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Which Pistons?

"Dear Ralph" November 28, 1999 0
Dear Ralph: I would like your thoughts on replacing pistons and which ones to use. I burnt a piston due to a carb freezing...

Dear Ralph:
I would like your thoughts on replacing pistons and which ones to use. I burnt a piston due to a carb freezing or something, it was not a lean set-up. It has been suggested to me to replace both pistons at the same time. The sled is a ’94 ZR 580 with almost 3,000 miles. Compression at the start of the season was 120 PTO and 119 MAG. Is it best to use OEM pistons, or Wiseco, or other aftermarket, jobber parts? What are the pro’s and con’s of those options? Can the cylinder plating take a light honing if needed?
Wes Moore
Prince Albert, Saskatchewan

Most important here is that you follow the specifications and recommendations based on the type of piston you install. Cast OEM pistons can be installed with much tighter skirt clearances than forged pistons (like Wiseco). Forged pistons tend to expand a bit more than cast, and need more skirt clearance. Forged pistons also tend to withstand heat better, and are typically more durable. OEM cast pistons are usually less expensive.
Considering that the engine has almost 3000 miles, it is prudent to replace both pistons. More important than mileage is the condition of the piston and cylinder; specifically the critical specifications like piston skirt to cylinder clearance and piston ring end gap. There are very specific service limits, and regardless of how the piston and cylinder “look”, these measurements should be performed and the decision based on that information.
The ’94 580 engine with plated cylinders would need to have any aluminum stuck to it removed, then it could be lightly honed with 400 grit silicon carbide SOFT stones (Ammco #3955 honing stones). The ’94 ZR 580 had a tendency to run at too high RPM in the midrange, which caused some piston seizures. Arctic Cat addressed this in Service Bulletin 94-08, recommending the installation of a new cam arm and drive clutch spring kit (p/n 0637-069) to keep the engine spinning at 8000 RPM.
Other areas to pay attention to on the 580 engine that scuff or seize pistons are the cooling system, it is extremely critical that the cooling system be properly filled. If air remains in the system, piston scuffing can start to occur when the water temp reaches 190 degrees. Check the outlet of the silencer, make sure it isn’t bent, partially closed from hitting a stump or something. This would cause piston heat to be excessive. You should also remove the silencer and shake it, checking for loose internal baffles. Loose baffles will have the same effect as a blocked outlet, restricting exhaust and causing piston heat.

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