What could of caused this piston to melt?

[DennisG01]

This is the spare engine for the boat so it's just sitting on a stand in my garage at this point. The guy I bought the boat from gave it to me as an "extra" and said that something is wrong with it, but wasn't 100% sure of what it was. I did a compression test and got 72 - 80 on all cylinders, except #2, which was 0. The engine has been sitting, unused, in his barn for about 7 years so maybe that 72-80 isn't entirely accurate, either (FYI, I used the same tester for the "good" engine and got 115-ish for all 6 cylinders).

After pulling the head, it's pretty obvious to see why the compression was 0. The cylinder wall appears to NOT be scored at all (from an initial inspection, anyways). Instead, there appears to be remnants of melted piston smeared onto the wall (not scored, "raised"). The outer perimeter of the head (not the dished area) looks like the edges of the piston, as well.

Any thoughts as to what could of caused this? I'm saying "melted" above, but that may or may not be accurate. Or... does this make more sense: could pieces of a ring have come loose and caused this damage? Especially since there doesn't appear to be any damage to the center portion of the piston (since there would have been space there at the top of the stroke due to the dished-out area of the head. I've never diagnosed this type of failure before.

 

[Slacktime]

Perhaps it was running too lean, or the fuel octane level was too low? Is it carbureted or injected?

 

[DennisG01]

It is a fuel injected, '97 250HP OX66. Wouldn't bad fuel cause damage to other cylinders, too? While I have not yet pulled any pistons out, the other 5 piston tops all look very, very good (I did pull the starboard side head, too).

 

[Doc Stressor]

There are several things that can cause one cylinder to run lean and burn up. A bad injector or a bent or damaged valve are 2 common causes.

 

[DennisG01]

Since I've never gone down this road of rebuilding an engine, is this a worthwhile venture with the anticipation of selling the engine when I'm done? If anyone out there has done this before (or know of someone who has), what are your thoughts? I feel comfortable doing most of the wrenching, but would obviously hire-out the honing of the cylinder. The throttle body assembly looks relatively easy to remove - I can work on that next.

-- Can the honing be done without taking the powerhead off?
-- Do I have to also bore out the other side of the block (the #1 cylinder) for balance issues?
-- What am I missing - I'm sure there is a decent amount!

 

[seasick]

Since I've never gone down this road of rebuilding an engine, is this a worthwhile venture with the anticipation of selling the engine when I'm done? If anyone out there has done this before (or know of someone who has), what are your thoughts? I feel comfortable doing most of the wrenching, but would obviously hire-out the honing of the cylinder. The throttle body assembly looks relatively easy to remove - I can work on that next.

-- Can the honing be done without taking the powerhead off?
-- Do I have to also bore out the other side of the block (the #1 cylinder) for balance issues?
-- What am I missing - I'm sure there is a decent amount!

Often when the mixture is lean, the piston will be damaged in iis center which yours isn't. It can not be a bad valve since it is a 2 stroke. Three things come to mind: A stuck ring that caused the piston grove to crack, a local overheat condition and finally a leak of cooling water into the cylinder. Whatever, the previous owner apparently continued to run the motor while the condition existed. The crap on the cylinder walls may hone off but also may be hiding wall pitting. The more disturbing thing is the low compression on the other cylinders. That tells me that there is more damage, perhaps piston ring failure on multiple cylinders. Carbon buildup ( not using something like RingFree) can lead to ring failure.

 

[DennisG01]

Just from doing a little reading... I suppose the reed valves are another thing to take a look at? Both from the possible overheat and low compression? Unless I'm mistaken, getting the pistons out to take a better look isn't all that hard?

 

[family affair]

As for your original question regarding rebuilding to sell... no way.
Considering the resale of old 2 strokes and Yamaha's outrageous prices for parts, you would be much better parting it out or keeping for parts.

 

[DennisG01]

As for your original question regarding rebuilding to sell... no way.
Considering the resale of old 2 strokes and Yamaha's outrageous prices for parts, you would be much better parting it out or keeping for parts.

As I said, I've never gone done this particular road before so I'm open to hearing and learning more info. Please correct me if I'm wrong or not thinking this through completely... From what I can tell, these motors have a value of somewhere between $3K and $4K. Considering I can do all the work myself, except for honing/boring... and assuming I'm calculating costs correctly (just looking at the first online place I found)... New head = ~$225, piston/rings = ~$150... even if I have to replace the other 5 ring sets = ~$300. Add in whatever a engine shop charges and I'm probably still under $1K and that's buying all new parts. I'm sure there will be some other incidental stuff, but does that estimate get me pretty close?

