block.<br /><br />Change the gasket, be careful to get a good seal around the spark plug holes. A little sealant goes a long way here.

 

[jakeboomer]

Re: 25 hp Merc - Leaking from water jacket?

I guess this is what I am confused about. The spark plug holes don't look like they remove with the water jacket (head cover.)<br /><br />The diagram on mercurypartsexpress.com only shows the block from the crankshaft side, so I cannot see the head config. <br /><br />Thanks again for all the info.

 

[Laddies]

Re: 25 hp Merc - Leaking from water jacket?

They don't, the piece your looking at that that is leaking, is a water passage that allows water to move from one cyl to the other

 

[BaysideMarine]

Re: 25 hp Merc - Leaking from water jacket?

Many Mercs are built with what is called a "blind cast bore". The plate over the top of the cylinder is simply a water jacket.<br /><br />The "lands" or "pads" where the spark plugs go are cast into the block itself.<br /><br />Often what happens is that over time these pads are corroded away and a gap opens up between the pad and the water jacket plate. The gasket that is designed to seal between the plate and the pads can no longer do so and a water leak results.<br /><br />If a gasket is not sufficient to fix the problem then a product such as Devcon (a metalised putty)is required in order to build up the area around the pad (it can then be sanded to the right shape) and then with a new gasket the problem should be solved.<br /><br />Another, more temporary fix, is to thoroughly clean around the sparkplug holes and then apply a thin bead of silicone around the perimeter of the hole. This will provide a short term fix to the problem.<br /><br />Cheers,<br />Nicholas