1990 Force 120HP #2 Cyl zero compression and upon opening piston shows damage

[gica]

New piston might be 3.365 /3.367 " at the rings.-----3.372"at the skirt.-----Aluminum pistons expand at around 0.000012"/ inch / degree F ------New pistons are tapered and sized with clearance when cold.------When engines run clearances , I do realize I cannot use standard size pistons on the oversized #2 cyl. I am trying to understand why I cannot use oversized pistons if I actually have the B or C model.

Thanks for that I think I understand it. But if I have the A model then the motor is junk. Bor at 3.312 is way below the .030 max oversize piston posibility. But the B or C models can be bored to within the oversized specs since the standard sizeis 3.375.
I did not see any listings for 120hps.
So new Force motor is out of the question.
I don't even think I would get another Force though I have now learned a bit of what it's all about as far as adjustments and tore down.

 

[gica]

https://maxrules.com/models/forcemodels/90_150.html
I have to look it up in the morning based on the serial number

 

[gica]

I couldn't wait so mine is a B so 3.375 standard. I should have all the way to 3.410 to install up to .030 oversize pistons right?
I could be wrong but this is what I read.

 

[gica]

1990 Force 120 Cylinder Bore HELP !

I have a 1990 Force 120 . I bought the motor and didn't do a compression check. The motor ran and shifted o.k. so I thought it was fine. Not knowing about doing a compression check, I found out the hard way . One piston had 0 compression. The others were fairly high140. I disassembled the motor...

forums.iboats.com

This thread mentioned what I stated
So now I want to know what I can do short of getting another engine.
The one thing I still don't know is the oversized rings, I goes to get closer to the ideal gapin an oversized cylinder bore you need a larger ring. Would that go hand in hand with the piston size say .010 piston .010 ring?

 

[jerryjerry05]

Probably need to go to .020 piston and rings.
.010 rings don't fit good on a .020 piston.

Buying another Force?? If they've been maintained right then they will last as long as any other motor.
Proper maint. is the key.
I had 4 88/85hp. They had over 2k hours and the transom gave out from salt water. They just don't hang good when used in salt.
Same as the 125 had 2 and had to retire them because of the weak transom material.
But buying a 35 year old anything isn't a good bet

The following is my OPINION
Buy any brand that makes a EFI
The HPDI is much better on fuel but crazy expensive to fix and pay the mechanic, a regular shade tree (like me) can't fix them.
Yam ,Merc, Suzy all make great motors as they use the same technology.
Good luck.

 

[racerone]

Measured a brand new piston for a Mercury.-----It has 0.008" taper.------If a piston is way too loose in the bore than crankcase compression can be lost right out the exhaust ports.-----Fitting new piston rings in old worn out grooves is a mistake as well.----This whole concept of a 2 stroke engine is not well understood.------Piston has to do double duty.-----Compression on the downstroke on the bottom.----Compression on the upstroke on top.------Proper fitting is important to get a " wow factor " when owner hears a rebuilt engine run !!!

 

[gica]

Measured a brand new piston for a Mercury.-----It has 0.008" taper.------If a piston is way too loose in the bore than crankcase compression can be lost right out the exhaust ports.-----Fitting new piston rings in old worn out grooves is a mistake as well.----This whole concept of a 2 stroke engine is not well understood.------Piston has to do double duty.-----Compression on the downstroke on the bottom.----Compression on the upstroke on top.------Proper fitting is important to get a " wow factor " when owner hears a rebuilt engine run !!!

I totally agree and

Measured a brand new piston for a Mercury.-----It has 0.008" taper.------If a piston is way too loose in the bore than crankcase compression can be lost right out the exhaust ports.-----Fitting new piston rings in old worn out grooves is a mistake as well.----This whole concept of a 2 stroke engine is not well understood.------Piston has to do double duty.-----Compression on the downstroke on the bottom.----Compression on the upstroke on top.------Proper fitting is important to get a " wow factor " when owner hears a rebuilt engine run !!!

I totally agree and speaking from experience I do remember installing pistons and rings the I thought were borderline good and the end result was not the best compression. I don't have much experience using oversized pistons.
That said what do you recommend in this case. Jerry mentioned .020 with the same size rings. Let me measure tye pistons I have. You say they will he 3.365.

 

[racerone]

They will be 3.365 or less at the rings ----They will be 3.368 / 3.367 at the bottom.-----Piston clearance is determined with the measurement at the bottom!

 

[gica]

At the top it shows 3.360
Bottom 3.373
So with a bore of 3.375 we can see the clearance that the factory accept.
Compression was 125 and 128psi

 

[racerone]

Recheck your measurement at the bottom.----Piston might be out of round.-----When it left the factory it would have been 3.371 / 3.372?

 

[gica]

Recheck your measurement at the bottom.----Piston might be out of round.-----When it left the factory it would have been 3.371 / 3.372?

yes bottom is 3.373 +- .001

 

[gica]

I think I need the .010 which will he bigger than my bore at roughly 3.3820. It will he bigger which means I still need to take of some material.
The bore has to he 3.385 right?

 

[gica]

Ok so I need to do this properly, so I am following this video to use a micrometer and dual gauge and see tye real status of each cylinder. I zeroed everything. I will set it at 85mm and see what's going on. I am sure it's out of round more than .002
So honing is in order. On #2 I want to see how much material I took of. The telescopic measure I took of 3.382 is ball park don't know if it's true 100%.
And with those measurements I can decide which way to go. I do need new stones though the 240s are 90% gone. I probably should have gotten 180s and 300s for more efficient work

 

[racerone]

Make sure you measure diameter at the very bottom of the cylinder.-----Using honing stones to take out 0.010" is not what is consider " best practice "-----Now I used to work in a factory where tolerances of + /- 0.000020" could be held.----I do know what measuring is all about !

 

[gica]

My dial gauge is not top notch so ball park set it at 3.375 and went up and down from there
On #2 some minor error adding .00005
Don't know how the telescopic dial found 3.381

 

[gica]

My dial gauge is not top notch so ball park set it at 3.375 and went up and down from there
On #2 some minor error adding .00005
Don't know how the telescopic dial found 3.381

So I am thinking to take the block to a machine shop to do proper boring but I need to buy the OS piston and rings first. Which size do I get based on the measurements, .010 or bigger?

 

[gica]

Anybody?

 

[Nordin]

Take the power head to a machine shop and have it measured by them.
Then order over size piston and rings from the measurement from the machine shop.

 

[gica]

Can I do on OS the rest standard

 

[gica]

Ok found a machine shop with a guy that told me to get the .020 since he didn't measure it he said that is safe to get the 0.020.
But I measured it and want to get the 0.010
Only number 2 has been hone to do understand the damage.
Also he said to do them all.
So do you guys think I can get away with .010 or jump straight to .020
I wanna have room in case pistons get damaged.