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Tech: 4V Desmo Adjustment


Often demonized and rarely understood, desmo valve adjustment is becoming a bit of a lost art as people are discouraged from touching their own machines because of the supposedly frightening complexity of desmodromic valves and the need for "special tools" to do the job.

Of course most of this is BS. Desmo valves are no harder to adjust than traditional valves, the only difference being that there is twice the number of everything - double the shims, double the measurements, double the rockers, etc. But with a little patience and the right techniques it's all very straightforward and not at as mind boggling as some people would want you to think.


- Opening note -

I'm doing this adjustment with the head off and on the bench. This is the easiest way to adjust 4V heads, but has some problems - first, you need to replace the head gaskets, and second you need a special wrench to take the heads off. On pre-1999 bikes, head gaskets are thick, fibre-composite items that cost about 25-30$ each (so 60$ total). This makes taking the heads off worthwhile. On post-1999 models (starting with the 996) a thinner, multilayer metal gasket was used - these run about 125-150$ each. Obviously if you are running a tight budget and have a later bike, leave the heads on and adjust them in place. I will not cover the procedure for removing the heads; this tutorial assumes you have the heads off, or at least have the timing belts removed with the heads in place.

If you take the heads off, it's recommended you lap the valves with grinding compound. Ideally this should be done every 6000 miles (10 000 kms) for optimum sealing. Every 12-24K miles (20 000 - 40 000 kms) is good enough for most people. Also think about setting your piston squish while you are in there, these motors respond well to careful squish setup.


- Special Tools -

To remove the heads (if you want to, see above) the special cylinder head wrench is a closed end 12-point 15mm with a ½ drive attachment for a torque wrench. You can buy cheap ones from DesmoTimes or Motoreva, for around $45.00 US. Compared to 300~ for the Ducati tool, that's a bargain and is well worth it if you plan on doing your own work.

You will also need a closing shim adjustment tool - a top hat shaped plug that fits into the closer shims to measure them with a micrometer. Aftermarket items sell for around 15$ US.

To remove the rocker pins, I use an M5 bolt with a nut, a washer, and a 5/8" socket. This actually works better than the expensive factory tool in most cases.




- Checking Clearances -

First, lets look at the desmo setup. We have a double overhead cam design driven by automotive style timing belts on the right hand side of the engine. There are two rocker arms per valve, one for opening and one for closing. Thus on each cam there are four lobes - two for opening, two for closing. The openers are the small lobes on the outside operating the top rockers. The closers are the large lobes on the inside operating the lower rockers. The closing rockers are aided by "helper" hairsprings that close them at rest or at low rpm. You can feel the tension of the helper springs when turning the cams. At higher RPM they become pointless, but when the engine is starting or at idle they are needed to maintain sealing with loose closing clearances.



There are two shims per valve as well - one small shim sitting on the top of the valve stem to set the opening rocker clearance and a large shim wrapped around the valve stem (securing the retaining collets) to set the closing clearance.*



The easiest way to check the clearances if you leave the heads in place is to loosen and remove the timing belts with the piston at top dead centre on whichever cylinder you are working on. TDC cam position and the default position for measuring the clearances is with the opening lobes (the small ones operating the top rocker) pointing towards the top outside corner of the head.

Checking the opening clearance is simple - at TDC, check the gap between the top shim and the rocker arm.



There are two ways to check closing clearance, and the best method is to use both of them. First, there is "loaded/unloaded" clearance. The "unloaded" clearance is the opening clearance you just measured. To measure "loaded" clearance, you take a screwdriver or a small socket on an extension and press down firmly on the lower rocker arm (the forked part poking out below the bottom shim). Now measure the gap between the top shim and the top rocker arm. You will get the opening clearance with the closing clearance added to it - the loaded gap. What you are doing is pushing the closing rocker against the tension of the helper spring to squeeze it against the cam, thus giving you your different measurement.



The second measurement is the clearance between the cam lobe and the surface of the closing rocker. This is tricky to measure because it is hidden behind the cam, and needs a set of long, angled feeler gauges. In a pinch you can disassemble a normal set of feeler gauges and use the individual leaves to measure the gap. I measure the closer this way after doing the loaded gap, as a double check for my first measurement.



So in this case I had:

0.127 mm opening clearance
0.178 mm loaded gap

Subtract opening clearance (unloaded gap) from loaded gap.

=

0.051 mm closing clearance

This is too tight on the closer side. Ducati recommends the following for most desmoquattros:

Intake
Opening - 0.05- 0.18 mm
Closing - 0.16 - 0.25 mm

Exhaust

Opening - 0.05 - 0.23
Closing - 0.11 - 0.20

So this means we have to open things up to replace the closing shim.


- Changing Shims -

Replacing opening shims is a simple process, but changing closing shims is a much more involved procedure and has a number of steps. I will detail how to replace each type of shim individually; it's best to avoid replacing closers unless necessary.

