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I have a 04 rhino 660 that needs to be rebuilt.Should I bore out the cyl and go with a oversize piston and rings or buy the 686 big bore kit that comes with everything? I am using the rhino for trail riding no racing or work.
 

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My recommendation would be to contact Chris sk21 at Benchmark Performance here on forum... see vendor section...

There is no point in putting a stock piston back in IMO...

And if you replace the piston there is NO reason not to go +2mm on bore imo...which is a 102mm...

However, you can go cheap and get a wiseco 11:1 100mm piston and keep the stock bore...

I highly recommend getting a good head gasket and head bolt stud kit...

Another good source for professional help and quality parts is Racers Edge AZ - Tucson, Arizona Motorcycle, ATV, Rhino Services

Stick with 11:1 compression or less for use with pump gas...

Stick with 10:1 for use with 87 gas...

You can have 11:1 pistons milled to 10:1 if you use a quality shop like Benchmark or Racers Edge AZ
 

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Bore it out to 102mm which will make a 686 as Craig mentioned. Benchmark, Wellers, Pro Shop, KMS and Racers Edge can help you out. DO NOT have a piston milled to lower compression. It will sit to low in the cylinder. I talked to CP and they recomend, at 11 to 1 to mix race gas with 91. I had them make a 10.5 to 1 piston for me that will safely run on 91 pump gas. With no detonation issues. Alot of guys are running 11 to 1 ratio with no problems, I just wanted to be safe and not have any issues with some of the crap gas out there!. Greg at Unleashed UTV helped me with getting the piston as he is a dealer. He had CP make a couple of extras and has them for sale. Give him a call, he also has the Cometic top end gasket kits for the different bores.
 
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