MC Garage Video: How to Check and Adjust Screw-Type Motorcycle Valve Clearances

Step-by-step instructions for checking—and adjusting—old-school screw-type valve lash.

Metal parts expand as they get hot, and the clearance between your engine's cams and valves is there to account for that expansion. If that clearance increases, you'll end up with a lot of valve-train noise. If the clearance decreases—as it often does on the exhaust valves as the miles stack up—the valves may not seat fully. If that happens, it won't be long before hot exhaust gasses burn the valve. And then you're really in trouble.

The service interval for checking the valves and the procedure itself is different for just about every bike, so as always, review your shop manual for your bike’s specifics, but this demonstration will give you a rundown of the steps involved.

With most bikes you’ll need to remove the bodywork, gas tank, air box, radiator, and other parts just to get to the cylinder head cover and valves. That’s one of the reasons most people don’t want to deal with checking their valves. The other reason is that on most modern bikes, adjusting the clearance means removing the camshafts, and that freaks people out.

On the Kawasaki Z125 we're using for this demonstration, however, you can access the valves by popping off covers on the top and bottom of the cylinder, and since the Kawi uses old-school screw-type adjusters the cam can stay where it is. So in this vid we'll be showing you how to work with screw-type adjusters, which can still found on a lot of bikes. But don't worry, we'll cover the more common shim-type adjustment and cam removal on our KTM RC390 project bike in another video.

First up, your engine needs to be cool, so let it sit overnight before you start working on it. Then, you need the engine to be at top dead center on the compression stroke, meaning the piston is all the way up and both valves are closed. To get there, remove the spark plug so the engine is easier to turn over, then remove the inspection covers from the side of the engine and put a socket on the rotor bolt and turn the motor counterclockwise until the “T” on the rotor lines up with the reference mark on the cover. That “T” stands for Top, as in Top Dead center. If you go past it, just keep going for two more revolutions. Don’t try to turn the engine backwards or you’ll be grinding it against the starter clutch. You’ll notice there’s an “F” on the rotor as well, that’s an ignition timing mark that you can ignore, but it does serve as a head’s up that the T mark is about to appear.

The goal here is to put the engine in a position where the cams are not pushing on the valves. If you’re lined up with the T but there seems to be some pressure on the valves, you’re probably at top-dead-center on the exhaust stroke. Turn the motor over 360 degrees and you’ll be at TDC on the compression stroke.

The intake and exhaust valve clearances will be listed on a sticker on the frame and in your shop manual. Select a feeler gauge that’s in the middle of the range and try to slip it between the adjuster and the top of the valve. Sometimes you have to bend the end of the feeler to get it to slide it parallel. Not sure how to use feeler gauges? Check out our video on the topic:

Okay, so you found a valve that’s out of spec. Don’t give up on the bike just yet! On engines with shim-type adjustment you’ll need to pull cams, but with screw-type adjusters you just need to loosen the locknut and then turn the screw in or out, depending on if you need to close or open the clearance. Not much, maybe a 1/16th of a turn will open it up quite a bit. Use trial and error until you think you have it.

Once you’ve got the clearances set it’s a good idea to turn the motor over by hand a few times and then recheck the clearances. If they’re good, you’re good. See, that wasn’t so bad! Obviously, most bikes are going to be a lot more complicated to work on than the Z125, with more cylinders, more valves, and trickier adjustment procedures. That’s why it’s really important to study your shop manual and get familiar with the process before you start working on the bike.

Okay, that’s it for this video. Hopefully that took some of the fear out of that valve-clearance service. Tune in next time for shim-type adjustment.