Desmo Demystified

Ducati is considered by many to be the Ferrari of motorcycles. The comparison is well deserved. In addition to a long racing history, Ducatis are a joy to ride, expensive, beautiful and a pain in the ass to work on.

Let’s focus on that last one.

Synonymous with the name Ducati is the term “Desmo,” short for desmodromic. A desmo valvetrain does not rely on springs to close the intake and exhaust valves. A cam lobe opens the valve, and another cam lobe closes the valve. This design has been used on almost all Ducati motorcycles since 1968.

TL:DR
Make your desmo service easier with these recommended tools and helpful video.

While they did not invent it, Ducati are the only manufacturer to offer it to the public in significant numbers. The concept dates back to racing cars as early as 1914, but it was Ducati’s implementation on their 1956 125 Grand Prix racing bike that ushered in a new era. Back then it overcame the problems of stiff valve springs, metal fatigue or springs that weren’t able to close the valves quickly enough, a problem known as “valve float.” These problems the desmodromic valvetrain was designed to solve have pretty much been eliminated by modern metallurgy and more advanced valve actuation systems.

But I digress.

From the routine maintenance standpoint, a desmo increases the complexity of a valve adjustment. In addition to the typical valve clearance (opening the valve) we also have a “closer” clearance. Simply put, there are twice as many clearances to check! And historically, since the closing cam clearance is less than the thickness of a human hair, a Ducati has required this maintenance much more often than most bikes. Add to this the marque’s infatuation with timing belts, instead of chains or gears, and you now understand the dread owners feel when they hear the words Desmo Service.

To be fair, in recent years the service intervals have increased and are now on par with many “conventional” motorcycles. But understanding how to properly maintain a desmo is required if, like me, you cannot resist the alluring sound of a big Italian 90-degree twin. So here are a few resources and observations you may find helpful.

There are a multitude of videos online that deal with servicing the Ducati valve clearances and timing belt replacement. Some of them are terrible. I found a few that have been helpful for me and created a playlist (see below). Most of them deal with the modern 4-valve Ducati, or Testastretta engine (literally narrow head, referring to the cylinder head shape). But first allow me to share a few random observations:

• Timing Marks
  The cam and drive gears have timing marks. The drive gear’s dot aligns with a hash on the crankcase, but the dots on the cam gears… ? What they line up with is a mystery to me and probably the reason for the “belt marking” method most mechanics suggest.
• Camshaft Alignment
  Here’s what really matters: the T-shaped indentions in the nose of the camshafts (see G in the photo above). When the belt drive gear is aligned with its mark, then you should see the inverted T on all four cams. The horizontal slots should line up with the gasket surface. When changing belts this is the first thing to check before removing the old belts!
• Timing Gears
  If you’re used to changing timing belts on cars the Ducati setup might look odd. First off, the gears are all the same size! Typically the cam gear is twice the size of the crankshaft gear (in a four stroke engine the crankshaft goes around twice for each revolution of the camshaft). But on the Ducati the timing belt isn’t on the crank, it’s actually driven off a jackshaft. The jackshaft is specifically intended to drive the timing belt and the 2-to-1 gearing happens internally.
• Belt Tension
  The tensioners on a Ducati are manual. There is no spring or hydraulic actuator like you’d find on a car. Each belt has two idler bearings- one static and one adjustable. The adjustable one sets the tension on the belt. Here’s the fun part- the tension is specified in frequency. That’s right, like a guitar tuner. I use an app on my smartphone called Frequency Measurement Tool and pluck the belt. It’s a crazy way to do it, but super accurate.

Next, here are a few of the tools I find makes this job easier. The full list of tools and common parts is available here.

Buona fortuna!

Metric Feeler Blades

Since the valve shims are supplied from Ducati in .05mm increments, it makes sense to measure the clearances in millimeters. It saves a ton of head-scratching and might spare you an extra trip to the dealer. Use the straight blades to measure the closers and the angled blades for the openers.

Crankshaft Turning Tool

I suppose you could consider this nice but not a necessity. You can certainly shift the bike into gear and turn the rear wheel to rotate the engine (use 6th if possible). But if you want to prop it through a couple of revolutions (and you should after fitting new belts) this will make it much easier.

14mm Spark Plug Socket

The spark plugs used in the Testastretta engines are very small with M10 threads. You’ll need a 14mm socket to fish them out, but a real spark plug socket is ideal because it protects the fragile porcelain insulator.

Hemostats

Yeah… here’s the secret weapon! When it comes time to change the closer shim you’ll want these hemostats to hold the valve fully closed. Buy two– they aren’t designed for this and will twist and wear out.

Magnetic Pickup Tool

A strong magnetic probe like this is indispensable for retrieving the opener shims and closing shim keeper rings. Keep this magnet on top of the valve when replacing the closer shim keeper rings, it will help “attract” them to the valve.

Last Words
Buy a new fuel filter. Since the fuel tank has to be removed you might as well change the in-tank filter. It’s also wise to block the oil return holes in each head to prevent the valve keepers from falling into the crankcase. Small bits of rag or ear plugs are ideal for this. Just don’t forget to remove them!

And one last tidbit of advice. If you don’t feel the need to own all the stuff required to perform this work, rent it! Ducati Tool Rental offers tools, shim kits and helpful reference material for DIY Ducatisti.