Blown/burnt piston

Mercury Black Max, 150 HP 1989 V6 motor, Running great, then quit. Wouldn't turn over, motor locked up.
Pulled heads, it has one burnt(melted) piston. #2 (center) on port side. What are the pros and cons of boring/honing .015 or .030 on just the one cylinder and replacing piston, rings, etc. I can't afford to replace the whole powerhead and everything else looks fine.

Thanks

Dan

Re: Blown/burnt piston

Oversized pistons are made to weigh no more than std-bore pistons and boring-out one or several cylinders will have minimal, if any, effect on engine balance.

If the cylinder is scored or has excessive wear/taper, have it machined to the next oversize that will clean it up. If that's .015", more's the better since you'll have one more opportunity to bore to .030" in the future if anything happens.

If it won't clean up at .015", pretty much no choice but to go to .030". Beyond that you're looking at re-sleeving that hole back down to std size or scrapping the block.

Best thing to do is get 'er all torn apart, run a glaze-buster/hone down the failed cyl and see what it looks like.

Last thought, besides fixing the mechanical failure, best to also address what caused it to fail in the first place (plugged/lean carb, timing or ign issues, etc), lest it just do the same thing again after you spend a lot of time and $$$ fixing.

HTH & G'luck...........ed

Re: Blown/burnt piston

be sure to check and see if you can find a reason why you burnt that cylinder down.... im no pro ... but i would suspect there is a reason for the burn down

Re: Blown/burnt piston

There are no cons to boring and replacing one piston. Even at .030, the difference in displacement is so small--less than one cubic inch--that the engine will not even notice it.

If only one piston is melted, then yes only one needs replacement. However, if more than one is damaged, say three on a six cylinder engine, then it is worth boring and replacing all. It takes the same amount of work to disassemble and reassemble the engine and essentially, you will have a new engine.

And remember: Even though the engine appears to be complex, it is really simpler than an auto engine. Very few special tools are required and you can do all the work yourself with the exception of boring the cylinder. A good manual and help from the people here on the forum are what you need. You will save gobs of money and have the satisfaction of a job well done. You will also learn all about your engine and be able to avoid future problems.

Record every step in disassembly with your digital camera. The engine will be apart for a bit of time and you will forget electrical wiring and hose routing. You download the photos and view them on your computer. Assembly becomes MUCH easier.

That engine is a multiple carb engine and undoubtably one of the carbs went lean. Find out why and correct it during the rebuild. Lean running is the PRIMARY cause for melted pistons in outboard engines.

If the engine locked up, there is a possibility that it is damaged too far to be rebored. In that case, a cylinder liner can be replaced. While it is more expensive than simple boring, it is not unaffordable. Probably around 350 bucks. Take the engine to a competant machine shop for evaluation. Ask around: Some shops will not have the expertise to properly bore a blind hole (outboard cylinder) or replace a cylinder liner.

Re: Blown/burnt piston

Thank you for your response to my burnt/blown piston on my Mercury 150hp BlackMax. I have some additional information and have some additional questions. I bought the boat second hand and original owner says no major repairs were ever done and he bought the boat new. A quick personal note: I used to make my living as a mechanic and have done many rebuilds of automobile engines, I have discovered that this is a relatively simple motor to work on ? once you get all the electric out of the way.
I have torn down the motor, and discovered something during the teardown which I found a little strange. First let me say that the piston that was destroyed was not #2 port side, but #3 starboard side (center piston on driver side of boat ? I am an old Marine and still have a problem with port and starboard.) There are rubber Bleed lines running to the top of the cylinders. According to the Mercury Service Manual, there should be a Bleed line running to the top of ?all? cylinders. My motor does not have a line going to #3 cylinder. There isn?t even a place drilled for a fitting and from all indications there never was a line installed on #3. Seems strange that this was the cylinder that burned the piston. I have already planned on rejetting the carbs larger, to compensate for possible lean condition, but am wondering about the lack of a bleed line. Is this a design flaw? I am not exactly sure what the bleed line does, any additional information will be greatly appreciated.

Re: Blown/burnt piston

i can check on my motor when i get off work to see about the bleed lines on mine... i know very little about these engines.... but if you need me to i can check to see were all my bleed lines go

Re: Blown/burnt piston

wondering about the lack of a bleed line. Is this a design flaw?

Click to expand...

No its not a flaw it was made that way....

I am not exactly sure what the bleed line does

Click to expand...

Routes excess unburnt/puddled fuel to other cylinders to be burnt...

If it won't clean up at .015", pretty much no choice but to go to .030"

Click to expand...

Thats only OEM piston size offered, In the aftermarket you can get a .010-.040 oversize...

Re: Blown/burnt piston

I appreciate that offer, but not sure if it is worth the time and effort on your part, especially if yours is a 1984 motor. From the feed back I am getting off other areas of the internet, it would seem that there isn't a bleedline on this year/model. Why there isn't, no one seems to know for sure. But, I sure wish I knew why the cylinder that burned the piston was the only one without a bleed.

JUST A GENERAL THANKS TO ALL THAT HAVE RESPONDED, I SERIOUSLY DO APPRECIATE IT ALL.

Re: Blown/burnt piston

Daniel1947 said:

I appreciate that offer, but not sure if it is worth the time and effort on your part, especially if yours is a 1984 motor. From the feed back I am getting off other areas of the internet, it would seem that there isn't a bleedline on this year/model. Why there isn't, no one seems to know for sure. But, I sure wish I knew why the cylinder that burned the piston was the only one without a bleed.

JUST A GENERAL THANKS TO ALL THAT HAVE RESPONDED, I SERIOUSLY DO APPRECIATE IT ALL.

Click to expand...

anytime buddy...

Re: Blown/burnt piston

I am having the same problem as you. I bougth a 87 black max 150 this summer. rebuilt the carbs engine runs great . Good compression in all cylinders. I have ben running it at the lake and then I check the plugs and piston wash. No 3 plug looks leaner or possibly less oil than all the rest. Also no 3 piston always looks dry on top of it. All the rest of the pistons look wet or oily. I also have no bleed line to number 3 cylinder. I am sure if I keep running it. My piston will burn and seize like yours. Anybody have any ideas.