HOW-TO: Install a Cam Shaft

Building engines is a fun thing—tough, but fun. We have assembled and tuned many motors here at Dirt Wheels, whether it be with a shop’s help or on our own. It’s also a well-known fact that a great way to get more power or tune your engine’s power characteristic is with a good set of aftermarket cams. We recently caught up with Travis Smith of Thumper Racing ([661] 424-1800) during a cam install on a YZ450F engine. The engine is very similar to the YFZ, and the cam install and valve-adjustment techniques carry over on to all shim-under-bucket-style head designs, such as on the TRX450R, YFZ450, YFZ450R, KFX450, DS450, LT-R450 and many more. We showed you how to adjust rocker arms for valve clearance last month, so this time we will concentrate on the more in-depth shim-under-bucket design.

You can do the job with the engine in the frame, but since this one was already out, we did the job on a bench. Start by removing the valve cover.

Remove the engine’s timing plugs and rotate the engine to top dead center (TDC) using the marks in the view ports. If you’re not sure how to do it for your motor, check a service manual for details. With the engine at TDC, none of the cam lobes should be compressing any valve springs.

Mark the cam chain and cam gears with a paint pen, and count the number of chain pins between the hash marks on the cam gears. Travis likes to do one cam at a time so he doesn’t have to fish for the chain when reinstalling. Remove the cam caps and one cam.

Lube up the lobes and bearing surfaces of the new cam, and install it back into the head, timing it correctly. Torque the cam cap properly per the owner’s manual spec, and grab a set of feeler gauges.

Measure the clearance between the bucket and the cam lobe and write it down; this is your valve clearance. When you have all the clearances measured, remove the cam again.

Carefully remove the bucket and shim from each valve, noting which part belongs to each specific valve. Here you will need a set of calipers to measure the shims.

Here you will have to measure the difference between the valve clearance you want and the shim. Say you measured 0.01-inch clearance and you want 0.07-inch; you will have to use a small enough shim to bring the bucket down to that 0.07-inch clearance. Use paper, take your time and double-check your math until all the buckets are shimmed to the correct tolerance.

When you have all the right shims picked out, reinstall the shims, buckets, cam and cam caps. Torque them down to the correct spec, then measure your clearances with the feeler gauge once more. The valve clearances should fall within spec if you measured the shims correctly.

When one cam is finished, move on to the other for DOHC engines. Once it’s all done, reassemble the top end.

Remember to give the new cam its proper break-in procedure per the instructions, then let it rip!

Thumper Racing (661) 424-1800,
Hot Cams (515) 402-8005,
Web Cams (951) 369-5144,