Top piston worth replacing? (120hp low compression)

[Warlord1337]

Hey guys Ryan here,,first time posting. Have a 1997 120 hp Force outboard on a quantum 170xd I started tinkering with. Have had it for a couple years but only have taken it out after I first bought it (ran fine) and then it was sitting for awhile. Motor was seized but after spraying some wd40 in the cylinders and set for 24 hours I was able to break them loose fairly easy. Turns over and runs good again.
Have cleaned the carb, checked thermostat and replaced impeller. Noticed the tell tale didn't have "great" flow so I checked compression. #1 120psi #2 135psi 3&4 had around 150psi pulled the cylinder head to do a once over and check gasket. Found top piston has more "play" then the rest. Cylinder walls look and feel good on all 4.. guess my question is how easy is it to change out the top 2 pistons? My guess is the rings are going out and I've found kits on ebay for $70 (ring and piston). Can you access the rod bolts from the intake side or would you have to break the whole block apart? And can I just hone the cylinders a little, I can feel no pits or any damage to the cylinders. Not going to spend a lot on this boat but if a can do some easy and low cost preventive maintenance I'm fine with that.
Also bottom piston looks like it is what seized the engine after sitting. Looks like moisture got between the piston and the head then by chance was fully compressed while sitting for a couple years, by the looks of the scoring. Am I looking at that right? Have worked on trucks and motorcycles, but first real outboard though.

Found one imperfection on the gasket and have no clue how it could of happened seeing as there is no hole on the the head for the water to go...it will be getting changed out anyway but figured someone might know..

This is a fresh water only engine by the way, overall a very clean engine just don't want to do more work then I have to with a engine that runs fine and could last me a couple of years....

 

[jerryjerry05]

IF??? you didn't destroy the gasket???? I'd try sealing with
silicone and doing another comp test.
Sounds like the gasket might have been the problem???

You can also pull the port covers and check the sides of the
pistons for damage.
The pistons can be replaced singly without breaking the block
down(not that easy)
One new tool I've added is a snake camera. $11 works great.
It even takes movies, my system is Android and the results are
real good.

 

[racerone]

There are broken / loose pieces inside the engine.-----Remove the bypass cover for the cylinder in picture #2.----Likely a broken piston ring.

 

[Warlord1337]

Thanks for the replies!
Will pull the port cover soon. A broken ring, or 2 does make a lot of sense. I just pray I don't have to machine the block, I have no clue where to go for that...atleast locally.
The metal rings are in great shape on the gasket so I don't think that was the compression problem. There is just a factory hole that ripped a little and I don't know what would cause that.
Will look at getting a scope, I've been meaning to get one for awhile, had no clue they have one for the android.

 

[Warlord1337]

Pulled the port cover. Only broke 3 bolts.
Think I may of found the broken ring on top cylinder looking at the open gap on the side.
On #3 found scoring marks near the top after the rings. Other then that the piston looks good, maybe some metal got in that one and left its mark. The top 2 pistons are much darker then the rest.
Will try pulling the top 2 piston tomarrow.

So do I have to change the piston when replacing rings or can I just install new rings?
Thanks

 

[racerone]

Fairly obvious what needs to be done.----Total teardown is in order.

 

[jerryjerry05]

From the new pics??? I'd find a machine shop that
can do the bore/hone job.
Then pull it apart.
IF??? there's ANY damage to the piston skirts I'd
change the piston.
Post piston pics after cleaning/removal.
If it really is a fresh water motor?? it should come apart
fairly easy.
I just rebuilt the same motor, not that big a job
, try to figure out why the motor failed?
bad reeds? sucking air/bad gasket?
carb set wrong etc???

 

[Warlord1337]

Hey guys, I posted this thread a little while ago and here is an update along with a question.
I pulled the powerhead completely apart like everyone told me to do and found I had 1 broken ring on the #2 cylinder along with the top carb Reed valve just about completely toast. So Ive had the block bored/hone on all 4 cylinders and head was a little warped so had that resurfaced. Bought a complete rebuild kit with 3 .040 over and one .030 over pistons along with new fuel pump rebuild kit, carb rebuild kit and new Reeds from TSM performance (nice guys to work with).

Im about ready to put everything back together and my question is, do I need any special gasket maker for the Intake manifold to the block? I believe those are the only parts that never had a hard gasket, just a type of gasket maker/glue.... My machine shop recommended PermaShield gasket maker but hes not a outboard mechanic so I wanted to hear from someone with experience....I have Permatec 80019 aviation I have on hand along with that old style Permatec in the brown tube with brush...
Any helpful advice would be great, have white lithium grease and Sta-Lube engine assembly grease I have used in the past for bearings.

Thanks

 

[jerryjerry05]

For the intake to block seal: I use the black RTV/Silicone

 

[puffitu]

I use blue RTV.

 

[racerone]

RTV is not to be used where there is gasoline present.----Simple as that.----Put a lump of cured RTV in some gasoline and see what happens.

 

[Warlord1337]

RTV is not to be used where there is gasoline present.----Simple as that.----Put a lump of cured RTV in some gasoline and see what happens.

So what do I use? the old stuff didn't seem like it was RTV so I wasn't going to use that thick stuff. The old stuff looked like a very thin clear liquid.

 

[Redbarron%%]

I use Hylomar blue. It is fuel resistant and is actually reuseable. The polymer "sets and fills voids and is easily disassembled.
It does not set like RTV as it just seals.
Rolls Royce uses it in their Jet engines.