Should I replace all 3 pistons?

[PatMurray]

This question may have been asked already, apologize if so. I have a 1985 mercury 60hp outboard on a 17ft Monark bass boat that I recently restored. In the process of making a shake down run I discovered that the fuel system had issues, and the fuel pump was failing. After rebuilding the pump and replacing the entire fuel system from the carbs to the tank, the engine ran great for a bit, so I loaded it up and was about to put it on the trailer when I heard a fairly loud ping in the powerhead, and it began to rattle. Having a good idea of what had happened I immediately shut the engine down and brought it home. I pulled the plugs and used a bore scope to have a look. As expected the top cylinder appeared to have run lean (I suspect from being run with the failed fuel pump). The damage to the piston was obvious. So I proceeded with a teardown of the powerhead to determine whether the cylinder walls were salvageable. Unbelievably there isn't a single mark on the cylinder walls and the crosshatch is still fully visible in all 3 cylinders. The top piston is obviously wiped out, but the other two are still in decent shape. My question is whether I should replace all 3 pistons at additional cost or reuse the 2 lower pistons and replace the top? Thoughts, concerns...

 

[racerone]

Did you install a new impeller ?-----Did you look at the poppit valve if so equipped ?-----You need to determine the cause of the failure or you will be doing this job again !----Did you find a problem with your fuel pump that you rebuilt, yes or no ?----Are you mixing fuel and oil at 50:1 on it ?

 

[PatMurray]

I did install a complete new water pump which was working fine. Oil mix was 50:1 as specified by the mercury dealer. No poppet valve on this engine, nor does it have a thermostat. I did find that the fuel pump diaphragm was very stiff and not at all flexible. Carbs were both thoroughly cleaned and rebuilt. However since the fuel pump was not working properly it was almost impossible to adjust the carburetors. The rebuilt pump made a huge difference and I was able to get them adjusted in the water without making any huge changes. I ended up being 1 and a half turns open on both low speed jets.

 

[PatMurray]

My belief is that the damage to the top cylinder was done on the initial run before the fuel pump and issues with the fuel system were fixed. I will certainly investigate any other causal factors before I attempt to run the engine again. I plan to replace the top and bottom seals and if possible do a low pressure test on the engine after assembly to ensure that there are no air leaks. My concern is based on not wanting to replace the other 2 pistons unless it is necessary. Seems that availability is limited and the cost of replacing all three pistons is a substantial issue in comparison to the cost of replacing the entire engine with a different engine. Adds a little more than double the cost to the rebuild. While I'm not completely opposed to replacing 3 I don't want to spend money unnecessarily. Granted I have pulled both bearings and they will also be replaced with new as will the wrist pin bearing.

 

[racerone]

Serial # is ?

 

[PatMurray]

A109898

 

[Faztbullet]

Unbelievably there isn't a single mark on the cylinder walls and the crosshatch is still fully visible in all 3 cylinders.​

Means totally nothing......need to take a rigid hone and hit the cylinders. This will show where any wear is around ports from side thrust wear. ther will be shadow areas where hone did not touch.

 

[PatMurray]

I agree, definitely would hone the cylinders and check for taper and out of round.

 

[jimmbo]

2 Stroke Cylinders wear a bit differently than 4 stroke, the Taper does occur, but the Area around the Ports also wears more too

 

[PatMurray]

Thanks for the information, I'm considering putting it on a shelf until spring. Looking at a 4 stroke upgrade, it's going to be late spring before I have another break from work and I will probably have a better opportunity to either buy a new motor or a very good low hour one which is something that I had already counted on anyway. The old mercury was sitting for 30 years before I bought the boat and it was in a couple of boxes until I acquired it. My thinking is that fuel economy, power, and dependability may be better with a newer 4 stroke.

 

[QBhoy]

Hi

Personally, if I was in North America, with the seemingly cheap price of parts, I’d do them all. Especially if you want to keep the boat for a while.
if it ran lean...it’s been lacking oil. Worst case might be that the crank has been damaged or marked too ? I think given you have a while before the season starts, I’d strip her right down for a look.

