Credit to Rob at Rockin Horse Web for this (2004/2005) Honda TRX450R Camshaft install. The pictures and write-up are all available courtesy of him. FYSA: Although not comprehensive, most of these steps also apply to 2006+ TRX450R's and CRF450R dirt bikes as well.
Disclaimer: I assume NO RESPONSIBILITY for your ATV, parts, or tools. Conduct these steps AT YOUR OWN RISK.
When removing or torquing bolts, if there is more than one bolt holding anything on that you are removing, follow this procedure. Example: There are 3 bolts holding the valve cover on; put your socket on bolt #1 and barely break it loose, then bolt #2, then bolt #3, then remove bolt #1, then #2, and finally #3. Do NOT remove #1 before you break #2 and #3 loose. If there are 4 bolts follow the same procedure using a crisscross pattern. The same goes for tightening, install all 3 bolts and hand tighten, then cut the torque spec in half and start with #1, then #2, finally #3. Then reset your torque wrench to the full amount and repeat. If the torque amount is high, like 40 ft/lbs, then start at 10, then 20, 30 and finally end at 40...do not just cut a large amount like that in half. Use this procedure anytime there is more than one bolt. This may take a little longer, but not in comparison to a broken bolt or stripped threads.
Remove the seat, rear plastic, and finally the front plastic.
Remove the gas tank by completing the following steps: ensure the petcock on the gas tank is turned off and remove the clamp/hose from the petcock. Remove the two bolts and two tank straps and lift the tank off the bike. Remove the plastic tank heat protector from below the gas tank.
Remove the spark plug boot and the spark plug. Remove the 3 bolts holding the valve cover on and remove the valve cover. Remove the crankshaft hole cap (pictured below) to set the motor to top dead center (TDC). Inside the crankshaft hole, use an 8mm allen wrench to rotate the crank manually to line up the timing marks on the crank gear and case.
TDC is achieved once the timing marks on the crank gear and case are aligned, as well as the timing marks on the cam sprocket and cam tower are aligned. Note: there are two TDC's on a 4-stroke engine, make sure your cam lobes are pointing to the rear of the quad and slightly up like in the picture below. Once you have achieved TDC you need to secure the cam chain to the sprocket with two large zip-ties.
Remove the two allen head bolts holding the sprocket to the camshaft. I found this to be very difficult as they are secured with loc-tite. Put quad in gear and have a helper step on rear brake to hold cam sprocket in place to break the bolts free.
Cam chain tensioner...there are different thoughts on the removal of this part. Here is what I did and I found it very simple. Take the end bolt out of the tensioner and insert a small flat head screw driver. Turn the screw driver clockwise until it stops. All of the tension should be removed from the chain and you can slide the sprocket off the end of the camshaft. Do not let go of the screwdriver - just ease it back turn by turn until it is free (its spring loaded).
At this point I would recommend you check the clearance on your valves. This is not necessary, but is recommended. See the valve adjustment article for instructions on how to do this. Moving on, remove the four bolts holding the cam assembly as shown below.
Steadily lift off the cam assembly taking care not to drop any of the shims into the motor. The assembly is positioned with bosses, just work it back and forth gently and lift up at the same time and it will come off. If any of the shims come off make sure you place them back on the correct valve. The shims are circled below.
Remove the decompressor bolt from the cam assembly. I used a pair of vice grips but some have used a vice.
Remove the snap ring and the cam shaft will slide out.
Slide the new cam in and re-install the snap ring. Apply loc-tite and install the decompressor bolt. Torque the decompressor bolt to: 18 lbf-ft. Re-install the cam assembly and torque the four bolts to 10 lbf-ft. Ensure the cam lobes are pointing to the rear of the quad and slightly up as pictured below. Use feeler gauges and check your valve clearances. Compare your clearances to the camshaft clearance specs. After you check all the valves you may need to make shim changes. The easiest way to explain this is find the correct thickness of gauge where it will slide in an out with just a little tension or pressure on the gauge. All of the shims are numbered according to how thick they are. It sort of works backwards, if you can properly fit a 0.220mm (exhaust valve example) gauge in, then you are too tight even though it is a smaller number than the required 0.280mm tolerance. Therefore, you add a smaller numbered shim to increase the amount of space so the larger 0.280mm gauge will fit. In the above example, if a 0.220mm fits then you probably need to drop 0.060mm on the shim. So if the factory shim is a 2.15mm then you need to get a 2.09mm. For a detailed write-up on how to calculate and adjust your valves, see the valve adjustment article.
Check the clearance on the decompressor arm and compare it to the camshaft clearance specs. For example, the HRC camshaft decompressor clearance spec is: a* + 0.25mm +/- 0.02mm, where a* = adjusted exhaust valve clearance. Basically, take the exhaust valve clearance you have and add 0.25mm +/- 0.02mm to get the clearance for the decompressor. If needed, adjust the decompressor arm clearance by loosening the jam nut and turning the decomp arm in or out using a flat screw driver. Don't forget to tighten the jam nut back down afterwards.
Mix your molybdenum and motor oil in a 50/50 solution and apply to the top off the shims, lobes on the cam, rocker arm and lifters.
Ensure you are at TDC and turn the screwdriver in the camshaft tensioner fully clockwise. With the tension off the tensioner, reinstall the cam sprocket and chain. Ensure the alignment marks line up, (there are two marks on the sprocket and two on the camshaft holder) apply loc-tite and torque the sprocket bolts to: 14 lbf-ft. You can now remove the zip-ties. Now begin reassembling your 450R. Torque specifications for reassembly: head cover torque: 7 lb-ft; spark plug: 17 lb-ft; crankshaft hole cap: 11 lb-ft.