Piston & Cylinder damage 1979 Mercury 70hp 3 cylinder

[mathguydave]

Lost compreesion on cylinder #3 so I took the engine apart to assess damage. Looking for advice on whether to replace pistons and if cylinders need shop work. Attaching pictures. Thanks!

 

[racerone]

Is that the top cylinder ?-----Run a hone in that cylinder to see what shows up as far as scoring.

 

[wrench 3]

Definitely replace #3 piston. On the other two, brake an old piston ring and use the broken piece to clean out the ring grooves and you should be good to go.
I think you'll find that most of the "damage" to the cylinder wall is only aluminium from the piston that will clean up with a hone. But as racerone mentioned, you won't know until you clean it up.
Have you determent the cause?

 

[Faztbullet]

That is the #1 cylinder(top)….usual failure do to weak impeller. Rings are coked due to excessive piston temps and possible questionable oil . Run a rigid hone down cylinder and see if there are shadows around ports where hone didn't touch, if so bore .020 possibly all 3.

 

[Award]

I am rebuilding my 78 merc 80 I am boring.020 over no need to half azz it do all 3 ....I took fatzbullet word
if you went that far to take it apart only makes sense to make it right .... A lot of great people on here to give great advice

 

[harringtondav]

I am rebuilding my 78 merc 80 I am boring.020 over no need to half azz it do all 3 ....I took fatzbullet word
if you went that far to take it apart only makes sense to make it right .... A lot of great people on here to give great advice

That's where I go with any major repair, boat or auto. Once you go through the pain of digging in that deep, fix it all. You'll get another 40 yrs out of that motor.

I replaced the front motor mount on my son's 2.0L Neon. After I tore off the front of the engine I was staring the timing belt and a good water pump in the face. I changed them both and buttoned it up. No appetite for a redo later.

 

[mathguydave]

Wrench 3 -- When I was doing the compression check I found the top cylinder completely dry. I just bought the boat and ran it an hour or so before checking compression. I know the boat had been sitting for years. My best guess is the carbs were dirty and fuel/oil wasn't getting to the top cylinder. I will be replacing the impeller and cleaning/rebuilding carbs. Let me know if you think I'm missing something.

 

[mathguydave]

Thanks for all the advice. I will take it to a shop and have the mechanic do the boring and honing. Any advice on where to purchase new piston(s), gasket set, etc?

 

[racerone]

Look for ----Wiseco-----Pistons that are I believe American made.------So it was the top # 1 cylinder that failed.

 

[Award]

Thanks for all the advice. I will take it to a shop and have the mechanic do the boring and honing. Any advice on where to purchase new piston(s), gasket set, etc?

I bought my powerhead rebuild kit from marine engine
com
it was 700.00 some dollars

 

[wrench 3]

Wrench 3 -- When I was doing the compression check I found the top cylinder completely dry. I just bought the boat and ran it an hour or so before checking compression. I know the boat had been sitting for years. My best guess is the carbs were dirty and fuel/oil wasn't getting to the top cylinder. I will be replacing the impeller and cleaning/rebuilding carbs. Let me know if you think I'm missing something.

It sounds line you've got it covered. As Faztbullet said, the usual cause for a top cylinder failure is poor water circulation. Also a lean fuel mixture on a two stroke with oil mix will not only deprive the cylinder of oil but can increase the combustion temperature enough to cause damage.
If you're taking it to a shop for cylinder work I'd get them to mike the cylinders first. Cylinder reboring isn't a maintenance item. Engine rebuilders only do it when necessary.

 

[wrench 3]

It sounds line you've got it covered. As Faztbullet said, the usual cause for a top cylinder failure is poor water circulation. Also a lean fuel mixture on a two stroke with oil mix will not only deprive the cylinder of oil but can increase the combustion temperature enough to cause damage.
If you're taking it to a shop for cylinder work I'd get them to mike the cylinders first. Cylinder reboring isn't a maintenance item. Engine rebuilders only do it when necessary.

Just to clarify, If the one cylinder needs reboring go oversize on all three.

 

[mathguydave]

Took it to a shop today. They said at least the #1 needs boring and they will either hone or bore the other two after checkimg the measurements. Thanks again for your help.

 

[Award]

Took it to a shop today. They said at least the #1 needs boring and they will either hone or bore the other two after checkimg the measurements. Thanks again for your help.

I took mine in shop yesterday he said it will be about 2 weeks before he can get to mine uggggg and funny thing is the guy use to be a merc certifiedmechanic and stopped doing it in late 80s
oh and I ordered my rebuild kit at marine parts . Com sorry on earlier post

 

[mathguydave]

I looked at pistons at marineengine.com and they are sierra brand and come in 0.015 and 0.03. At other sites I saw Wiseco brand and they were 0.02 oversize. Is that odd that different manufacturers would have different measures? I guess that is why a mechanic wants to see the piston before doing the boring.

 

[racerone]

Manufacturing is different today than in the past.-------I rebore cylinders myself.-----0.020" O/S pistons require a bore that is 0.020" bigger than the standard bore !!

 

[Faztbullet]

Here is what you need.....WSM kit for 3 cylinders, ignore HP's posted as it will fit yours.
https://www.ebay.com/itm/202289208612?roken=cUgayN