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Distributor Timing & Cam Timing

Jan/Feb 2005

Last issue we looked at the piston and valve events of a four stroke cycle engine like our old Fords use.
We can use this information to help us do the initial timing on a distributor, as noted in the column last issue.
What if we suspect that the cam timing is wrong in a freshly rebuilt engine? Perhaps it starts and idles well, but it will not easily get higher RPM. If so, we may suspect that the cam is advanced from its proper timing to the crankshaft. How can we tell without removing a head, intake manifold, or the front cover?
We could remove one spark plug and use a wire through the hole to find top dead center (TDC) of the piston. Remembering that the piston comes to TDC two times per firing cycle, we want to check this on the non-firing, or overlap, TDC.  If the cam is timed correctly, and the piston is at the overlap TDC, each valve in that cylinder should be off of its seat the same amount.  That is sometimes called "splitting the overlap". This will tell you whether or not the cam timing is correct. It does not depend on how the gear is indexed when pressed onto the cam, or whether the correct teeth on the gears are meshed. Nor does it depend on whether the cam was ground indexed properly to the bolt pattern or mark.
You can also use the diagram of these events to see where the camshaft should be in its rotation to set valve to lifter clearance. This valve clearance may be called valve lash to differentiate it from valve to guide clearance.  To set the valve lash as far as possible from the point of maximum lift, you can set the intake when the exhaust valve on the same cylinder is just starting to open. Set the exhaust valve lash when the intake valve is just closed. This is done rotating the engine in the normal direction.

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