Help! Mystery starting problem

[andyak]

Since I had to grind and cut off exhaust pipe at collector there is a chance some aluminum dust got in. But I doubt it. And every cylinder? Other engine went through the same and is fine.

I did compression back in boat just now and got over 185 psi after adding a little oil.
Does everyone agree reringing is the logical next step?

 

[andyak]

TAken with oil in boat.

 

[Big Shasta]

My other question relates to re-ringing a nykasil (sp) coated cylinder. Normally you would also hone the cylinder again to get a good new crosshatch for the new rings to seat to. But how does that work with a coated cylinder? Pretty sure you can't just hone it to get a good crosshatch.

Also, I would get an inside micrometer and check the bore to be sure it's within spec (at top bottom and middle of stroke at 2 spots each 180degrees out from each other.

 

[Zeus2013]

I am a car engine kind of guy, so these engines are a mystery to me. If I were in this situation till now, I would tear the entire engine down and do a full rebuild, get every part measured and replace anything I could.

I feel your pain. I hope you get this worked out.

 

[andyak]

It was my understanding that only the VX Yamaha engines were Nickasil coated. Does anyone know an exact list of which engines are coated?

 

[andyak]

Got completely apart the Pistons are actually scored. The rings are flush with the Pistons not stick out at all. All I can say is wow!

I don't know how it ran before.

They sell oversize oem piston's so I am assuming this is not a Nickasil plated engine.

 

[andyak]

Here is a pick of piston and cylinders

 

[Wayloncle]

Wow, I didn't expect them to look that "worn".

 

[andyak]

I don't even want to reuse the pistons! the cylinder scratches seem very minor to the feel - hopefully I won't even have to go oversize - I will bring to the machine shop and see what they say when I pickup the other crankcase.

 

[CrankyGypsy]

have a theory on the cause?

 

[andyak]

Unless cranking can erode rings like that it must have happened while running. Even if some fine aluminum dust got dumped into all intake tubes for my cutting there is no way it wore all that just cranking a few times.

 

[Big Shasta]

No that didn't happen suddenly. Wow....I'm glad you got down to it. If they aren't coated, I would get a machine ship to bore for the oversized pistons and go from there. Make sure you mic all the crank journals....with metal shavings running around in the engine from the pistons, you could have wear in other places too. Basically when you get it back together it'll be like a new engine.

 

[Zeus2013]

No that didn't happen suddenly. Wow....I'm glad you got down to it. If they aren't coated, I would get a machine ship to bore for the oversized pistons and go from there. Make sure you mic all the crank journals....with metal shavings running around in the engine from the pistons, you could have wear in other places too. Basically when you get it back together it'll be like a new engine.

I agree with you. Even if the cylinder is slightly out of round with the slightest damage, over time it will get worse. The rings will not fill the gap and it will be an area for blow by.

 

[andyak]

The standard oversize says .50mm. Does a machine shop need the Pistons first or can I just tell them the .50mm oversize and the original spec size for the cylinder? Haven't done this yet and want to know what to expect from a good machine shop.

So what would be the final bore size tolerance range for the oversize piston and rings ?

Thanks

 

[Big Shasta]

Seems like you should be able to take the original spec bore size, add .50mm and then use the same tolerance as before. It's going to depend on the extent of wear. You're talking about 1/64th of an inch oversize. That's only 1/128th of wear on each side. You might have more than that. I would talk to the machine shop first and see what they say. It might make sense to overbore and sleeve the cylinders. A good engine machine shop would be able to tell you the best route.

 

[Zeus2013]

I would have all the parts in hand and after they bore the cylinders, let them install rings, pistons, crank and bearings. This way they can check everything, including ring gap and all tolerances.

 

[subysti]

Just curious about how many hours on this engine. I keep hearing these engines are bullet proof but that is pretty bad wear

 

[CrankyGypsy]

i'm wondering if this was caused by an overheat problem?

...which has me scared. i'm reinstalling both of my engines right now and the starboard engine had a temp sensor with a broken wire that i replaced mid-season (i think it was broken for a very long time). i'm worried because once i replaced it, i had at least one instance not soon after where it did go limp - hope it didn't fail to go into limp mode prior and do the same thing as andyak's.

 

[andyak]

I hear you CrankyGypsy - I am wondering about my twin engine that still seems to run fine.... be nice if there were some kind of port to see rings - I could put a view cam (need really small cam end) down the spark plug hole but then could really only barely see cylinder walls.

 

[andyak]

The engine had 320 hours on it. I put on about 20 minutes since I bought it