Have you been curious as to what valve shimming is on your dirt bike? Perhaps you’ve heard your mechanic talk about it or your mates when out riding.
For a straight and concise answer:
Valve shimming on dirt bikes, and any other motorcycle for that matter, is the practice of adjusting the valve clearance inside your engine. By using a shim, you create space between the bottom of the cam and top of your engine’s valve bucket. This clearance is necessary for valves which are wearing out, but they too need to be replaced at some stage.
This is generally to be done by your motorcycle mechanic, though some riders who have pulled apart their engines before can fit these themselves with some helpful YouTube videos.
Dirt Bike Valve Shimming
It’s good pratice to use valve shims on your dirt bike if you’re using it regularly and are trying to prolong the life of your valves. These adjustments help in that they help compensate for the internal wear from high RPMs for those who love to twist the throttle hard.
It’s most common to use valve shims on 4-stroke dirt bikes and these shims are helpful. Some say they’re bad (as the shims need to be replaced soon enough) while others say they’re great to help prolong the life of your valves. After all, a valve replacement on a dirt bike can cost $300 to $1,000 depending on availability of parts.
Those riding older dirt bikes where parts are less likely to be readily available domestically are those who generally use valve shimming to prolong the life. Thus, if you do come across a used dirt bike online which says it has had the valves shimmed, it’s because the owner may have trouble sourcing replacement valves and valve train.
A Limited Life
Keep in mind that the use of valve shims on a dirt bike isn’t permanent. It’s literally just a band-aid and something to get you by for now. The shims could last for 700 miles or 12,000 miles, depending on your quality of shims, make/model, how hard you ride and other variables.
You can only use shim your valves once or twice in the lifecycle of your valves. Prolonged shimming can cause the valves to break and cause extensive internal damage, potentially leading to a top-end rebuild on your dirt bike.
Once you’ve used up your ‘shim lives’, we would recommend you replace your valves as well as springs, keepers, all seals and even the seats. This is an expensive and time-consuming process, but luckily, you probably had more than ample time to go riding before then. So understandably, this top-end rebuild is expected after some time.
Sourcing and Installing Shims
It’s always best to get the mechanic from your local dealership to do this for you, though some are reluctant to do this. Dirt bike shims are used on valves which are wearing out anyway, so they may just recommend a full replacement of your valves…especially if they know you’ve got the budget for this. After all, they will make much more money through your purchase of new valves and the labor than just replacing the shims.
To source shims yourself, you can easily find these available on eBay or Amazon, or even your local dealership is likely to have them. For the older dirt bikes (more than 10 years), you may have to dig around online on a few sites to find the right size and thickness for your dirt bike.
Installing shims on your dirt bike is an entirely different topic and one that is painstakingly slow. It’s something you need to do properly, however there are some great tutorials which explain it well for KTMs, Yamaha’s, Suzuki’s and Honda’s.
Going forward
Dirt bike valve shims are often used on older motorcycles as well as those who ride very frequently. You’ll typically see motocross riders using these as they want to save their expensive valves from wearing out fast.
For the weekend trail rider, the use of shims is less important and certainly not necessary if you can afford regular valve replacements. Some manufacturers do send their dirt bikes out from the factory with shims fitted and if they do, it’s best to keep an eye on these regularly and replace them as needed.
