Join Date: Nov 2015
Location: Portland, OR, USA
DESMO comments FWIW
DESMODROMIC VALVES utilize opening and closing camshaft lobes.
If you don’t understand the operation of the DESMO system, YouTube search for “Ducati DESMO Valve System”. Watch the operation at 1/4 speed.
The Opening cam lobe cams the rocker arm which pushes the opening shim, which in turn pushes the valve down into the cylinder away from the valve seat.
The Closing cam lobe cams the rocker arm which pushes on the closing shim, which in turn raises the valve up toward the seat. Air flow and compression close it the rest of the way. The helper springs probably assist more during starting than anything else.
Different from conventional:
The DESMO design is very different from a conventional valve spring system in which the closing part is handled by the spring causing the valve to follow the closing segment of the cam profile. The only strict requirement, clearance wise, is that some be maintained so that the valve can close all the way and not be held open by the camshaft lobe. The spring continues to assist the closing all the way but its strength is determined to follow the cam profile, not to seal the valve. Compression seats the valve just like the DESMO system. As the valve wears into the seat on a conventional spring system, the clearance diminishes.
The DESMO opening shim is held in place by limited clearance to the rocker arm, which rides on the opening cam lobe. Therefore, this clearance should not be excessive. Performance will suffer from reduced lift as well. It must also not be insufficient because then it could become an obstruction to the closing operation. This could be catastrophic.
The DESMO closing shim is held by collets. When properly installed and not worn out, they will restrain the closing shim. The closing cam lobe-rocker arm combination pulls the valve against gravity back up into the head toward the valve seat. If the clearance is too great, the valve seating could be delayed or insufficient. If the clearance is insufficient, the valve will contact the seat while still being moved by the camshaft and a catastrophic situation can result.
Operation within specification:
The manufacturer has determined what clearances will guarantee performance and avoid obstruction w/ damaging results.
The RANGE, however, does not guarantee that the valves will stay in clearance for the duration between clearance checks. This is a subjective area where the dealer/mechanic has great authority to maximize or minimize the potential to stay within specs during the inspection interval. An unscrupulous dealer/mechanic can pretty much guarantee you will need a valve adjustment at every inspection if he chooses to to increase revenues or because he doesn’t understand the system. If this last statement doesn’t get you wanting to understand the DESMO system, nothing will.
I am assuming that most road bike owners will want to maximize the duration between valve adjustments, or at least minimize the amount of time that the clearances run out of spec, and accordingly, how far out of spec they run. (Note: A race bike would not be adjusted this way)
To maximize the effectiveness of valve clearance shim adjustments, we must look at what happens as the valve and seat wear into each other over time.
The valve will seat farther into the head over time. This will raise the valve stem which the shims ride relative to.
— pay attention ---
When the valve stem end gets higher due to valve/seat wear, the opening shim is higher. That means that its clearance to the opening rocker is REDUCED.
When the valve stem collets get higher due to valve/seat wear, the closing shim is higher. That means that its clearance to the closing rocker is INCREASED.
If the opening shim clearance gets too tight, the valve may not close, even though there is no conflict with the closer which has gotten looser. Valve failure will result eventually due to the valve overheating. Performance will be dismal.
If the closing shim clearance gets too loose, the valve does not get returned close enough to the seat to close in a timely, efficient manner. Performance suffers.
Note that in general, the factory specification are for cold and the valve length is considered to lengthen as it gets hot relative to the head. You probably should not try to second guess how much, just stay inside specifications cold.
—now, finally we get to what this means to making an adjustment —
Since the opening clearance is going to get tighter as it wears and when it gets hotter, set its clearance as close as you can to the MAXIMUM range value without exceeding it.
Since the closing clearance is going to get looser as it wears and gets hotter, set it’s clearance as close to ZERO as you can without any detectable rubbing during camshaft locations.
It should be apparent that the conventions valve system meaning of “tight" and “loose" do not apply to the DESMO system in the same way. You must always be relative to whether you are talking about an opening or closing clearance. For example, if you are talking about a closing clearance, saying that it is tight would actually imply that it is less than zero.
I machine ground my current bike's shims to be exactly on spec. as described directly above. I also learned what to do during reassembly to make the inspection repeatable. I have run about 4 thousand miles so far with excellent consistent starting and performance. With no other changes, I found that the torque curve extended down about 500rpm, making the bike a pleasure to ride around town. The top end feels stronger but that’s just seat of the pants. Technically, all of the clearances were within spec before I started. The main reason I adjusted them was that two of the rockers had the beginning signs of chrome flaking and needed R&R. I decided to use the disassembly to maximize the duration to the next adjustment.
I attached a copy of the data plate. It differs from the factory manual. I would always take info off the bike over a manual.
Readers: Please correct any errors. I was up entirely too late already when I started writing this. Thanks.
I try to be thorough. I rarely return to a thread. If you have a question for me, PM me.