Mercury 40 Classic (Mystery water leak)

[MuscleOnTheRidge]

Hey guys, Can you help me? I have a 1992 40hp Mercury Classic and noticed a small water leak while tank running it a couple days ago. Right next to the the top spark plug is a very small hex head plug or screw that has developed a very small trickle of water. It looks like it goes right into the cylinder, but that can't be. Any clues as to what this plug is, and what is the best course of action? The motor serial is: OC233564

Thanks for any advice.
MOTR
"Introduce your kids to the outdoors, and they will have a friend for life!"

 

[MuscleOnTheRidge]

Re: Mercury 40 Classic (Mystery water leak)

One of my buds explained that it is just a small plug and the leak is caused from a worn out gasket. He suggested pulling off the back plate to access and replace the $20 gasket. What problems should I anticipate when removing that cover from the motor? Help please.

MOTR
"Introduce your kids to the outdoors, and they will have a friend for life!"

 

[Laddies]

Re: Mercury 40 Classic (Mystery water leak)

He is right it's the water jacket gasket leaking, just use care then removing the cover bolts they do break some times

 

[TriadSteeler]

Re: Mercury 40 Classic (Mystery water leak)

Laddies is so correct. I broke 4 of them and finding the correct bolts is not the easiest thing in the world. (Finally found them at a Fastenall about 1/2 mile from my house). I didn't take any chances and replaced them all.

I don't know if the 1992 is the same as a '74 merc 500 but the bolts on mine had nuts on the other end that were held in place with ridges coming up from the casting. Getting these things started proved to be quite the challenge. Especially the ones at the bottom. Get yourself one of those telescoping magnets. Trust me it will come in handy to find the nut you dropped that you can't see. Also it helps to hold the bottom ones in place while you get the bolt threaded in.

After you get the cover off you will need to completely clean all old gasket material off of the surfaces. This is tough because the aluminum is very easy to score using most traditional techniques. Look at some solvents to dissolve the gasket.

When reassembling you must use a torque wrench and I think that model requires 180 inch pounds (Laddies correct me if I am wrong. I'm guessing that his 92 Classic with that Serial is an American 4-cyl.) If you don't have a torque wrench to get that low, Hopefully you have a buddy who has a torque wrench you can borrow. The ones at AutoZone will not work. They are all in ft/lbs and can't get that low. I happened to order one off of Harbor Freight for 19.99. NAPA had them via special order for 258.00. Make sure you test the torque wrench to make sure it clicks at the desired torque on some other bolt before you start cranking away on the aluminum case. Cracking that thing would be disastrous. The bolts also need to be torqued in their proper order and be done at 1/3 the torque value each time i.e. 60 on all - then 120 on all- then 180 on all. I don't have the torquing order diagram with me right now, but I will post it tomorrow.

 

[MuscleOnTheRidge]

Re: Mercury 40 Classic (Mystery water leak)

Thanks for all the great information. I will take all of the advice, and heed your warnings on the cover bolts. Luckily for me, Fastenal is based here in town (Winona, MN) and they have an outlet store a few blocks away. The torque diagram would be a great help. Thanks again.

MOTR
"Introduce your kids to the outdoors, and they will have a friend for life!"

 

[MuscleOnTheRidge]

Re: Mercury 40 Classic (Mystery water leak)

Very hesitant to throw this suggestion out there, because I don't want to get mauled. Just how critical is this leak? Like I described earlier, it is that very small hex head port or plug that is less than an inch from the spark plug #1 port. It is a little more than a drip, but I am not sure how it is affecting the engine performance, nor do I know what might happen if left unchecked. I know you guys are not about short cuts, but just some objective advice on the severity would help. Is there a way to seal it externally without removing the cover?
MOTR
"Introduce your kids to the outdoors, and they will have a friend for life!"

 

[TriadSteeler]

Re: Mercury 40 Classic (Mystery water leak)

I can understand your reluctance. Taking bolts out of your powerhead isn't exactly trivial.

However, Water is the universal solvent. It has a unique way of finding its way into small crevices fairly easily. Over time that drip will accumulate and work its way around your spark plugs and foul them out. The void in the gasket has the potential to continue to get larger. Then the water will continue to wind its way down the threads of the plugs. The blunt end of that spark plug is millimeters away from your pistons 1000-4000 times a minute. Pistons, rods, rings, cylinder walls and bearings are like cats. They hate water. It is a much easier job to take the water jacket cover than the crankcase cover. A $20 gasket and two days worth of labor is cheap insurance.

The option is always available to attempt a quick fix and patch it with RTV or some other sealant, but in the boating world a quick fix more often than not results in a major expense.

Oh, and here is the torque sequence for the water jacket bolts.

 

[MuscleOnTheRidge]

Re: Mercury 40 Classic (Mystery water leak)

Thanks for putting it out there straight. I will round up a torque wrench and get this project done! Thanks for the diagram as well.

I really appreciate the help.

MOTR
"Introduce your kids to the outdoors, and they will have a friend for life!"

 

[CharlieB]

Re: Mercury 40 Classic (Mystery water leak)

Before you start taking this thing apart, take a closer look at that 'plug'.

If that is a small pipe plug into the water jacket simply recoat the threads with liquid teflon to seal the threads and be done with it.

 

[MuscleOnTheRidge]

Re: Mercury 40 Classic (Mystery water leak)

Thanks Charlie.. I will check on that. It looks like a small hex head screw to be honest, and it is a very low profile head so it might be tough to get a tool on. Thanks again for the advice.

MOTR

 

[oldtruckface]

Re: Mercury 40 Classic (Mystery water leak)

different motor but sounds like water pressure fitting location