Wednesday, September 26, 2012

The right shim: Blueprinting the valve train

Today we will discuss the valve train of the engine and the importance of fitting the correct shims. To start we'll have a quick anatomy of the system so that everyone is on the same page regarding names of parts. In the photo below the labelled parts are as follows (with explanations):

A - Cam lobe - Rotates to push the cam opened and closed
B - Valve - Opens and closes to allow fuel to enter or exhaust to exit the engine
C - Valve Seat - The surface the valve closes on
D - Shim - Disc of metal available in calibrated thicknesses
E - Valve guide - Tube that the stem of the valve rides in
F - Shim Bucket - Holder for the shim 
G - Valve spring - Closes the valve as the cam turns

Now with the pieces identified we can move on to what happens when a full re-build of the heads is done. This procedure includes:

- New valves
- New valve seats
- New valve springs and retainers
- New valve guides 
- Pressure testing of heads 
- Cutting of seats to seal with valves
- Full checking of measurements with machining as needed
- Setting of valve clearances

It is the final point that is for discussion here and the following diagram should illustrate things a bit better: 

Valve or 'tappet' clearances are required because the solid valve system requires a small amount of play between the cam and the shim when cold because if there were no space, when the engine heats up, the expansion of all of the metal parts in the valve train would cause a valve to stay open just a little bit reducing engine efficiency and possibly damaging the valve. There are many designs to adjust this clearance and Ferrari's use metal discs called shims that go on top of the bucket setting the distance to the cam lobe. As the valve train wears the bucket gets closer and closer to the cam requiring a thinner and thinner shim to stay within spec. 

The Ferrari parts book shows shims available between 3.25 mm to 4.90 mm spaced in tiny increments of about .02 mm. Despite this broad assortment you cannot use them all and, although conventional wisdom would dictate to place the thickest shim possible (to offer the most future adjustment), this too is also problematic. The fact is that shim thickness is limited in the following way:

Thinner than about 3.40 mm and the cam can clip the edge of the cam bucket rather than ride on the cam.

Thicker than about 4.00 mm and the weight of the shim starts to play a role as it becomes heavy enough to move between the bucket and the cam with the possibility of the shim dislocating at high engine RPM's that could result in engine failure

When everything is new it is essential to blueprint (ie. machine to spec) the components of the head so that the shims used are as close to about 4.00 mm as possible. Too thick and you run the risk of a shim moving in the engine and any thinner and you are taking adjustment range away from the engine in the future as it wears. Very few engine builders take the time to get this step right and even fewer use the correct tools and techniques to achieve the desired result. 

Essentially there are 2 adjustments to be made:

If the shims are too thin: The tip of the valve stem needs to be trimmed in a special machine.

If the shims are too thick: The valve seats need to be cut to push the whole valve deeper into the head and closer to the cam lobe. Some engine builders will recommend cutting the valve head (often because they do not have the proper machine cut the seat) but this is bad practice because you want to preserve as much material on the valve as possible to maintain its strength and ability to dissipate heat and stress. 

In our case engine guru Gord Bush from GB Performance was entrusted to machine and assemble our heads. His meticulous attention to detail and well equipped shop ensured that, in the end, all of our shims were between 3.95-4.00 mm. Gord really took the time to do a good job and remarked that the Dino was his very favorite car. Who were we to argue :)


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