Polaris Sportsman 850XP Engine Rebuild: Engine Reinstall

Welcome to the fifth and final part of our project to rebuild a Polaris Sportsman 850XP engine: reinstalling the engine and connecting all of its components after reassembling the top end

Watch the video above and follow the steps below to reinstall a rebuilt Polaris Sportsman 850 engine. Use our Polaris Sportsman 850XP parts diagrams for additional reference.

Tools and Parts Needed - Polaris Sportsman 850 Engine Reinstall


  • Ratchets and extensions
  • Sockets set
  • Pry bars
  • Hammer
  • Torque wrench

Polaris Sportsman 850XP engine reinstall

Reinstalling Polaris Sportsman 850XP Engine

Step 1. Grease the transmission input shaft and install a new drive train gasket onto the engine.

Polaris Sportsman engine rebuild reinstall

PRO TIP: Put some gasket sealant onto the new drive train gasket to prevent it from falling off the engine as you remount it.

Step 2. Carefully reinstall the engine into the chassis and remount it to the transmission, being careful not to damage any hoses or lines.

Polaris Sportsman ATV engine rebuild reinstall

PRO TIP: Pay particular attention to the hose leading to the foot brake that runs under the engine and avoid crushing it.

Step 3. Reinstall the bolts that hold the engine to the transmission and torque them to 18 foot-pounds.

Step 4. Reinstall the engine mounts and brackets. Lift the engine with a pry bar to slot the mounts and brackets back into place.

Polaris Sportsman ATV engine rebuild reinstallation

Watch the clip below to see the engine installation segment of our Polaris Sportsman 850 engine rebuild video.

Step 5. Reinstall the fuel rail and fuel injector assembly to the intake plenum, then reinstall the throttle bodies and intake air duct, and reconnect all their wires and sensors.

Polaris Sportsman 850 engine rebuild fuel injection

Step 6. Reinstall the prop shaft by sliding it onto the transmission shaft first, then knock the prop shaft forward so it slides onto the input shaft, and push the prop shaft pin back into place.

Polaris Sportsman 850XP engine rebuild reinstallation

NOTE: Make sure the prop shaft pin is properly aligned with the hole in the input shaft before trying to push it back into place.

Step 7. Reinstall the fuel tank and reinstall the fuel lines.

Step 8. Reinstall the upper frame strut.

Watch the clip below to see the fuel system, prop shaft and strut reinstallation segment of our Polaris Sportsman 850 engine rebuild video.

Step 9. Reinstall the exhaust system, first tightening the four exhaust header bolts, then reconnecting the exhaust shield at the rear of the exhaust.

Polaris Sportsman engine rebuild exhaust system

Step 10. Reconnect the upper radiator hose to the top of the engine.

Step 11. Reinstall the intake plenum boot, the transmission air intake and the airbox.

Step 12. Reinstall the coil pack trigger/CDI box to the side of the airbox.

Polaris Sportsman 850 engine reinstall coil pack trigger

Step 13. Install new spark plugs and reattach the spark plug leads.

Step 14. Clean and reinstall the air filter or install a new air filter.

Watch the clip below to see the exhaust system and air intake system reinstallation segment of our Polaris Sportsman 850 engine rebuild video.

Step 15. Refill the cooling system and the fuel tank, then reconnect the battery and refill the engine with two quarts of Polaris PS-4 engine oil

Polaris Sportsman engine rebuild oil change

Watch the clip below to see the refilling segment of our Polaris Sportsman 850 engine rebuild video.

Breaking In a Rebuilt Polaris ATV Engine

After completing the engine rebuild, take it easy for the first 20 hours of running to get all the new internal parts nicely seated. Use varying throttle levels, and avoid running the motor at a constant RPM for too long. Avoid using wide-open throttle until the new engine parts have fully bedded in.

Watch the video above for professional tips on how to break in a rebuilt engine.







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