First, you need to know that each rocker pivots on a pin inserted into the side of the head. There is one pin for each rocker, with small oilways through the centre of the pins to lubricate the pivots. To remove the cams, and to remove either of the shims, you need to withdraw the pins out far enough to move the opening rocker off to the side off the top shim.

To withdraw a pin, you first need to take the central side covers off the heads. These are the triangular covers located on either side of the head between the cam end caps. On the left side, there is an external oil line that needs to be disconnected before removing the covers.

You will see four pins on each side of the head, the lower ones are for the closing rockers (and should be left alone) the upper ones are for the openers. Place the socket over the pin and extract it with the bolt and nut as shown; be sure to have a long bolt so you can screw it fully into the end of the pin, otherwise you risk stripping the inner thread of the pins.

Slowly tighten the nut to pull the pin out; it will feel tight at first but will gradually loosen as the pin is extracted. As the pin comes out, you will notice that you can slide the opening rocker arm sideways. Keep pulling the pin until you can slide the rocker off the top shim and to the side. You don't have to remove the pin completely unless you want to remove or replace the opening rocker.





Now you can remove the top shim. If you are only changing the opening shims, this is all you have to do. Once you replace the shim, reposition the rocker arm over the shim and pound the rocker pin back into position with a hammer.

(Cam removed for clarity)



To replace a closing shim, things are a bit more complicated.

First, you need to remove the camshaft. To do this, you need to withdraw the rocker pin of the opening rocker on the right hand side so you can slide the rocker off the shim. Once this rocker is moved, you can manoeuvre the cam out. Undo the two retaining bolts on the right hand pulley cap and firmly pull the assembly out, being careful to turn the cam lobes so they don';t get caught on the rocker arm.

Now you can see the rocker arm faces clearly. Check the cam surface of the rockers for wear or signs of chrome flaking, a common and serious problem on any post-1995 4V Ducati. Any flaking rocker arms need to be replaced with improved aftermarket items, available from MBP Ducati or Megacycle in exchange for old rockers. If not, you risk damaging the cams, throwing off the valve clearances, and sending chrome flakes into the oiling system. In the case of this head, the rockers are showing the early signs of flaking, with grey patches along the cam contact surface.



To remove the lower shim, use a flat head screwdriver to lever the closing rocker down. Gently push on the edge of the closing shim to move it down and expose the retaining collet. Use a magnet to extract the two halves of the collet and then slide the closing shim off the valve stem.



Please note if you are removing the closing shims with the heads in place on the bike, you need to secure the valve stem so the valve doesn't drop into the cylinder. The easiest way is to wrap a zip tie tightly around the stem above the oil seal, and be careful not to push down on the top of the stem.

The half-ring collets are pretty unimpressive, and they are the reason for the frequent adjustment intervals of Ducati engines. The stock half rings deform and crush easily, and will often shatter at high rpms, so don't be too shocked if your collets come out in more than two pieces. A solution is to get larger, hardened collets from MBP or EMS, but on the 4V this requires using specially machined opening shims. It's an expensive process to install the upgraded collets and shims, but it will pay for itself by allowing you to go from adjustments ever 6 000 miles (10 000 kms) to every 12-18 000 miles (20 000 - 30 000 kms).



Ducati recommends replacing the half rings whenever you remove them. Most people agree that re-using the already deformed half rings will make it less likely for the clearances to shift. If your half rings aren't shattered or obviously damaged, consider re-using them. Supposedly the best way to re-install the rings is in the exact position they came out - so if you plan on re-using them, mark the stem and the rings with marker before removing them so you can reposition them in the same spot.

Reinstallation is the reverse of disassembly, as they say. One key thing is to make sure the half-rings are properly seated - there is a "special tool" for this, but as with most special tools you can do without it. Reinstall the half rings and closing shim the same way you removed them, then snap the rocker arm against the shim by levering it down and yanking out the screwdriver with the arm under moderate tension (about ½ inch of travel). This will snap the shim into place and seat the collets snugly. To double check, make sure the top of the valve stem is flush with the top of the closing shim, and gently lever down the rocker arm again to make sure the shim doesn't slide down on its own.

If you do use new half-rings, you may have to redo the clearances from scratch. Hence why most people are happy to reuse the half-rings.

Always double-check the clearances on reassembly. If anything is off, fix it. The cost of being lazy and ignoring improper clearances is very high. When run with improper clearances, Ducatis will shatter collets, flake rocker arms more rapidly, damage the valve seats, and mushroom the valve stems. They can also break and drop valves, as the valve is designed with a welded carbon-steel length on the top of the stem where the shims and collets sit - guess where they are most likely to break under stress.

Hope this helps demystify the process a little.

Happy riding,
Jason Cormier
Moto Montréal Cycle

President, DOC Ducati Desmo Clan Montréal
DOC of Montréal
Ducati Owners Group
'97 916

Ducati