'84 Mercury 115 #3 cylinder

I recieved a free parts boat with an '84 Merc 115 on it, I think. Serial #6525418. Number 3 cylinder has 20 psi, rest are good. I'd like to get this motor up again for use on a project pontoon boat. My internet connection is horrible and I've spent all morning "trying" to dig through these forums for good info at an impossibly slow speed. I would greatly appreciate any direction from all of you. I own a small automotive/speed shop I lease to a couple mechanics. They are good friends with great tools, so I think the tools and labor won't be an issue. I just need some good direction including the tips and tricks I don't want to have to learn the hard way. Thanks again.

Re: '84 Mercury 115 #3 cylinder

izzybird said:

I recieved a free parts boat with an '84 Merc 115 on it, I think. Serial #6525418. Number 3 cylinder has 20 psi,

Click to expand...

First you should find yourself a manual covering the classic inline six engines......

Then you can start tearing down the engine, one special feature on tease engines are their blind bore cylinders ( no lose head ) + their needs for special tools to install pistons... ( the correct way to install them are to mount the whole rotating assembly and then use some special tools to put them in all a once.. )

But your 84 should have the later rods, they can be easily be installed one at the time without the need for tools



When teared down inspect the parts for wear and damage ( specs are in the manual ), badly worn or damaged cylinders can be bored ( 0.015 or 0.030 ).
Pistons can be easily found ( Mercury , Sierra, ProMarine ( last 2 from Persan ), Wiseco doesn't belong in a inline 6 engine.... be sure to use 6 indentical pistons ( you should have low dome power port pistons )

Mercury STD pistons 774-9137A9
Mercury 015 pistons 774-9137A10
Mercury 030 pistons 774-9137A11


Gaskets can be Mercury or Sierra, same with bearings.
New rings on the good cylinder can be Sierra or Mercury, unless someone have installed Wiseco ( might not be compatible with oem spec/parts )...

Rebuild the carbs when your at it, parts for the fuel pump are included in the oem gasket kit.

Last part to be replaced are the impeller....AND the propeller..

When done, prop it for 5600 > 5800, the reason for your low comp on #3 are probably to big a propeller ( engine didn't come up to it's sweet spott.)

As for timing and adjustment... take a look in the FAQ

Online parts breakdown.


Arne Kjetil

Re: '84 Mercury 115 #3 cylinder

Thanks for your input Droll. Took a look and a chunk of the top of piston is gone exposing part of the top ring. I'm looking for a donor motor now and going to rebuild the 115 powerhead in my spare time. (Ha, I think I just made a joke) I read some info from Clams that stated info on swapping powerheads, and what I got is you can swap distributor inlines with the same and adi ignitions inlines with the same. Did I just interpret that wrong, or will they all bolt up to my '84 leg? I've found a couple donors, but they were both older distributor type.