Valve Shim Calculator

Enter your current shim thickness, the clearance you measured with a feeler gauge, and the clearance your manufacturer specifies, and the calculator returns the exact shim thickness that would put you dead on spec.

A valve shim calculator works out the replacement shim thickness you need during a shim-under-bucket valve clearance adjustment — the maintenance job required on most modern motorcycles, ATVs, and some cars with direct-acting overhead-cam valvetrains.

Beyond the quick single-valve version, this tool also includes a detailed mode that rounds your result to a shim size you can actually buy, and a multi-valve worksheet that handles a full valve adjustment (four, six, eight, or more valves) in one pass and builds a shopping list of the shim sizes you'll need.

Valve Shim Calculator Formula

Required Shim = Current Shim + (Measured Clearance − Target Clearance)

Where Current Shim is the thickness of the shim presently installed under the bucket, Measured Clearance is the clearance you checked with a feeler gauge before removing anything, and Target Clearance is the single value you're adjusting toward — often the middle of your manufacturer's specified range.

Why a Thicker Shim Reduces Clearance

In a shim-under-bucket valvetrain, the shim sits between the bucket and the camshaft lobe, taking up part of the clearance gap directly. Installing a thicker shim leaves less room in that gap, so clearance goes down; a thinner shim leaves more room, so clearance goes up. That's why the formula adds the difference between measured and target clearance straight onto the current shim thickness: if your measured clearance is larger than target, the formula adds a positive number and calls for a thicker shim, and if it's smaller than target, it subtracts and calls for a thinner one.

Worked Examples

Example 1: Exhaust Valve Needs a Thicker Shim

An exhaust valve has a 2.20 mm shim installed. You measure 0.30 mm of clearance with a feeler gauge, but the target is 0.20 mm. Required Shim = 2.20 + (0.30 − 0.20) = 2.30 mm. Because the measured clearance is larger than target, a thicker shim is called for — installing the 2.30 mm shim takes up the extra 0.10 mm of gap.

Example 2: Intake Valve Needs a Thinner Shim

An intake valve has a 2.50 mm shim installed. You measure only 0.05 mm of clearance, against a target of 0.15 mm. Required Shim = 2.50 + (0.05 − 0.15) = 2.40 mm. Here the measured clearance is smaller than target, so a thinner shim is needed to open the gap back up.

Detailed Calculation: Finding a Shim You Can Actually Buy

The exact math rarely lands on a shim size that exists. Shim kits are sold in fixed steps — commonly 0.05 mm, though some manufacturers use 0.025 mm or 0.10 mm — so the detailed mode takes the exact required thickness and rounds it to the nearest real shim size, then reports what clearance that actual shim would produce.

Shim Increment

Choose the increment your shim kit is sold in — 0.01 mm, 0.025 mm, 0.05 mm, or 0.10 mm — or select Custom to enter a different step size. This setting only changes which shim sizes are considered as candidates; it doesn't change the exact required thickness itself.

Available Shim Range

If your shim tray or kit only covers a certain range, enter the minimum and maximum shim thickness you actually have on hand. The calculator will only recommend a shim within that range, rather than suggesting a size you'd need to special-order.

Specification Range and Status

Enter your manufacturer's minimum and maximum clearance specification (not just a single target) to see whether the recommended shim's predicted clearance falls inside or outside that range. The "Use Specification Midpoint" button fills the target clearance field with the midpoint of the min and max you entered, which is a reasonable target when your manual doesn't call out a preferred value.

Worked Example

An intake valve has a 2.43 mm shim, a measured clearance of 0.33 mm, a target of 0.20 mm, and a specification range of 0.18–0.22 mm. With the shim increment set to 0.05 mm: Exact Required Shim = 2.43 + (0.33 − 0.20) = 2.56 mm. The nearest real shim size is 2.55 mm, which the calculator recommends. Installing that 2.55 mm shim predicts a clearance of 0.33 + 2.43 − 2.55 = 0.21 mm — inside the 0.18–0.22 mm specification range, so the result reports "Within Specification."

Multi-Valve Worksheet

The Multi-Valve panel adds a row per valve (up to 16), each with its own valve type, current shim, measured clearance, and target, then calculates every valve's exact and recommended shim in one pass using the same increment and rounding logic as the detailed mode.

Adjustment Summary

After calculating, the worksheet totals how many valves need a thicker shim, how many need a thinner one, and how many need no change — a quick way to see how much of the adjustment is actually needed before you start pulling shims.

Shim Requirements (Shopping List)

The worksheet also groups the recommended shims by size and counts how many of each you'll need across the whole job, so you know exactly which sizes to pull from your shim kit or order before reassembling the head.

Metric and Imperial Units

All three modes can be switched between metric (mm) and imperial (in) using the unit toggle. Values you've already entered are converted automatically when you switch, and the shim increment options always represent the same real-world shim sizes in either unit — a "0.05 mm" increment is still exactly the same physical shim step when it's displayed as its inch equivalent.

Frequently Asked Questions

Does a thicker shim increase or decrease valve clearance?

A thicker shim decreases clearance, and a thinner shim increases it, because the shim physically occupies part of the clearance gap between the bucket and the camshaft lobe in a shim-under-bucket valvetrain.

Why do intake and exhaust valves often need different target clearances?

Exhaust valves run hotter and expand more than intake valves, so manufacturers commonly specify a larger clearance for exhaust valves to prevent them from closing up completely once the engine reaches operating temperature. Always use the target and specification values from your own service manual for each valve type.

What if my engine doesn't use shims at all?

Some valvetrains use a screw-and-locknut adjuster instead of a shim, in which case you turn the adjuster screw directly to set clearance rather than swapping a physical shim. This calculator applies specifically to shim-under-bucket (and similar shim-adjusted) designs.

What increment should I choose if I'm not sure what my shim kit uses?

Check the shim kit or the individual shims themselves — the size is usually stamped or printed on each shim, and a full kit's box or listing will state its step size. 0.05 mm is the most common increment across aftermarket kits, but always confirm against what you actually have before ordering a replacement.