Valve Shims

Was your valve clearance out of tolerance at the last check? Engine shims are thin, precisely sized discs used to adjust the clearance at the valves. We stock both refill kits with five identical shims and complete kits for those of you who maintain or build your engine yourself.

What valve shims do in the engine

Valve clearance is the small gap in the valvetrain that ensures the valve can close fully, even when the engine is hot. If the clearance becomes too small, the valve can be held slightly open, which can cause loss of compression and overheating of the valve. If it becomes too large, the valvetrain can get noisy, and the valve typically opens less than intended. In engines with shim adjustment, you change the clearance by switching to a shim of a different thickness. In this category you will find, for example, complete shim kits from Hot Cams for models including the Honda CRF 150 R and Yamaha YFZ 450, and brands such as Wiseco, Supertech, COMP Cams and GSC Power Division are also represented.

How to choose the right shim

Three factors determine which shim you need:

• Diameter: must match your engine's valvetrain, e.g. 7.48, 9.48 or 10.0 mm.
• Thickness: calculated from the measured clearance, the thickness of the current shim and the clearance specified by the engine manufacturer.
• Kit type: a refill kit with five identical shims supplements an existing set, while a complete kit gives you several thicknesses to choose from.

Measure with a cold engine and feeler gauges according to the manufacturer's instructions, and check the clearance again after installation. If you have changed the camshaft or valves or machined the valve seats, the clearance typically changes, so it must be measured from scratch.

Frequently asked questions

How do I calculate the shim thickness I need?

Typically, you add the difference between the measured and specified clearance to the thickness of the current shim. So if the clearance is too small, you need a thinner shim. Measure the old shim with a micrometer instead of relying only on the printed marking.

What do dimensions like 9.48 x 1.70mm on a shim mean?

The first number is the shim's diameter and the second is the thickness, both in millimetres. The diameter must fit your engine's valvetrain, while the thickness is chosen based on the clearance you have measured.

Can I reuse the old shims during a valve adjustment?

Often yes, if they are not worn or damaged and have a thickness you can use. With a complete kit it is easier to swap between valves, while refill kits top up the thicknesses you are missing.