I'll know more once I tear it down further. Just "thinking out loud" here.

 

[family affair]

If your bearings, crank, and rods are still within spec, it might be that simple. If that cylinder was running lean or sucking water, I would be concerned with the previously mentioned.
Also dont overlook the cost of gaskets. They don't come cheap.

 

[DennisG01]

Anything in particular for me to look for as I tear it down?

 

[family affair]

I've rebuilt dozens of water cooled 2 strokes, but never an outboard. I can only speculate one would check similar wear areas like piston skirt wear, wrist pin wear, big and small end connecting rod wear and connecting rod side play. If all that checks out you will still need to determine with great certainty what fried that cylinder.

 

[GulfSea]

Looks like something dropped in the cylinder too hard/large to slide down the side of the piston. And ran long enough to beat the ever' livin' crap out of the piston. What did the head look like? It should have marks similar to the center of the piston.

 

[DennisG01]

Looks like you're dead on. Last night I talked to my brother and he remembers the owner saying that he thinks a ring had broken. Looking closer at the piston and the head... sure enough, there's little nicks all over both of them. That's obviously not an overheat - plus, nothing else I've seen so far indicates an overheat -- no discoloration and there was still some gas/oil mixture in the intake runner, about the same as the other cylinders.

So, about an hour ago I tore it down some more and removed the throttle body. Yup - little pieces of metal in and around the reed valves and intake runner. Knocking on wood right now, but I "think" this is a good thing? Meaning, I don't have to worry about trying to diagnose an overheat condition - the pumps, injectors, reed valves should all be fine?

To remove the connecting rod cap - can I just use a 12pt socket? The service manual doesn't mention a size, but appears to maybe be an 8mm or 9mm (based on how 6pt sockets fit over the bolt head). That's a 1/4" screwdriver in the pic.

 

[DennisG01]

Pulled the piston (12pt 8mm socket). Obviously that's shot. But, I have to admit, I was surprised at the rest. Both journals (is that the right term for what the bearings ride on?) were super clean and shiny with absolutely no marks. The rod end/cap (inside bearing surface) was nice and shiny, too. Even both sets of bearings were great (I'm looking at/judging the journals and bearings as I would wheel bearings/races). The reed valves, while I've never had ones in my hand before, looked an awful lot like they were practically new. Everything still had a nice film of oil on it, too.

So... I guess the next big question is whether or not I can find a shop that can hone/bore the cylinder and whether or not I'll need a bigger piston/rings. For what it's worth, I could still see the "cross hatching" in the cylinder wall... where there wasn't pieces of piston squished/melted onto the wall, that is.

 

[family affair]

I suspect you have had more than one issue to cause this damage. If you google detonation images you will find photos with piston damage similar to your condition, but not as bad. As seasick mentioned a lean condition will typically cause hole through the top of the piston. Detonation/ pre-ignition are closely related and can be caused by excessive heat from carbon and/or a stuck ring from carbon, etc. The pre-ignition would cause the pitting and melting near the circumference of the piston eventually causing the ring to fail. The chunk of ring trapped in the cylinder could have caused the other marks. That said, pre-ignition is brutal on the connecting rod. You might want have that rod inspected at the machine shop while you are at it in case it is bent.

According to my service manual 81 psi is the minimum compression for an ox66 250. I didn't dig up the details, but certain years of the 225/250 powerheads actually had lower compression by design on some of the cylinders to minimize pumping losses when cylinders shut off at low rpm. So if you see inconsistencies on cylinders, this could be why.
Good luck.

 

[DennisG01]

Thanks for continued advice, it's much appreciated. I'm with you on the connecting rod - it "looks" perfect, but obviously that's only going to tell me if there's major bending. I would definitely get it checked out.

Concerning the lower PSI in the other cylinders... What are your thoughts (or anyone else) on these possibly being low due to not being run in 6 or 7 years? Skipping ahead into the future and pretending that I do rebuild this to run it... If I put it all together and then still find the other cylinders are low, it's not that hard to tear it apart again and pull those pistons out. But, maybe it would be best to pull them out now and throw new rings on them?

Everything else on the engine (including what I can see of the other pistons/cylinders) appears to be in really great condition. There's not even any real corrosion to speak of. I also just pulled the VST tank and that thing was as clean as a whistle - no junk or anything in it.