I bought this engine in pieces and cheap due to it "needing a piston". Visual inspection shows a very clean engine, two other normal-looking pistons, and undamaged cylinders. Just the top (#1?) piston is damaged as shown.
Just wondering what might have caused it. Don't want to repair it, only to have it happen again. The previous owner spoke of "last-year's gas/oil mix as having gone bad".
Re: 1986 Mercury 60HP Burnt Piston - What Caused it?
2 things it could be or a combination of both.
Overheating or detonation.
Possible for a motor with a good impellor to get air trapped in the block on certain models cooking #1 while still having a good telltail.
Also blockage in the carb will cause it to run lean.
Looks like your piston is damaged from the center out suggesting the plug is running too hot or detonating.Start by checking impellor and cleaning carbs and setting timing.
Re: 1986 Mercury 60HP Burnt Piston - What Caused it?
Overheating; bad gas; too much timing; overloading the engine by propping it too "tall" for it to achieve maximum recommended W.O.T. rpm.
Any one of these or combination thereof could have caused that type of damage.
These 'small-bore' 3-cyl's are very overstressed and don't handle large loads or lugging the motor very well.
Check the top cylinder very carefully for cracks. You'll want to remove the water jacket cover over the spark plugs to inspect the outer side of the combustion chamber's dome for damage.
I've seen quite a few of the old 650 and 700 Triples crack #1 cyl when they lose an impeller and overheat underway. The 60 is just a 'prop rated' version of these motors.
From what I understand there is also supposed to be a mod performed to the blocks, to relieve a potential 'steam pocket' at the top of the block which can crack #1 cyl. The water telltale hose should be located at the top, which will vent the block and ensure it's completely flooded with water. If the fitting is not already there, you can <very carefully> determine the right spot to drill the block and tap for a 1/8"-NPT fitting for the telltale hose. Then plug up the original location of the telltale fitting.
Make sure the water pump and impeller are A-1.
When you get 'er back together, set the timing for no more than 21 deg BTDC. Use midgrade or premium, non-alcohol fuel if possible. Use a quality TCW-3 oil. Prop the motor to be at the top of its rpm band at Wide Open Throttle, with a lightly-loaded boat. Probably not a bad idea to replace the impeller yearly, too.
That should help keep it from going thermonuclear again!
1986 Mercury 60HP Burnt Piston - What Caused it?
Linear Mode
