Valve Shim Calculator

Determine the precise shim thickness required to bring your engine valves back within specification.

The current gap measured with a feeler gauge.

The desired clearance listed in your service manual.

The size printed on the shim currently installed.

Calculation Result

Valve Status:

Adjustment Needed:

Required New Shim:

function calculateValveShim() { var measured = parseFloat(document.getElementById('measured_clearance').value); var target = parseFloat(document.getElementById('target_clearance').value); var currentShim = parseFloat(document.getElementById('current_shim').value); var resultDiv = document.getElementById('shimResult'); // Validation if (isNaN(measured) || isNaN(target) || isNaN(currentShim)) { alert("Please enter valid numerical values for all fields."); resultDiv.style.display = 'none'; return; } // Calculation Logic: // New Shim = Current Shim + (Measured Clearance – Target Clearance) var difference = measured – target; var newShim = currentShim + difference; // Determine status var status = ""; if (Math.abs(difference) < 0.0001) { status = "Perfect Spec"; } else if (difference 0 ? "+" : "") + difference.toFixed(3); var formattedNewShim = newShim.toFixed(3); document.getElementById('valveStatus').innerHTML = status; document.getElementById('adjustmentDiff').innerHTML = formattedDiff; document.getElementById('newShimSize').innerHTML = formattedNewShim; resultDiv.style.display = 'block'; }

Understanding Valve Shim Adjustments

Correct valve clearance is critical for the longevity and performance of any 4-stroke internal combustion engine. This calculator is designed for mechanics and enthusiasts working on engines equipped with shim-over-bucket or shim-under-bucket valve trains, common in modern motorcycles, sports cars, and high-performance engines.

How the Calculation Works

The logic behind selecting the correct shim is based on the relationship between the gap (clearance) and the thickness of the shim occupying the space between the cam lobe and the valve bucket (or rocker arm).

The fundamental formula used by this calculator is:

New Shim Thickness = Current Shim + (Measured Clearance – Target Clearance)

Interpreting Your Results

• Too Tight (Measured < Target): If your measured clearance is smaller than the spec, the valve is tight. This is dangerous as it can lead to burnt valves because the valve face cannot make contact with the head to transfer heat. You need a thinner shim to increase the gap.
• Too Loose (Measured > Target): If the measured clearance is larger than the spec, the valve is loose. This typically results in a ticking noise and slightly reduced lift/power, but is generally safer than tight valves. You need a thicker shim to reduce the gap.

Metric vs. Imperial

This calculator is unit-agnostic. Whether you are using millimeters (mm) or inches (in), the math remains the same. However, you must be consistent. If you measure your clearance in millimeters, ensure your target spec and shim thickness are also entered in millimeters.

Standard Shim Sizes

While this calculator provides the exact mathematical thickness required, factory shims typically come in increments of 0.05mm (e.g., 2.40, 2.45, 2.50). You should select the commercially available shim size closest to the calculated "Required New Shim" value to bring your clearance within the acceptable range specified in your service manual.