Pistons, Rings & Piston Kits
About Pistons, Rings & Piston Kits
The piston and rings seal combustion pressure and carry the force of each burn down to the crankshaft. They wear with every mile and every hour, and the signs are hard starting, blue smoke, oil use, low compression and lost power. Riders buy a piston kit to fix those, to rebuild after an engine failure, or to raise compression and displacement in a performance build.
The key number is bore size. It is listed in millimeters, with standard (STD) and oversize steps such as +0.50, +1.00 or +2.00. An oversize piston is for a cylinder that has been bored out; measure the cylinder against the limits in your service manual to find out which you need. Compression ratio is also named in some listings, for example 9.2:1, 11:1 and 12:1. A higher ratio can make more power but can call for higher-octane fuel, so stay with the stock ratio unless you are building for performance. A piston kit holds the piston, rings and pin hardware. A top end kit is built around the same parts and adds the gaskets, so read the contents list.
Piston brands here include Athena, Vertex, SDI, WSM, ProX and Wiseco. Pick your machine in the Ride Finder at the top of the page, because bore and ratio vary by model and year. Pistons rarely go in alone, so also look at Complete Cylinders, Connecting Rods & Parts, Engine Bearings and Gaskets, Seals & Kits. The whole group is Engines & Engine Parts.
FAQs on UTV and ATV pistons
What does +0.50 or oversize mean on a piston kit?
It means the piston is larger than the standard bore size by that many millimeters. Oversize pistons are used when a worn cylinder has been bored out to a larger diameter. Choose the piston size your machine shop or service manual specifies for your cylinder.
Do I have to bore the cylinder when I fit a new piston?
Not always. Measure the cylinder against the wear limits in your manual. If it is within limits, it may need only honing so the new rings seat. If it is out of round, tapered or scored, it needs boring to an oversize piston or replacing.
What is the difference between a piston kit and a top end kit?
A piston kit has the piston, rings and the pin hardware that mounts it. A top end kit is built around those parts and adds the gaskets needed to put the top of the engine back together. Check the listing's contents to see exactly what is included.
What is compression ratio and should I raise it?
Compression ratio compares the cylinder volume with the piston at the bottom of its stroke versus the top. Raising it can add power, but it increases heat and can call for higher-octane fuel and careful tuning. For work and trail machines, the stock ratio is the safer choice.
How do I break in a new piston and rings?
Follow the break-in steps from the piston maker and your manual. In general that means moderate, varied engine speeds, no sustained full throttle at first, and fresh oil and filter after the first period of use. The rings need time to seat against the cylinder wall.
