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Yamaha F115

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I have a 2006 yamaha 115 four stroke on a 18.5 ft xpress bass boat. Last week a ring failed and scared 2 cylinders. My mechanic said that nothing 'caused' it that sometimes the rings just go out. He said that the 115 is a solid motor but he has seen a couple come in with this problem. I was wondering if anybody else has run across this. Motor has less than 150 hours on it and has never been run hard.
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Rings
Do you use pure gas of ethanol blend?
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Real gas
I try to only put real gas in it. Only had ethanol a couple times and the few times it did it was treated with sea foam.
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low hours
i will say that 150 hours in a 8 year boat is sometimes not a good thing. It needs to be used regularly. Let it sit for too long and things start to build up. I run my 2003 yami hard. use the cheapest and most convenient fuel which always contain etanhol, but I run it almost every week. Beside the yearly maintenance items, it been abeast of a motor.
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Ring Failure
It's all a crap shoot now-a-days. You are better off using pure gas than ethanol. But, I know folks who run ethanol and no additive and have motors with hundreds of hours on them... and I know folks who used pure gas and additives and had problems. But the statistics from the outboard motor companies tests say that engine problems go way up when using ethanol. Four stroke motors seem to tolerate the problems better with ethanol than 2 stroke motors, but they are not entirely immune. I saw a test result from Mercury Marine on using ethanol in 4 stroke motors. The completely tore the motors down after every 100 hours to inspect all components. They were very surprised to see initial bearing damage on one motor after only 200 hours. Every outboard manufacturer recommends using an additive with every fill up now.... of coarse they recommend their own additive. Most of these additives contain a component that helps the rings.
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Had it been ran hot ?
If a motor has been run hot, the piston ring stake pins, the ones that keep the ring from rotating in the land, can come loose. The stake pins are steel and the piston aluminum, the alum hole for the stake pin will open up before the steel pin has time to heat and expand in timely fashion, thus letting the pin come out. Once the pin is out, it will do damage itself, the ring will then work itself around inside its land till a end of ring gets into a port, then its a done deal.

More motors have been run successfully on ethanol than folks want to admit to. You can't let it set up as mentioned, the alcohol will absorb moisture. The worst thing about ethanol is it eats up the older style non-alcohol hoses, seals, etc., then those particulates get into carbs, stops up injectors which will toast a nice motor..
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Never
Motor has never bren run hot, at least in the four years i have had it. I put a water pressure gauge on it that i watch more that any other gauge on the boat. When you get one rebuilt, is there a break-in period like on a new motor?
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Yamaha Ring free Oil is a must
Yamaha told me to use their Ring Free Oil in my gas. Not may want to use it but you need to use Yamaha Ring Free Oil in Gas. I have used it my Yamaha motors for years.
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Check the bad piston
The ring free and like products (sea foam, etc.) do a good job for keeping carbon build up from behind the rings, inside the lands.

Your mechanic should be able to tell you exactly why the thing went south. A lost pin is pretty evident to the naked eye. Carbon build up in the ring lands are too.
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Reason
My mechanic told me when he got it broken down that the yamaha f115 is a very solid motor with little to no problems but he has seen a few the come in where the rings have come apart for no reason. Said he is not sure if yamaha got a bad batch of rings for some reason or what. Im definately going to ask him again when i go pick it up. Hopefully he will call this week.
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