Yamaha YFZ450R Top End Rebuild: Reassembling the ATV

Welcome to the third part of our YFZ450R top end rebuild project, which shows you how to reassemble your Yamaha ATV and get it running again.

Watch the video above and use our step-by-step guide below to learn how to rebuild the top end of a Yamaha YFZ450R. Use our Yamaha YFZ450R exploded parts diagrams for additional reference.

NOTE: The video above features the entire YFZ450R rebuild from the beginning, starting with the engine teardown. Watch the video below if you want to jump ahead to the YFZ450 top end rebuild. 

Tools and Parts Needed – Yamaha YFZ450 Top End Rebuild


  • Ratchets and extensions
  • 8mm through 17mm sockets
  • 3mm through 14mm Allens
  • 10mm, 12mm wrenches
  • Flat-blade and Philips screwdrivers


  • Needle nose pliers
  • Torque wrench


Yamaha YFZ450R Top End Rebuild - Reinstalling the Ancillary Components

Step 1. Reconnect the radiator hose and the cable harness bracket to the top of the cylinder head.

Yamaha YFZ450R top end rebuild

Step 2. Reinstall the upper mount bracket holding the engine. Reconnect the two top bolts first, then the lower 8mm Allen that connects to the cylinder head. Torque the upper bolts to 24 foot-pounds and the Allen to 29 foot-pounds.

PRO TIP: You should’ve already reinstalled the crankcase inspection bolts and O-rings, but if you haven’t, do so now.

Step 3. Reinstall the breather hose that connects to the valve cover and lower down to the crankcase, then refasten its mounting brackets.

Watch the clip below to see the hoses and brackets reinstallation segment of our Yamaha YFZ450R top end rebuild video.

Step 4. Reattach the air intake to the throttle body. 

NOTE: There’s an indentation on the clamp that must be aligned with the corresponding section on the rubber intake boot. The clamp uses a 4mm Allen.

Yamaha YFZ 450 top end rebuild

PRO TIP: Spraying parts cleaner on the intake boot makes it easier to slide back over the throttle body.

Step 5. Reinstall the airbox, and reattach it to the throttle body. Use parts cleaner to help the boot slide over the throttle body, and use a 4mm Allen to fasten the clamp. Tighten the airbox mounting bolts.

Yamaha YFZ450R engine rebuild

Step 6. Reinstall the breather box to the side of the frame with its 10mm mounting bolt.

Watch the clip below to see the air intake system reinstallation segment of our Yamaha YFZ450R top end rebuild video.

Step 7. Reinstall the bracket across the top of the frame, but don’t fully tighten the bolts because you’ll have to remove them to install some of the bodywork later.

Step 8. Inspect the copper exhaust gasket, and replace it if necessary. Reinstall the exhaust header pipe, but don’t tighten the bolts yet. Next, reinstall the muffler section and once it’s in place, fully tighten the header mounting bolts and the muffler mounting bolts.

Yamaha YFZ450-R top end rebuild

Step 9. Reinstall the plastic shield that sits under the fuel tank and the fuel tank itself. The fuel tank is held by two clips at the front and a single bolt at the rear. Reconnect the fuel line and the electrical cable to the fuel tank.

Step 10. Refill the radiator with coolant/antifreeze and top off the reservoir.

Yamaha YFZ-450R top end rebuild

PRO TIP: If the old fluid in the reservoir looks old and contaminated, now is the ideal time to replace it with fresh coolant/antifreeze.

Watch the clip below to see the exhaust, fuel and cooling systems reinstallation segment of our Yamaha YFZ450R top end rebuild video.

Yamaha YFZ450R Top End Rebuild - Initial Engine Startup

Step 1. Reconnect the battery, and run the engine on idle for about 30 seconds. Next, check the oil level and coolant level, and top them off if necessary.

Step 2. Run the engine on idle again until it gets up to full operating temperature (about 5 minutes) while watching for any leaks and listening for any unusual sounds. Wait for everything to cool down again, then check the oil and coolant levels one more time and top them off if necessary.

YFZ450R top end rebuild

PRO TIP: Make sure the ATV is outside or in a well ventilated space when you run the engine for an extended period.

Step 3. Reinstall the seat, and all the body panels and plastics.

Watch the clip below to see the initial engine startup segment of our Yamaha YFZ450R top end rebuild video.

NOTE: Following a top end rebuild, the new engine parts will need to be “broken in.” To do this, be progressive on the throttle and avoid rapid acceleration or wide-open throttle for the first hour of riding.

Yamaha YFZ450R ATV top end rebuild


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