what could cause this issue?

[JoshKeller84]

backstory - started off with a 1994 omc 90/65 jet. an older friend that fishes in the area asked me to help him out with an engine issue. he had his motor rebuilt and it lasted a year and the same exact issue happened again. the two starboard side pistons had issues where the rings wallowed out the ring grooves, and eventually broke. after having the original block and rebuilt block fail in the same exact manner, i found him a completely different block and helped him install it. new waterpump and thermostats were tested in water.

fast forward 2 months later, and he calls me and tells me its popping a little at idle and he lost 200 rpm. i run a compression test and on the port cylinders get 110, 109, and the starboard, 95 and 100. i pop off the head and see the pistons have a small damge spot each. pull the exhaust plate and see the rings have wallowed out their grooves. none were broken yet, so i tell him we can rebuild this motor after we spec and mic it to ensure theres no ovaling. the cyclinder walls look good, no damage to head. after 3 failures on two powerheads, on the exact same two cylinders on the same side - what could be causing this??

 

[Scott Danforth]

my guess the ports are catching the rings. did they break the edge of the ports after boring and honing. the scoring on the piston says there is an issue with the bore. the fire-sloting of the piston says the rings broke first, than the piston ate itself.

 

[JoshKeller84]

what are the odds of this happening again with a completely unopened, unbored second powerhead?

 

[alldodge]

I think either
The bore was a tad to tight and thermal expansion of the piston started things.
Ring gap to tight
Not enough oil in the mix or somehow some water is leaking in that side

 

[racerone]

I think it will destroy itself again.----Is there a tachometer in the boat , what roes it rev out at ?----Water pressure gauge ?------Temperature gauge ?----Oil injection in use or mixed at 50:1 on this machine ?

 

[JoshKeller84]

this happened on two seperate powerheads, same failure on both. 5300 rpm at wot (jet motor. cant change the prop), no water pressure guage, but the water pump is fed by the jet impeller - ive ridden in this boat, and the pee stream is very strong at all speeds.

 

[racerone]

Sorry, ---A strong pee stream does NOT mean that motor is cooling properly.---There are more question in post #5 too.

 

[Rustywrench]

Are those Wisco pistons? They need lots of break-in before being run hard. Jet application can be hard as a no load application. Where are the locating pins to keep the rings in place? Can't be sure from the pictures if the top one is there on that bad piston shown? Like racerone one said. Cooling is critical. It's what keeps the locating pins in place.

 

[F_R]

The failure seems to be aligned with the port. I wouldn't expect the locating pins to be aligned with the ports, but rather 12 and 6 o'clock. Does that sound right?

 

[racerone]

Very much correct F_R----Lots of heat and little lubrication appear to be a culprit here.-----There are question in post #5 but no answers yet.

 

[JoshKeller84]

sorry guys was taking a break on the road and had to call him to verify - no temp or water pressure guages. no oil injection, he claims to be using a mixing bottle from walmart and supertech oil.

 

[racerone]

Most outboards rarely get a chance to wear out.----They end up on the work bench because owners make mistakes.----This may well be an example of that.

 

[Rustywrench]

Since we are still guessing at this point this is what Pistons look like when locating pin come out & end of ring rotates to port. All heck breaks loose then. Seen thousands of them since 1977 when OMC first tried high ring pistons. It has been hard to keep pins in Pistons of all brand motors for a long time. Number one killer of two stroke motors. All the owner has to do is move the ring around & look for the pin. Top & bottom like y'all said on this model. If either is missing (top one in this case) then he has his problem. A mechanical failure of the piston, but is aggravated by overheat.

 

[JoshKeller84]

sorry for taking so long, was helping him tear the motor down. luckily there isnt any damage to the block. hone, pistons and rings and should be good to go. i went over the carbs completely, verified all jets were clean and the proper size. all were. here is however what i did find - the thermostats were operating property (tested in 150 degree water), however the poppet valves were gunked up with sand, silt, and some metal shavings - the starboard side was worse than the port. i went to examine the starboard side head for any blockages, and the temp sensor wire fell right out. does this appears to be an overheat? thinking the motor was overheating due to the restricted flow and lack of a temp sensor, causing the pistons on the starboard side to expand and catch the rings on the ports.

 

[racerone]

Are you sticking new standard pistons in possibly worn / out of round / tapered bores ??----What type of pistons do you plan to use.-----Using a bore gauge to check the cylinders ?

 

[JoshKeller84]

we took the block this morning to the machine shop and had them gauge it out. he recommended a hone job and standard 3.50" pistons. we were planning on using oem pistons and rings. is there a better choice?