MC Garage Video: How To Adjust Shim-Type Motorcycle Valve Clearances

Step-by-step instructions using our KTM RC390 project bike.

Checking and setting your valve clearances is a hugely important piece of maintenance, and it's one that a lot of people dread because they know it's going to be expensive to get it done at the dealer or because they're not looking forward to doing it themselves. The job does entail working with vital engine components and screwing up any of the steps could be catastrophic, so now more than ever, it's really important that you study your shop manual!

We've already covered the simpler screw-type valve adjustment in this MC Garage video and we went over why you need to adjust your valves and how to get the motor to top dead center, so we're going to skip all those fundamentals and get to it.

Okay, with the engine cool and at top dead center on the compression stroke, it’s time to pop off the valve cover. But before you do, wipe down the engine so it’s clean. You don’t want any dirt getting into the engine.

Now select an appropriate feeler gauge and slide it between the cam and the top of the valve, or in this case between the cam and the finger follower. On the KTM RC390, make sure you slide the decompression tab over before you check the right exhaust valve. You want slight drag on the feeler gauge—it shouldn't feel tight and it shouldn't slide it super easily. Measure the exact clearances for all the valves and write them down on a sheet of paper, then compare the figure to the spec listed in your service manual.

Okay, so you found a valve or two that are out of spec. Don’t dump the bike on Craigslist just yet. When it comes to shim-type valve clearances, you set the gap using different thickness shims. Some setups have the shim in a recess on top of the valve spring retainer, while other times the shim sits under a cover called a bucket.

In any case, to swap out the shims you’ll need to remove the camshafts. This is the part that freaks people out the most. And it should. You could really screw things up if you drop a shim or bolt down into the engine, don’t tighten something to the proper torque, or worst of all, don’t time the cams correctly when you put them back. So read your manual, take your time, and check your work.

To remove the cams, double check that you’re at top dead center by making sure the reference marks on the rotor and the cam sprockets line up and then remove the cam-chain tensioner to slacken the cam chain. Now loosen the cam-holder bolts—making sure to do it gradually and in a criss-cross pattern—and then lift the holder straight up and off.

Now throw a zip tie or wrap some safety wire around the cam chain to keep it from falling into the chain channel and remove the cams and set them aside on a clean rag.

If you’re paranoid about dropping anything into the engine—and you should be—stick some paper towel in the spark-plug hole and the cam-chain channel.

Pluck the shims out using a magnet and put them on your sheet of paper, writing down the shims’ thickness, which will be printed on one side. If the numbers are worn off then you’ll need to measure the thickness yourself using calipers or a micrometer.

Compare your measured clearances to those listed in your service manual to determine what’s in spec and what’s not. To figure out what size shims you need to install to bring the clearance into spec you can either reference the chart in your service manual or do a little math using the measured clearance, the shim thickness, and the desired clearance. Shims come in 0.05mm increments, so you won’t always have the exact size you need, even if you bought a full kit. As a rule, it’s better to be slightly loose than slightly tight, so let that guide your decision.

With your shims swapped out, make sure the piston is still at TDC by checking to see that the mark on the rotor lines up, then reinstall the exhaust cam and then the intake cam with the timing marks oriented correctly. Put some tension on the cam chain with your finger and recheck the marks on the cams. If they look good, install the cam holder and tighten the bolts gradually in a criss-cross pattern, torqueing them according to the spec in your manual. Reinstall and set the cam-chain tensioner, then turn the engine over two revolutions and recheck the clearances. They should be spot on. If not, you’ll need to recheck your math and maybe swap out another shim.

With the clearance set, go ahead and give yourself a pat on the back, and then give yourself a minute to think about anything you might have forgotten. All good? Good. Then slap on the valve cover on and reassemble your bike. And you’re done!

So if you watched the video we did on the Z125 Pro and its easy to adjust screw-type valves, you may be wondering why the heck most bikes use shim-type valves. Well, with shims the cams can act directly on the valves, which means no rocker arms, which means less reciprocating weight and a faster, higher-revving engine. Also, because there are fewer parts to wear, clearances on shim-type valves remain stable for much longer than on screw-type setups. We’re talking two to five times as long between adjustments.

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