Cylinder 2 Piston/Compression Issue Force 85

Thebevis

Cadet
Joined
Sep 5, 2015
Messages
10
Well the troubles continue with this motor. It is a 1989 #856X9H. Although the powerhead is stamped 1991.(See pic) I posted a while ago about a compression issue I was having and turns out the Head gasket was bad. Replaced. I ran great after that. Good compression in all Cylinders. Until....... About a month ago. Was having issues with it starting. Started out with the basics pretty much. Fuel, Spark, Compression. Umm Why is cylinder 2 compression only 30psi? Maybe blew another head gasket? Nope. The first photo is The top of Cylinder #2. You can see there is a indent in the piston right at the cylinder wall(almost looks like someone took a screwdriver and pryed on it). Well damn!! My only guess is that the piston ring broke off and got caught one of the ports and the piston caught it on its upstroke. That is a wild guess. Have any of you seen this before? I really don't want to have anymore issues with this motor. I'm thinking of just pulling the powerhead off and putting in new piston and rings in all three cylinders. Or should I? The compression on all three Cylinders is 1-93psi 2-30psi 3- 90psi. Or should I just replace the 2 cylinder piston and not pull the powerhead? But my only concern is that if the ring did break, where did the piece go?? Again that's just a guess as I haven't taken the piston out yet. If I do a rebuild, expect me to be on here a lot. I have rebuilt engines before, but never an outboard. So it will be a bit of a learning experience for me. Thanks again!!
 

roscoe

Supreme Mariner
Joined
Oct 30, 2002
Messages
21,657
Guess you will have to pull the piston and inspect.

Its ok to replace just one.
 

jerryjerry05

Supreme Mariner
Joined
May 7, 2008
Messages
17,923
Looks like #2 is shot.
​The cyl. wall, can you catch a fingernail in the grooves?
​If so it needs to be bored.
Like roscoe said you can do just one cyl.
The damage around the exhaust can be cleaned up.If the main part of the cyl. isn't damaged.

But you need to figure out why it went bad??
You can rebuild and have the same thing happen??

Where you located?
Sent you a PM
 

Thebevis

Cadet
Joined
Sep 5, 2015
Messages
10
Well I?m finally getting back around to working on the motor. After weighing my options and although I still haven?t found out what could have caused it, I probably will tear it out and go through the whole thing. I replace parts as nessesary. I inspected the #2 cylinder a bit closer and my fingernail does not catch in the walls anywhere. So I think a little honing will probably do it as well as chamfering the ports to get rid of the sharp edges. I haven?t measured the bore yet, but have measured the piston diameter. It is 3.30 inches which has me a bit confused. I know this engine has never been gone through( unless whoever did it painted the engine really good) as there is no indications. The standard piston size should be 3.312. Am I correct? So basically the piston I have is .20 over standard. Am I thinking right on this? Was it an option in these motors when they were mfg to come bored .20 over stock? Or am I completely off in left field with all this? Also the #1 is stamped on top of the piston.
 

Nordin

Commander
Joined
Jun 12, 2010
Messages
2,428
No the bore is 3,312.
You are thinking in a wrong way.
Your engine is standard bore 3,312 in, pistons are 3,30 in.
The 0,012 in is the clearance for the piston because they will "swell" when get hot.
 

jerryjerry05

Supreme Mariner
Joined
May 7, 2008
Messages
17,923
The pic of the piston shows it's a stock piston.
It would have .020 or the size stamped on the top.
 
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