Desmodromic Valve Adjustment for your motorcycle

How and why

Adjusting the valves on a desmodromic engine actually requires just a few basic tools. The crank-turning tool makes the job easier but is fully optional. A magnetic probe is handy, but you can improvise with a magnet and a screwdriver. The closer-shim caps and a micrometer are mandatory if you want to show up at your local Ducati dealer with a correct list of the shims you need. Alternatively, you can bring your old shims, along with your clearance measurements, to the dealer. Feeler gauges should already be in your toolbox, and T-handle allen drivers are much nicer to use than the cheapie L-shaped allens that you got in Cub Scouts. You need socket wrenches and a big flat-head screwdriver. And finally, a dental-style pick is handy.

Checking and adjusting desmodromic valves on ’88-and-newer two-valve Ducatis

Ducati Monster Side Stripped View

1. Access to the valves on most Ducatis is fairly straightforward. On this Monster, the tank unsnaps in front and tilts backward onto a hinge. It is supported by a prop built into the frame by those crafty Italians. Reaching the valve covers requires removal of the battery and dropping-but not removing-the oil cooler.

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2. The battery box is designed with a cutout which permits access to the intake valve. The signature ladder-style frame and stressed-member engine hang the front cylinder right out in the open. The hardest valve to access is the exhaust valve on the rear cylinder. On an older 900SS, the shock has to be removed to service this valve. We took the editorialΒ libertyΒ of working on an engine which had been removed from a bike because it would have been impossible to get a pair of hands, a camera and lights into the bike.

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3. Remove the spark plugs and bring the cylinder to TDC on the compression stroke (TDCC). Finding TDCC is easy if you have the crank-turning tool, but if you don’t have one, put the bike into first gear and turn the rear wheel. Put your finger over the spark-plug hole as you spin the engine, and the air being forced out will make TDC fairly apparent. If you don’t have the crank-turning tool, you can open the access hatch used to see where the pistons are lined up. The two notches that the tool fits into line up with the cylinders. You can reassure yourself by looking for the piston through the spark-plug hole with a penlight. Remove the valve cover with an allen wrench.

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4. Measure the clearance of the opening (top) shim; it should be 0.004 inch. The feeler gauge should push and pull about as easily as a business card stuck into the bottom pages of a thick phone book. Valves tend to sink into their seats as the motor wears in, so the clearance should get looser.

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5. Turn the crank until you reach TDC Overlap when both valves are held open, 360 degrees from TDCC. Push down the rocker arm with your thumb or perhaps a screwdriver and measure the gap again. Subtract the gap you measured for the opening shim from what you find now with the rocker depressed to determine the clearance for the closing shim. The factory recommends that the gap on the closer shim stays between 0.000 and 0.002 inch. The critical thing to avoid here is a negative tolerance, which means that the valve is being pulled to a point of interference with the head. Valves don’t like to get stretched.

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6. If you need to adjust the shims, pull off the retaining clip with the hook end of a dental-type pick. Don’t use pliers. Turn the crank forward 90 degrees, and the rocker will slide off the valve with finger pressure. The factory says the valves should be inspected every 4500 miles, but Steve says that if you’re doing your own maintenance, you can get away with just checking every 6000 miles once the engine has gone past the 10,000-mile mark.

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7. Measure the thickness of the shim with a micrometer, and write down the size. Writing down the sizes of all your shims is a smart thing to do no matter what kind of bike you have. As with most valve adjustments, access to shims is the hardest part of this operation. No matter who you are or how many shims you already have, you always have to go to the shop for at least one shim when you adjust valves. Don’t try to grind them down yourself unless you have a precision micrometer milling machine, since the upper and lower surfaces need to be absolutely parallel.

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8. The closer shim should get looser as it gets older. If it is getting tighter, you are accumulating carbon on the valve and have other problems to investigate. When you take out the closing shim, keep in mind that once the shim comes off, the valve is free to drop into the cylinder if the piston isn’t at TDC. Don’t turn the crank at this point or you’ll be sad and sorry. Block the oil-return hole so you don’t lose the little shim-retainer collars. (An allen wrench works well to block the hole.) Square the tip of a big screwdriver against the wall of the cylinder head and lever down on the closer arm. The closer shim looks like a little top hat with a hole on top and will slide down the valve stem. It is held in place by two half-circle collars that sit in a groove on the valve. The surface tension of the oil will usually keep them in place for awhile, but have a magnet ready to pull them out.

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9. In order to measure the closer shim, you need the special closer-shim caps. Don’t worry, they’re cheap. Do some math and get the right shim. Installing the closer shim is the hardest part of the job, but a dab or two of grease will hold the little collars in their grooves if you don’t have three hands.

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10. Once the closer is installed, buttoning up is easy. The opener rocker goes right onto the valve when you turn the crank through a rotation. Make sure you put the rocker-arm retainer clip between the two thick shims, and don’t forget to unplug the oil-return hole. Turn the crank through at least two full revolutions before rechecking your clearances to ensure that the shims are fully seated. According to E.C.S., the opening shims typically go for at least 6000 miles without needing adjustment, and the bottom shims generally go 18,000 miles before needing adjustment.

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