Cylinder Head Shimming
Ducati cylinder head shimming on the earlier 2-valve air-cooled is an important but relatively straightforward procedure. A number of specialized strategies are helpful, and the earlier Pantahs, F1’s and Alazzurras also benefit from some later updates.
When beginning with a clean, bare head, I start by replacing cams in the heads. Don’t forget the shim on either side of the cams, between the lobe areas and the head casting. There is one shim for either side, between cam and cam cover. Bolt your pulley to the cam to lock the cam in place. Note that you should either use a new cam pulley nut every time to lock the nut on the camshaft, OR you should use a little dab of low strength Loctite on cam pulley bolt threads. Otherwise, you may find these bolts loose or missing like I did one year, 15 minutes before my race at Daytona….
I pre-lube all bearings and pre-lube cam lobes with assembly grease, especially during a cam swap. This prevents cam and followers from galling or being damaged during break-in. I also clean and pre-lube valves in their guides with assembly grease if that hasn’t been done. You can also replace valve seals at this point. Seals pop on and off easily with the help of a small flat blade screwdriver…just be careful not to damage seals during installation.
For racing heads, I remove the return springs and run a pure desmodromic system. The soft closer return springs are intended to improve engine behavior for very low RPM (near idle) conditions. The springs are completely unnecessary for a motor that spends its time from 5k-10k RPM and idles at 2k RPM like these race bikes do. At these speeds, the cam action, cylinder pressure and valve inertia help seal the valve against the valve seat. Removing return springs eliminates some minor resistance in the top end. After eliminating return springs, make sure that you modify your technique when you adjust valve clearances - you no longer have to push down on the closing rocker arm to maximize closer clearance- closers will sag to the maximum clearance automatically now. You will actually have to prop closers UP to measure your opening clearances!! Yeah, clearance measurement is slightly different when you remove the soft valve return springs.
After cam installation and prep, you are ready to start shimming rocker
arms up. Take a minute and inspect your rocker arms before installation.
For racing or high mileage units, you’ll want to inspect for wear
on the cam or valve shim sides of the rockers- I have seen radical cams
such as NCR#7’s beat up rocker arms and leave indentations a few
thousandth deep, so replace your rockers if there’s any visible
Next, shim up the opening rocker arms. This is basically the same process as with closers. Center the rocker arms over the valve and the cam lobe while ensuring that there is no interference between rockers that are opposite of one another. I like to use the later rocker arms in the earlier heads. Later rocker arms (starting with F1B, Paso and later- from the late 1980’s on) have a clip that can be removed to adjust valve clearances. These later “clip” style rocker arms setups make valve adjustment a snap. [Earlier opening rocker require that you pull the entire rocker pin out to change valve shims, which is a pain in the butt as the rocker arm shims can be uncooperative during replacement.] You could buy some later clips and modify your early rocker arms for these, but you will be grinding hard rocker arm material away for a long time - there is a lot of material to remove if you use older rockers. If you use later rocker arms, you can grind off a few millimeters of rocker material and the later rockers will fit the earlier heads. This modification is one of the services that Bimotacagiva.com provides. Also note that when shimming up opening rockers with the later clips, the clips provide a little bit of spring force in the sideways direction when they are installed. In other words, here’s a little “give” in them in the direction of the rocker pin axis. You shouldn’t shim the openers up until you have to force the clips on. The clips should pop on easily and after installation, the rockers should feel smooth in action and should not bind.
During the shimming process, it definitely helps to have a stack of extra shims. I got my stack of shims from old engines. The thick ~3 mm closing rocker arm shims come in very handy, too, especially for closer arms, so I horde these, as I’d rather use one thick shim instead of a stack of thinner shims.
Stuff to watch out for: make sure that none of your rocker arms interfere with rockers on the opposite side of the head. Bad things happen when rocker arms contact each other due to insufficient clearance. Also, make sure that there is no binding ay any point in the system.
When you’re done with rocker arm shimming, adjust your valves. I adjust openers to .004” and closers to .000-.001”. The tighter closer spec is achieved when you can just spin the closing shim by hand but not measure an appreciable closer clearance with a feeler gauge. Just being able to spin the closer by hand indicates that you have some clearance and are in good shape. Remember that valve adjustment clearances that are too large are bad, but adjustments that are too tight are very, very bad. High stress situations and failures will result from opening or closing valve shim clearances that are too tight…especially for closers that are too tight.
So that’s it. I can usually shim up a set of heads in less than an hour using these techniques.
A few tips.
- If you have a rocker arm shaft that is hard to remove or replace, dress
the bores in the head casting with 220 grit sandpaper and then clean out
with solvent, air and a cotton swab. It’s usually the blind holes
at the far end of the head that can get gunked up, which makes rocker
arm extraction difficult. You can use a little antiseize upon assembly
to assure easy removal next time.
- Thoroughly check the oil drain holes in the cylinder heads while they
are off the bike. It’s easy to lose half circle valve shim keepers
or other stuff down these holes.