Hot Cams/Mud Buster
Setting Proper Valve Clearances
Shim and Bucket
Rocker arm and Tappet
Converting Inches to MM
Shimming Example

The proper setting of valve clearances is necessary to get the best performance and component reliability from your machine. Whether it is “Shim & Bucket” (cam follower & shim under style) or Rocker Arm & Tappet (CORA designs), it’s very important that the clearances be set correctly.

CORA Design
Cam On Rocker Arm, or CORA for short, is used primarily in older style air cooled engines, although Honda is currently using a modified version of this design in their UniCam design engines. The proper adjustment relies on the mechanic’s ability to tell the difference between correctly tightened and too tight or too loose adjustment. The feeler gauge used in this application must enter into position between the tappet and valve stem at a right angle (90°) to the valve train to get the correct feel. The correctly adjusted valve will have enough drag to keep the feeler gauge from falling out and not too much to keep from pushing the gauge forward without bending it. Width, length, and angle of the feeler gauge is vitally important to get the correct positioning under the tappet for the correct feel. It must not have contact with any other portion of the engine. If it is not a 90° approach to being between the valve and rocker arm tappet, or it is touching anything on the cylinder head other than the valve and rocker arm tappet, make it so the fit into the valve is correct.

We see, all to often, over tightened rocker tappets on cams. The give away detail to this fact is wear on the base circle of the camshaft lobe and the rocker arm and camshaft have oil burnt to the surface and wreaks of a burnt odor, plus the discoloration of the rocker arm & camshaft themselves. That means the rocker arm was never allowed to leave the camshaft, it continually had pressure against the rocker arm foot and the tappet where it contacts the valve. This does two bad things to the cooling of the valve train, it wipes the cooling and lubricating effects of oil from the lobe and rocker foot and it transfers heat from the valve stem into the rocker arm and then into the camshaft. This creates an overheating of the camshaft lobe, the end results are that the rocker arm foot and camshaft are destroyed.

Also, since the rocker arm foot is continually in contact with the camshaft and the oiling is being wiped away by the rocker arm foot, a lot of friction and heat is generated. There is only one oiling port at the base of the opening ramp of the lobe to ensure adequate oiling at the highest loading point for the rocker arm foot. Once the heat builds up in the metal of the cam and rocker, it flashes off the oil as it contacts the two pieces. This feeds on itself as the less oiling and cooling compounds the effect and failure is eminent.


Shim and Bucket Design
There are a lot of unknowns with this design, most of which are ignorance to the design. It is thought that since there is no adjustment point, with respect to the CORA design, there is no way of adjusting it and it must be set already. Which is completely untrue, manufacturing tolerances are NEVER that good and wear will always exist. Every camshaft installed into an engine should have the valve clearances checked to be sure they are in the correct range of adjustment.

Normally this design is subjected to a range of clearance that is within an acceptable limit. For instance, the spec will call for .004” to .006” or .005” + or - .001”. This design is usually subjected to a “Go, No Go” on the feeler gauge fitment as well. Meaning that if a .006” gauge fits and a .007” gauge does not, that means it is within spec, if the spec maximum clearance was .006”. Again it comes down to feel and the observation of the valve bucket as the gauge is being pushed into place. And the larger the clearance, the stiffer the feeler gauges become and the more likely a mistake is to occur.

It is vitally important to have a “straight”, unobstructed, shot into the clearance point of the valve train. The same can be true of this design as it is to the CORA design from the standpoint of feeler gauge fitment into the clearance gap. In many cases a normal width, straight bladed, feeler gauge will not work to give you the correct clearance. Case in point is the Honda CRF450R. Using a “normal” feeler gauge on the intake valves will give you an incorrect feel for the valve. The fit into the gap is both odd angled and narrow. To get the correct fitment of the gauge into the gap, you need a narrow, short blade length, 45° angled, feeler gauge blade to measure it correctly. If the blade is too wide it drags on the cam carrier going in. If the blade is too long, you can’t get the correct angle into the gap as it hits the camshaft’s exhaust lobe and the cam carrier. If it’s not bent at a 45° angle it won’t go over the front of the carrier and into the bucket at a 90° angle to the gap or parallel with the top surface of the bucket.


Converting Inches to MM
Going back and forth between thousandths of an inch and tenths of a millimeter can be confusing. To make this transition takes some simple math. One (1) inch = 25.4 mm. Roughly translated, to give an idea of the differences, .001” = .0254 mm. Since most shims come in .05 mm increments, that means there is a .002” difference in each shim size that WE furnish in both the HCSHIM01 and HCSHIM02 shim kits. The size of these shims range from 1.20 mm to 3.50 mm in .05mm increments. These shims have a tolerance as well and are + / - .025mm or .001”. If you are the type of person that needs to have the setting exactly to one specific point in the range of adjustment, a micrometer can solve the problem by finding either a “thick” or “thin” shim of the size needed to put you where you want to be.


Shimming Example
Here is an example. Let’s say that the measured clearance is .007” on the intake. You know that it needs to be around .005” to put it in the middle of it’s range, or .002” tighter than the measured clearance. You will need a shim that is .002” thicker to take up that difference in clearance. You also now know that .002” means that the shim will need to be .05mm higher in number. Let’s say the bike had a 1.90 shim in it, you will now need a 1.95 shim to get the clearance you desire. Make sure the new shim is completely seated on top of the valve stem and it is able to rotate freely in the pocket of the spring keeper. Install the camshaft, torque the cam cap bolts to spec, install the cam chain tensioner or just release the tensioner (whichever the case may be) and roll the motor over slowly feeling for binding or catching, do this several times to ensure a good shim seating. Check your clearances again to be sure they are correct. Repeat if necessary.

Short of making your own custom feeler gauges for each machine, Motion Pro Tools has an assortment of feeler gauges. www.motionpro.com or at (650) 594-9600.

Good luck and thank you.