How to Calculate Valve Shim Size: Worked Examples for ATVs and Dirt Bikes

Measured your valve clearances and found one outside the service-manual range? The next question is which replacement shim to install. On a direct-acting shim-over-bucket or shim-under-bucket system, a simple calculation turns the measured gap and existing shim thickness into a starting point for the replacement.

This guide explains the math with fictional examples. The example clearances are not specifications for a particular ATV or motorcycle. Use the manual for your exact engine, model year and measurement temperature.

The valve shim calculation formula

New shim thickness = current shim thickness + measured clearance − desired clearance.

A thicker shim reduces clearance; a thinner shim increases it. This relationship applies where a change in shim thickness produces an equal, opposite change in clearance. Rocker-arm systems and other geometries may require a different calculation.

Open the free Elite Engineering valve shim calculator to enter the existing shim, measured clearance, specification range and available shim increment.

Example 1: A valve with too little clearance

Suppose the existing shim measures 2.40 mm, the measured gap is 0.08 mm, and the fictional specification is 0.15–0.20 mm. Choosing the midpoint gives a target of 0.175 mm.

2.40 + 0.08 − 0.175 = 2.305 mm.

If the correct-diameter shims are available in 0.05 mm increments, consider a 2.30 mm shim. Its predicted clearance is:

0.08 + (2.40 − 2.30) = 0.18 mm.

The predicted result falls inside the fictional range. The replacement is thinner because the original gap was too tight.

Example 2: A valve with too much clearance

Keep the same 2.40 mm existing shim and fictional range, but change the measured gap to 0.28 mm.

2.40 + 0.28 − 0.175 = 2.505 mm.

A 2.50 mm shim predicts 0.28 + (2.40 − 2.50) = 0.18 mm. This time the replacement is thicker because the gap needs to become smaller.

Check the predicted gap before choosing an available size

The exact calculated thickness may not be sold. For any candidate shim, use:

Predicted clearance = measured clearance + current shim thickness − replacement shim thickness.

Compare that result with the entire allowed range. Do not round blindly to the nearest shim if it predicts a gap outside the specification. If no available size works, check the measurements, supported shim sizes and service procedure.

Keep a separate record for every valve

Label each intake and exhaust position before taking anything apart. Record its measured clearance, original shim thickness, selected replacement and final clearance. The calculator's multi-valve worksheet lets you add or remove rows, including a three-intake, two-exhaust arrangement. Enter the intake and exhaust ranges separately.

Common calculation mistakes

• Mixing inches and millimeters: convert the clearances or select the matching input units. Shim thickness remains in millimeters.
• Entering zero when no gauge fits: an unmeasurable gap is unknown. Follow the engine's service procedure to establish a usable measurement.
• Using another model's specification: a similar engine name does not establish the correct range.
• Reading a worn marking as a measurement: verify the old shim thickness with a suitable measuring tool.
• Confusing diameter with thickness: the calculated thickness must also be available in the diameter required by the engine.

Verify the adjustment after installation

The calculation predicts a result; measurement confirms it. Follow the manual for camshaft removal, timing, fastener tightening and engine rotation where applicable. Recheck each adjusted valve under the specified conditions before returning the machine to service. A valve already inside the permitted range does not automatically need a new shim.

Calculate a replacement shim or build your multi-valve worksheet.

Technical reference

ProX Racing Parts also explains the direct-acting calculation in its shim-under-bucket valve adjustment guide. Engine-specific specifications and assembly procedures must come from the applicable service manual.