Guys, got the engine pulled from my ATV (Yamaha Grizzly 660) and tore it partway down. I'm not sure which way to go with it from here.
Here's the deal.....the ATV has 2700 miles on it, which I've been told isn't a lot. My other one doesn't have a speedo/odo so I have no idea how many miles I've ridden it since I got it around 10 years ago.
The previous owner was riding it on a road at 68 mph (nutty) when it suddenly lost power. He turned around and went back to their mountain cabin and it was knocking when he shut it off. He took it home, started it up and drove it into his garage where he was going to rebuild it. He decided he didn't have the time or the desire to do it so he sold it...I bought it. He concluded that it needed a bottom end rebuild and advertized it as such based on his evaluation.
Yesterday I tore it down and one of the first things I did was drain the oil. It had about a half a cup in it and the oil capacity is 2 quarts. I saw no metal in the oil but it was pretty black. I connected a battery to the starter to see if it would knock when it turned over...it didn't as far as I could tell and it seemed to turn over smoothly. Using the pull-starter it seemed to have some compression. I tried a compression check but my gauge end wouldn't go far enough down in the deep spark plug well on the head. So I put my thumb over the hole and cranked it over a few times. It had compression but not much.
I pulled the cylinder head and the piston has a little carbon but it's not bad. The valves all look good...it has 5 valves even though it's a single cylinder engine. Weird.
Anyhow, I pulled the cylinder and the piston has obvious scoring/galling and the cylinder sleeve is scored pretty bad on the thrust side. The cam looks fine as does everything else in the top end of the engine. The rings are pretty tight in the piston grooves. The rod bearing (roller) seems pretty tight other than having some side play which I believe is normal. It turns over great and the crank is also on roller bearings, which seem to have no wear. I can see no damage looking down into the crankcase and the timing chain and everything looks good and was still covered with oil.
So I'm concluding that he ran it low on oil and the piston started to seize in the cylinder causing the rapid loss of power. The bottom end seems to be okay since everything there is on roller bearings, and what little oil it had was enough to preserve the bearings. He said the transfer case worked fine as well, and it's all roller bearings too. I'm assuming the knock was from the piston, not the bottom end.
I'm really hesitant to split the case and get into the engine any further, because I don't think I need to. I think the bottom end is fine and a top end rebuild should take care of it.
So my plan is to flush the engine with diesel fuel to see if I can find any metal shavings or anything that didn't come out with the oil. I plan to remove the oil filter and split it open to see if there's anything in it. Then if all looks okay I'll buy piston kit and have the cylinder bored and honed and replace it with all the new top-end gaskets. Hopefully once I put it all together it will run fine without the knock. I assume the knock was from the piston, not the bottom end.
Does this sound reasonable? Or am I wasting time and money. I figure if something IS wrong in the bottom end I haven't wasted anything but gaskets if I have to tear it down again. A bottom end kit is almost $400 and the piston kit with gaskets is $165 plus maybe $75 for boring and honing. I'd have to do the top end in any case. There could be a few odds and ends I need too by the time I'm finished.
What do you think? I don't want to have to do this again, but I hate to spend the extra $400 or more and the added time if I don't need to.