Polaris 550 engine burndown...

[Xc700edge]

Bought my 12yr old son a used 2004 polaris 550 supersport. It had 1200 miles on the clock and was/is in mint condition. So he put about 600 trouble free miles on it riding local trails and fields so I figured he was ready to do some riding on the groomed trails up north. Got him up north and headed out for a day ride and the sled fires right up and is running great as usual. After about 30 miles cruising at an average speed of about 40mph, I pulled over to take a break and see how he's doing. As my son pulls up and parks behind me, when his sled idled down it sounded like the engine was "rattling". My first thought was a crank bearing but figured if it was, the whole engine was going to have to come apart anyways, so we continued on to our first stop for lunch. He told me that at speed going down the trail, it felt fine. Had power as normal but only made that. "Rattling" noise at idle. After lunch we headed back toward the truck, which at this point was 45-50 miles away. Well, to no surprise of mine, we got about 5 miles from the truck and his sled list power , spit and sputtered, then died. It didn't seize but when you pulled it over it just sounded like metal on metal clunking around in there.
So we towed it to the truck and hauled it back home. I pulled the head off just to see if I could see anything wrong, expecting that I wouldn't as I thought it was a crank issue for dome reason. What do I find? Mag side piston is completely melted around the outside and a huge blob of melted aluminum stuck to the top of the piston on the exhaust port side. Also, quickly mic'd the cylinders to find that the mag side has been bored to 73.5mm with the pto side still at the stock 73 mm spec. So it's burned down before on the previous owner and he didn't tell me. Now I'm thinking that there might not be any issue with the crank at all. Now that the head is off, it pulls over nice and smooth, no more banging around.
I've read some of the links on here about the horror stories of these motors and I still have some questions.
1.) how do I tell if the crank is ok or not without tearing down the entire engine?

2.)was the noise I was hearing normal for a melting down piston that had a blob of aluminum on top of it?

3.) do I just put this thing back together and sell it?

4.) do it completely rebuild it and add heat shields and cooling ducts and raise the needles to fatten up the low end?

5.) do i spend $1000 on a salvage yard engine?

Go ahead and ask me questions and give me as much opinion and feed back as you can.

Thanks

 

[whoisthatguy]

You can grab your drive clutch and wiggle it while watching the crankshaft at where it exits the crankcase. If it moves or rattles then the PTO bearing is shot. If the engine sounded like it had popcorn in it before it stopped running, then it was throwing metal filings all over the place and some may have made it into the lower crankcase and got into the rod and crank bearings. In either case, your crank would be toast.

However, replacing a piston, both sets of rings, new high temperature O-rings, cylinder base gaskets and one cylinder is pretty easy and only takes a few hours, once you get the parts. It might run you $300 in parts with a used cylinder. Don't worry about minor dents on the bottom side of the cylinder head.

The cause may have been the oil pump, or perhaps a carburetor was not seated properly in the air box and sucked in more air, leaning it out and burning it up. You would want to find the cause and fix it. The jets may have been significantly different. If you were sledding at lower elevations than what it was jetted for, then you have the cause as improper jetting. Check the throttle needle clip positions, to see whether they are the same or whether something came loose.

Your son was probably really working the engine hard so the main jets are more likely to be the culprit, than the needle clip position.

I would attempt to fix it before writing the sled off.

 

[sno*jet]

my mom stuck a piston on her 06 with like 600 miles. it is a known problem and polaris has a solution invoving a oil line vent or something. i guess air bubbles got in and stopped the flow of oil. hers has been fine since the rebuild and oil line update, partially paid for by polaris, made dad mad and said never another polaris tho...
anyway, i would rebuild what is broken and not worry about the crank, and if it was mine i would see if you can get away without oil injector on that motor and just mix 50 or 40:1. would save some weight too.

 

[Xc700edge]

Yeah, I've been thinking about that "oil pump bleed kit" that polaris created. For. $45 I suppose it's a little insurance. Has anyone on here bought one of those " fleet model intake ducts"?

 

[Xc700edge]

So, just tore the engine down. No piston skirts on the mag side piston and the exhaust is coated with melted aluminum. Funny thing is the cylinder walls are unharmed. Even worse news though..... Lower connecting rod bearing has about ten thousandths of vertical play in it. The crank is toast.

 

[blueyoda]

Find a 488 and or 440ho and forget the 550 forever!