07-08-2009, 11:00 PM #1
'98 GP800--no compression on rear cylinder
I'm helping a buddy fix his '98 GP800. I'm a Polaris guy and have done a top end job on mine, so reworking a cylinder(s) is pretty much straight forward. However, the YPVS is something new to me so I appreciate any help you can offer to guide me through this. I have downloaded and printed the Service Manual so I have all the torque specs and related information.
Last year he was doing some low speed running, went to take off, and had no power. At the ramp we found almost no compression on the rear cylinder.
I haven't torn it down yet but I did shine a small LED light in the spark plug holes to get a first look. The front cylinder is perfect, very nice piston wash and clean looking cylinder walls.
The rear cylinder had great wash pattern but is missing a small chunk of piston right near the exhaust valve. Also it has some dents in the dome where the piece of shrapnel bounced around before it left. I'm certain at minimum we'll need a piston, rings, and hone the cylinder. This big question is the condition of the exhaust valve, cylinder wall, and exhaust port.
Here are my questions:
1) I have read something about a small clip failing on the exhaust valve allowing it to drop down and tag the piston. Does this sound right, or am I just dreaming?
2) Are these cylinders plain steel or are they Nikasil coated?
3) While I have the top end torn down, what "usual suspects" should I look for or replace as preventative maintenance?
4) Are there any tricks to the exhaust or other components I should know as I remove them?
07-09-2009, 05:53 AM #2
07-09-2009, 09:15 AM #3
Beer, you da man. Thanks for the linky to the retainer clips.
I'm hoping to get the exhaust pulled and the head off this weekend and get a first hand look at the damage.
I know we're in for a piston and rings at minimum. What brand of piston works best in these 800 engines? Also, what's the best source for Yammie parts? I have purchased some things from Parker Yamaha in the past and they seem okay. Is that a good choice?
07-09-2009, 09:24 AM #4
07-10-2009, 01:17 PM #5
What's the verdict on the cylinder walls, are these nikasil plated? If so, can that be honed or does any cylinder wall work require them to be replated?
07-10-2009, 01:23 PM #6
Yes they are plated. If there is any aluminum smeared on the plating use muriatic acid to remove the aluminum. Then a light hone.
08-16-2009, 10:42 PM #7
So I finally had a chance to dive into this ski. I spent too much time getting my Polaris ready for the Midwest Nationals at Starved Rock, so the GP800 took a back seat.
Anyway, I pulled the cylinder today and it's not pretty.
There's a good size gouge above the exhaust port and you can see where the rings snagged the bottom edge of the port.
This a little blurry, but you can see where the rings also snagged the top edge of the exhaust port.
Here you can see the upper gouge pretty well. It also looks like the rings clipped the exhaust valve as well.
Here's what the exhaust side of the piston looks like
Here's the dome where the chipped off piece bounced around like a jumping bean:
Here's what the head looks like. The MAG side looks perfect.
The big questions are:
1) Can this jug be bored and replated? If so, where can I get this done?
2) If the jug is shot, where do I get a used one? I'm gonna contact Zigler, but are there other sources that readily have one available?
3) What about the piston? Can I get oversize if I bore this?
4) Can I use a die grinder to smooth the head and reuse it? This is recreational ski, so little lower performance isn't a big deal.
08-17-2009, 12:26 PM #8
In searching for a replacement cylinder I've found Wiseco offers a cylinder sleeve for the 66E motor. What's that about? Can you oversize this jug and then install a sleeve? So far I've been quote around $300 just to repair and replate this cylinder. That's a lot more spendy than I had anticipated on this repair!!!
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