Any way to change just one piston and rings on 1988 Mercury 115 L6 2 stroke?

tomconnolly

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So I pulled the power head and removed exhaust cover on my engine today to fix a water leak into bottom cylinder. On inspection I find the top piston is missing it's top ring entirely. That cylinder ran 100 psig on compression test and other 5 ran 120 last week before I started this project.

Is it even possible to replace only the top piston and rings since 120 isn't too bad compression for the other 5 cylinders? Or must I remove the entire crank and therefore should I just buy the other 5 sets of rings and replace them all?

And if I have to take off the entire crankshaft, should I go ahead and have the cylinders tested and honed if needed?

What I rl am tempted to do is put the exhaust cover back on and run it as is, with the missing ring. The cylinders and pistons, including top one with missing ring, all look fairly smooth with no significant scoring. There is some barely visible scoring on piston #5...

It doesn't appear that this engine has ever been opened by looking at the bolts all with paint on them. Other than these recent discoveries the powerhead and lower unit all seem to be in good shape.
 

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racerone

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Crank has to come out to remove the piston.-----Replacing one piston is common practice.-----But do a complete inspection once motor is apart.----Lower crankshaft seals must be replaced as well.
 
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tomconnolly

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Looks like pistons for sale on this site are only 2 ring type. Mine are 3 rings. Anyone know where to find oem replacement 3 ring piston?
 

racerone

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I have seen 2 ring pistons and 3 ring pistons mixed.----But you should do a complete inspection / measurement of bores.-----Inspection of labarynth seals before ordering any parts !!
 

tomconnolly

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I have seen 2 ring pistons and 3 ring pistons mixed.----But you should do a complete inspection / measurement of bores.-----Inspection of labarynth seals before ordering any parts !!
Labyrinth seals? What's that? Something like making sure my cylinders aren't egg shaped so the rings stay fully compressed full stroke?
 

racerone

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They are the sealing grooves on the 3 reed blocks.------Found way inside around the crankshaft.----They collect oil and separate the 6 crankcases in this 2 stroke motor.----If crankcase compression leaks between cylinders then they just wont start and idle well.
 

tomconnolly

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They are the sealing grooves on the 3 reed blocks.------Found way inside around the crankshaft.----They collect oil and separate the 6 crankcases in this 2 stroke motor.----If crankcase compression leaks between cylinders then they just wont start and idle well.
Gotcha. I thought about reed valves, but checked and have no blowback thru carbs and engine always starts and idles ridiculously well for its age and the fact that it had one cylinder sucking water and one with reduced compression. But I'll inspect.

Thank you @racerone for responding to all my noobie questions on this forum! With your input and the overall good condition of this engine, I'm considering pulling crank, taking to machine shop, honing, installing oversize rings as needed.

Another noobie question: if I have cylinder honed, I'll have to buy 6 oversize pistons, not just rings, right? $$$
 

racerone

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Stop-----Inspect and measure before making decisions.-----Honing a cylinder to break the glaze rarely changes the size by much.-----Standard rings can then be re-used if ring gaps are good.----Boring a cylinder O/S means new pistons and O/S rings.----Rings are expensive and kits are available with a new piston and rings.----If you want a simple , smooth running motor for the next 20 years then I recommend 6 new pistons with new rings.----Best to find a local shop with the special ring compressors.-----They may be able to install the crank assembly for 1 hr labor.-----That is if you can find a shop with the tools.----An old time mechanic to go along with those tools.----Seems shops doing this kind of work are becoming scarce.
 

Chris1956

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Replacement pistons are the two ring style. The rings are keystone shaped, so no swapping is possible. I am not sure if you can buy the older style (3 ring) replacements. You need to check as the price difference is a lot, if you have to buy all new pistons.

You can remove the crank to replace the piston ring. A shop may have the six ring compressors the OEM used. If not, be aware that all the loose roller beatings from the connecting rods and their cages will fall out, when the connecting rods are unbolted. You need to capture them and keep them isolated, so you can put them back on their individual connecting rods. You can stick them on with grease when reassembling. use new connecting rod bolts, or they will fail, and the motor will throw a rod.
 

tomconnolly

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Replacement pistons are the two ring style. The rings are keystone shaped, so no swapping is possible. I am not sure if you can buy the older style (3 ring) replacements. You need to check as the price difference is a lot, if you have to buy all new pistons.

You can remove the crank to replace the piston ring. A shop may have the six ring compressors the OEM used. If not, be aware that all the loose roller beatings from the connecting rods and their cages will fall out, when the connecting rods are unbolted. You need to capture them and keep them isolated, so you can put them back on their individual connecting rods. You can stick them on with grease when reassembling. use new connecting rod bolts, or they will fail, and the motor will throw a rod.
Oh crap. Thanks for that warning! Can I leave crank in place, unbolt connecting rod push the one piston out the head end and avoid removing crank?
 

racerone

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I clearly stated that there is no cylinder head and intended to tell you that piston can not come out the head area.
 

tomconnolly

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Oh crap. Thanks for that warning! Can I leave crank in place, unbolt connecting rod push the one piston out the head end and avoid removing crank?
@Chris1956 Or can I leave all rods and pistons connected to crank and not have to deal with any needle bearings except the ones for the one piston? Is the special ring compression tool for reinstall really available online? What parts should I replace in addition if I year down powerhead to this depth?
 

Chris1956

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You can pull all pistons out of the block, still connected to the crankshaft. Merc uses six special ring compressors to install the crank/piston assembly. Can you do it without those special tools, I have no idea.

I know that you cannot install the crank/piston assembly YOURSELF without those special ring compressors. If you had someone to hold the crankshaft/piston assembly, you may be able to install it without ring compressors.
 

tomconnolly

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You can pull all pistons out of the block, still connected to the crankshaft. Merc uses six special ring compressors to install the crank/piston assembly. Can you do it without those special tools, I have no idea.

I know that you cannot install the crank/piston assembly YOURSELF without those special ring compressors. If you had someone to hold the crankshaft/piston assembly, you may be able to install it without ring compressors.
Sending this in case someone else might find it useful. I'll prolly try this method if I end up removing the crank shaft.
 

Faztbullet

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1) You will need to remove crankshaft...period
2) Being a 1988 it will have the new style cracked rods
3) Remove as a assembly, inspect and mike block especially around power/transfer ports
4) If it mikes ok (90% dont) you can remove rods from crank and install back in block one at a time, the reinstall crank.
5) Any grooves/scoring/scuffs of cylinder liner will require boring.
 
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