To calculate the valve shim size for a dirt bike (or any four-stroke engine), you need to measure the current valve clearance and determine the required clearance based on the manufacturer’s specifications. The goal is to adjust the valve clearance to the correct specification by replacing the existing shim with one of the appropriate thickness.

Formula for Calculating the New Shim Size:

The formula to calculate the new shim size is:New Shim Thickness=Current Shim Thickness+(Measured Clearance−Specified Clearance)

Where:

• Current Shim Thickness : The thickness of the shim currently installed (usually measured in millimeters or thousandths of an inch).
• Measured Clearance : The actual valve clearance you measure using a feeler gauge.
• Specified Clearance : The target valve clearance specified by the manufacturer.

Step-by-Step Process:

1. Measure the Current Valve Clearance :
   • Use a feeler gauge to measure the gap between the valve and the rocker arm (or cam lobe, depending on the engine design). Record this value as the “Measured Clearance.”
2. Check the Manufacturer’s Specifications :
   • Refer to your dirt bike’s service manual to find the “Specified Clearance” for the intake and exhaust valves. These values are typically different for each type of valve.
3. Remove the Current Shim :
   • Carefully remove the shim from the valve bucket or retainer. Measure its thickness using a micrometer or caliper. Record this value as the “Current Shim Thickness.”
4. Calculate the New Shim Thickness :
   • Plug the values into the formula above to calculate the thickness of the new shim needed.
5. Select the Closest Available Shim :
   • Shims are usually available in standard sizes (e.g., increments of 0.05 mm or 0.002 inches). Choose the shim size that is closest to your calculated value.
6. Reinstall the New Shim and Recheck Clearance :
   • Install the new shim and re-measure the valve clearance to ensure it falls within the specified range.

Example Calculation:

Let’s say:

• Measured Clearance = 0.20 mm
• Specified Clearance = 0.15 mm
• Current Shim Thickness = 2.60 mm

Using the formula: New Shim Thickness=2.60+(0.20−0.15)New Shim Thickness=2.60+0.05=2.65mm

So, you would need a shim with a thickness of 2.65 mm .

Important Notes:

1. Precision Matters :
   • Always measure clearances and shim thicknesses accurately. Even small errors can lead to improper valve operation.
2. Use the Correct Tools :
   • A high-quality micrometer or caliper is essential for measuring shim thickness.
   • A feeler gauge is necessary for measuring valve clearance.
3. Follow Manufacturer Guidelines :
   • Always refer to your dirt bike’s service manual for specific procedures and tolerances.
4. Avoid Over-Tightening :
   • If the valve clearance is too tight (below the specified range), it can cause valve damage due to insufficient clearance during thermal expansion.

By following this process, you can ensure your dirt bike’s valves are properly adjusted for optimal performance and longevity.

New Shim Thickness=Current Shim Thickness+(Measured Clearance−Specified Clearance)​