Polaris Sportsman 850XP Engine Rebuild: Cylinder Head Install & Set Cam Timing

Welcome to the fourth part of our project to rebuild a Polaris Sportsman 850XP engine: reassembling the top end after tearing down and rebuilding the bottom end

Watch the video above and follow the steps below to reinstall the cylinder head and set the cam timing of a Polaris Sportsman 850 engine. Use our Polaris Sportsman 850XP parts diagrams for additional reference.

Tools and Parts Needed - Polaris Sportsman 850 Cylinder Head Install and Camshaft Timing


  • Ratchets and extensions
  • Socket set
  • Allen set
  • Torx set
  • Feeler gauge


  • Torque wrench


Polaris Sportsman 850 top end rebuild

Reinstalling Cylinder Head Polaris Sportsman 850XP

Step 1. Make sure the cotter pin on the camshaft is pointing upward. This will verify the engine is on the compression stroke.

Polaris Sportsman 850XP top end rebuild

Step 2. Make sure the mating surfaces of the crankcase, the cylinder head and the new cylinder head gasket are completely clean and free of fingerprints.

Step 3. Set the crankshaft to top dead center by aligning the TDC mark on the flywheel with the point where the upper and lower halves of the crankcase join.

Polaris Sportsman 850 engine TDC

Step 4. Reinstall the dowel pins and the cam chain guide into the crankcase.

Step 5. Install the new cylinder head gasket onto the crankcase.

Polaris Sportsman engine rebuild cylinder head

PRO TIP: Hold the cylinder head gasket by its edges to avoid getting dirt or fingerprints on it.

Step 6. Reinstall the cylinder head onto the crankcase.

Polaris Sportsman engine rebuild cylinder crankcase

PRO TIP: Tie a piece of wire or string around the cam chain to hold the chain up as you reinstall the cylinder head.

Step 7. Oil the tip of the threads on the six cylinder head bolts and reinstall them. Torque the bolts to 43.5 foot-pounds in the sequence shown below. Next, tighten each bolt again by an additional 90 degrees in the same sequence.

Polaris Sportsman ATV engine rebuild

Reinstalling Sportsman 850XP Camshaft and Rocker Box 

Step 1. Oil the camshaft’s lobes and journals, then reinstall them into the cylinder head.

Step 2. Verify the crankshaft is still at top dead center by aligning the TDC mark on the flywheel with the point that the two halves of the crankcase meet.

Step 3. Align the camshaft so that the small dowel pin on the end of the shaft nearest the cam chain is facing upward.

Polaris ATV engine rebuild top end

Step 4. Reinstall the rocker box, then torque the ten bolts to 13 foot-pounds in the sequence shown in the image below.

Polaris Sportsman engine rebuild rocker box

Setting Polaris Sportsman 850XP Cam Timing 

Step 1. Loosen the valve adjusters and verify the camshaft is set with the small dowel pin facing upward. Use a wrench to rotate the camshaft if necessary.

Step 2. Reinstall the cam sprocket and cam tensioner. Once reassembled, the two marks on the sprocket should be level with the cylinder head and the TDC mark on the flywheel aligned with the crankcase halves.

Polaris Sportsman 850XP cylinder head rebuild

Step 3. Adjust the valves.

NOTE: Watch the video above for reference on how to adjust valves. 

Step 4. Install a new valve cover gasket and reinstall the valve cover.

Completing Polaris Sportsman 850XP Top End Rebuild 

Step 1. Reinstall the thermostat and the starter motor.

Step 2. Reinstall the rubber drive gear onto the crankshaft, followed by the stator and stator cover assembly.

Polaris 850 Sportsman ATV top end rebuild

With the engine, reassembled all that remains is to install it back into the Polaris ATV. Watch how to do that in the fifth part of our Polaris Sportsman 850XP engine rebuild video below.



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