 

[racerone]

QBhoy-------Post # 1 clearly indicates that the motor is apart and spread on his workbench.

 

[QBhoy]

QBhoy-------Post # 1 clearly indicates that the motor is apart and spread on his workbench.

A terrible habit of mine...not reading the full details. Apologies.
In that case...it’s a no brainer. Or at the very least get the callipers and vernier out and check for tolerance. At least replace the rings and top end Bearings.

 

[Texasmark]

A terrible habit of mine...not reading the full details. Apologies.
In that case...it’s a no brainer. Or at the very least get the callipers and vernier out and check for tolerance. At least replace the rings and top end Bearings.

"Or at the very least get the callipers and vernier out and check for tolerance."

Interesting choice of words.

I do a lot of crossword puzzles to keep my mind functioning. Seems the ones available are written by a Brit. and he has his British colloquialisms of which I am unaware as to meaning. Makes it tough to get 100% without cheating.

Back when I was in the formal learning thing, we had Micrometer Calipers and Vernier Calipers, both "calipers" meant for multi-decimal measurements with Mics used for the more acute measurements. Today it's the digital and you have the convenience of mm or inches with the flip of a switch. Neat.

Noticed that you didn't include Vernier as a type of caliper. For us Blokes, if you don't mind, what are you talking about?
Thanks,
Mark

 

[QBhoy]

"Or at the very least get the callipers and vernier out and check for tolerance."

Interesting choice of words.

I do a lot of crossword puzzles to keep my mind functioning. Seems the ones available are written by a Brit. and he has his British colloquialisms of which I am unaware as to meaning. Makes it tough to get 100% without cheating.

Back when I was in the formal learning thing, we had Micrometer Calipers and Vernier Calipers, both "calipers" meant for multi-decimal measurements with Mics used for the more acute measurements. Today it's the digital and you have the convenience of mm or inches with the flip of a switch. Neat.

Noticed that you didn't include Vernier as a type of caliper. For us Blokes, if you don't mind, what are you talking about?
Thanks,
Mark

Haha. Hi mark.

so to me, a vernier would be a vernier calliper to be specific. Just callipers would be a set for internal or external measurement to then be set and checked against a rule or similar.
a micrometer as you mention would be of a u shape with handle and adjustment on handle...similar shape to a c clamp
digital vernier are great as you say, but need “zero’d” every time if not looking at the manual scale from habit...as you’ll no doubt know. Haha.
hope that makes sense. I’ll try and find a picture of what I would naturally call callipers. Won’t be anything new to a guy like yourself.

 

[QBhoy]

"Or at the very least get the callipers and vernier out and check for tolerance."

Interesting choice of words.

I do a lot of crossword puzzles to keep my mind functioning. Seems the ones available are written by a Brit. and he has his British colloquialisms of which I am unaware as to meaning. Makes it tough to get 100% without cheating.

Back when I was in the formal learning thing, we had Micrometer Calipers and Vernier Calipers, both "calipers" meant for multi-decimal measurements with Mics used for the more acute measurements. Today it's the digital and you have the convenience of mm or inches with the flip of a switch. Neat.

Noticed that you didn't include Vernier as a type of caliper. For us Blokes, if you don't mind, what are you talking about?
Thanks,
Mark

My idea of standard callipers. Just noted that some would call them spring callipers

 

[racerone]

I use telescopic gauges and good micrometers to measures bores.------Those and a steady hand with experience.----Not sure if I would trust readings from calipers pictured above.

 

[QBhoy]

I use telescopic gauges and good micrometers to measures bores.------Those and a steady hand with experience.----Not sure if I would trust readings from calipers pictured above.

To be fair. Anytime I used similar, it was to check tolerance on serious big engines. Things like big sulzer slow speed 2 stroke diesels etc. Wee while ago now.

anything normal like outboard motors or jet ski motors...it’s just always been a case of new pistons and and rings to suit a top end rebuild or similar. You’re right. Wouldn’t be much use for wee things like we all know and love in our spare time.
handy for roughly checking progress when messing about on the lathe though.