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402 water coming out spark plug hole

the gasket on the waterjacket cover is a common failure, remove - replace - torque to 100in/lbs with locktite 271.

there is no head gasket so if its anything else its done for...jb
 
omc_10 said:
^sounds right. and why is there no head gasket on this one? what engine is this?
Click to expand...
the head is a one piece cast which is part of the block, standard merc design, this is a merc 40hp 2cylinder 2 stroke
all the small merc are this way
 
For many years Yamaha built the powerheads for Merc (and didn't use them on their own models).

Merc's design was for a "headless, split block" the cylinder is completely within the upper half of the block itself - this allows the water jacket to completely surround the cylinder (with the exception of the little hole for the sparkplug).

So instead of a "head" holding in the combustion pressure, you simply have a "water cover" where the head would be on any other motor which only has to hold back a max of 15 psi of water pressure.

Of course this design does have it's downsides - one of which is machine work is a little more difficult if a cylinder requires a bore and on some models the allowance was never made, so if you toast a cylinder, the block is scrap (just added incentive to maintain your motor)....
 
well i did think it was the waterjacket cover gasket and replaced it and it still leaking
its around spark plug so i remover the spark plug and started it and water was forsure coming out of it
 
so you cant get it bored out and sleived if its cracked if so this may be $200 wasted
ive been looking around and i dont see any rebuilt or new power heads so if its bad could i put a 35 hp power head
 
Two years ago I decided to replace the above mentioned gasket on my 1983 25HP Merc since it had a small leak (and water showed up around spark plug too.) The cover is secured with six stainless steel (SS) bolts, which are metric - smaller than 1/4" and thread finer that 20 threads per inch, i.e., bolts are smaller than a 1/4 20 bolt. SS and aluminum is not a good mix; I broke two of the bolts and eventually bolt extractors (made of hardened steel) and drill bits in each broken bolt. This task turned into a several month's nightmare. Finally got it back on in a very unorthodox way after coming close to it being impossible.

Be careful!:mad:
 
water in the cyl can also come from the exhaust cover/cooling plate. It has 2 gaskets that can fail and the cover itself can crack. I would think if this was the case both cyl's should show some water damage. I don't know for sure how far back you can go with the 402, but the 35 and 40's are the same, the 402 just being older.

Could also back up through the lower powerhead gaskets with enough backpressure I guess
 
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The exhaust gaskets and base gasket JB mentions total less than 10 bucks at a Merc dealer - so that and an hour of your time and you can replace them.

The motor could well last another 10 or 20 years if you correct the problem - you are liable to kill it in less than a season if you don't...
 
I was thinking over a drink, if you have access to a leak down tester you could remove the water cover, charge the cyl with air and spray it with soapy water. Any crack would show up, if not it must be a gasket or something else. You wouldn't be over here in St. Louis Mo. I got the tools, if your buying. I think Graham is under estimating the time involved to get the exhaust and head off though, but I don't do this for a living. jb
 
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yeah im going to take it apart and hope for just gasket stuff i was thinking about the all thing to ill try that first got xter gasket anyways think you all for the help
them dome cyl's was weird when i seen it the first time
 
If you haven't tried to remove that exhaust cover yet, try this when you do: Use an impact wrench set on the LOWEST setting that will turn the bolts, and take your time. I've only broken off one bolt with this method, which is stunningly successful!! (ask others who have tried to simply unscrew the bolts.)

Now, when the cover is off, look for a hole in the aluminum inner baffle. If you see one, great! Make up a new cover from 1/8 " aluminum and go boating. But if you don't see a hole, scrap the block. Why? It's cracked, and I've seen one of these already. (Se photo.)

Jeff
 

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so i got it all about think the exhaust cover gasket was shot looks like someone used it in saltwater to me any was the pistons look good for 1974 but still only gaskets looked oily
and bolts was loose so im going to clean it up and put new gaskets is the 1/8 aluminum plate a good ideal looks to me it won't get water there anymore
 
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