This is mainly for the 1999 – 2002 400EX and Fourtrax. Newer years will be similar with maybe a few differences. Check manual for torque and specs on newer years just to make sure they’re correct.
Metric Socket Set
6 and 10mm Hex Sockets
Needle Nose Pliers
Standard and Phillip Screwdrivers
Micrometer or Caliper
A Good Straight Edge
Valve Spring Compressor
Hydraulic Press (for servicing the camshaft)
Molybdenum Oil (Molybdenum Grease mixed with engine oil in a 1:1 ratio)
NOTE: There are some special tool that you may need too, but you can take the parts to a machine or motorcycle repair shop to have checked or serviced if needed
REMOVING THE HEAD COVER:
The first thing you need to do is remove the front and rear fenders and give your quad a good bath so you don’t have any dirt or debris falling in the engine while it’s apart. Then the Air Box, Gas Tank and Heat Shield. Don’t forget turn off the gas and disconnect the cable at the front of the heat shield. I usually like to close the choke (on position) on the Carburetor and put it in a clean plastic bag to help keep debris from getting into. You don’t need to disconnect the throttle cable from the carb either. Just route it around to the front and set it onto the frame.
Remove the Spark Plug Wire and disconnect the Breather Hose from the top of the head cover. Next remove the upper Engine Hanger Bolt and Spacers that go through the top of the Head Cover.
On the left side of the crankcase, remove the Timing Hole Cap, Crankcase Hole Cap, and at least one of the Valve Adjusting Hole Caps. Using a 17mm socket, rotate the Crankshaft counterclockwise to align the “T” mark on the Flywheel with the Notch on the inside the timing hole. (The “T” mark will be sideways with a straight line above it.) The Piston should be at TDC (Top Dead Center) of the compression stroke. To make sure that it is, try to wiggle the Rocker Arms; they should be loose. If they aren’t then rotate the Crankshaft one more time and re-align the “T” mark.
NOTE: Always rotate the crankshaft counterclockwise. If you pass up the mark then rotate the crankshaft one more time and try again.
TIP: To help know when it coming up on TDC of the compression stroke, watch the rocker arms to start moving down then start watching for the “T” mark. The Camshaft will rotate twice for ever rotation of the Crankshaft so the mark may not always be right there. There is also an “F” mark (Fire) a little before the “T” mark (TDC), so look closely at which one it is. Removing the Spark Plug can make it a little easier too.
After you get it at TDC of the compression stroke then remove the (12) 6mm bolts and the (1) 8mm bolt to the Head Cover. Do this in a crisscross pattern in 2-3 steps. For the first step I usually like to turn them just till you hear the pop of them breaking loose. Then go through them again with about a half to full turn, and the final of fully removing them. Then remove the Head Cover.
TIP: Some of the bolts are longer then the other so here’s a tip to remind you were they all go a little easier. Here we have three different length bolts so for all the longest ones I put a green mark on the head of the bolt and a green mark next to the hole it goes in. Then for the second longest bolts I put a black mark on the bolt head and the hole they go in. For the short ones I just left them alone. This is good for when you’re not sure how long it will be apart for and don’t have to go through dropping the bolts in the holes trying to remember which one went where.
Also with the cylinder being at a slant some oil tends to hang out in the left front corner, which will run down the front of the engine when the cover I is removed. To avoid this I usually lift the front of the quad up to where the cylinder is vertical or close to vertical.
Next remove the gasket and then using a pair of needle nose pliers remove the (2) Dowel Pins. Sometimes the Dowel Pins will stay in the Head Cover.
On the back right side of the cylinder, loosen the Cam Chain Tensioner Plug and remove the (2) bolts to the Cam Chain Tensioner and remove it and the gasket.
Back up top, note the position of the Cam Sprocket to the Camshaft, the index mark next to the bolt is pointing up while the lobes are all pointing downwards. There are no marks on the Camshaft or the Cam Sprocket to line it up to and the Cam Sprocket can be bolted on in either direction, but should only go one way. Remove the bolt from the Cam Sprocket and then rotate the Crankshaft counterclockwise to reveal and remove the other bolt and remove it. Pull the Cam Sprocket off of the Camshaft Flange and then while holding the Cam Sprocket and Cam Chain remove the Camshaft with the Bearings and then remove the Cam Sprocket. Attach the wire to hold the Cam Chain from falling into the crankcase.
NOTE: The Bearings on the Camshaft are not pressed on and can side off so be careful not to catch the one Bearing on the Cam Sprocket while pulling it through.
Remove the small Plunger and Spring on the right side and the (2) Bearing Setting Pins. The Plunger and the Bearing Setting Pins should just pull right out loosely. The Spring usually gets held in the hole from the oil. It could be removed after pulling the Head by turning the Head upside down. Just don’t forget about it if you leave it in there.
REMOVING THE CYLINDER HEAD:
Remove the Exhaust System and the Carburetor. You can leave the Throttle Cable on the Carburetor and just route around to the front of the quad to rest on the frame inside of a good clean plastic bag or rag.
Next remove the (4) nuts and washers. Be careful with the one next to the Cam Chain so that you don’t drop it down into the crankcase.
Tip: A small magnet usually works real good for removing the washers. The one large washer in the front can sometimes be pain, but it could stay and removed after pulling the Head.
Remove the Cylinder Head and remove the washer(s) and the spring if you didn’t get them before hand. Then remove the gasket and the (2) Dowel Pins.
REMOVING THE CYLINDER AND PISTON:
Remove the front Cam Chain Guide and then remove the (2) 6mm bolts on the lower right side of the Cylinder. Then remove the (4) 10mm bolts and washers on top and remove the Cylinder.
Remove the (2) Dowel Pins and place a clean rag around the Connecting Rod and down inside the crankcase.
Then with a pair of needle nose pliers remove the Piston Pin Clip from each side of the Piston and then push the Piston Pin out from the Piston and Connecting Rod.
Clean the gasket off using a gasket scrapper being careful not to get any pieces inside the crankcase. I like to use an angle grinder with a light scrub pad on it for the final cleaning.
NOTE: A razor blade can also be used to clean the gasket off, but you half to careful not to dig the blade into the metal and mar it up.