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Cams, Cam Gears, and Crankshaft Gears

Sept/Oct 2005

Our camshafts are lazy and backwards why is that? 

They turn only half as fast as the hardworking crankshaft, and in the opposite direction.  Why is that?

For a four stroke cycle engine, the cam ordinarily turns half the crankshaft speed because the valve events, that the cam lobes control, must only occur every other rotation.

The four strokes are usually named intake, compression, power, and exhaust.  Sometimes you hear "suck, squeeze, bang, and blow".  On the compression and exhaust stroke, the piston is moving up. On the intake and power stroke, the piston is moving down. Ignition normally occurs only every second time that the piston comes to the top of the cylinder.

You can also see that these camshaft lobe controlled events have to be timed correctly.  If the intake valve was open when the piston was coming up in the bore, it would push the air mixture out instead of sucking it in.  You may also note that the distributor is turning only half as fast as the crankshaft also, and that this must also be a timed event.  That is why it is usually driven by the camshaft.

The way that the Ford engineers chose to accomplish these two things on our Flathead V-8's is with timing gears.  One is on the front of the crankshaft, the other is on the front of the camshaft.  The cam gear has twice as many teeth on it as the crankshaft does.  That is how they make the cam turn only half the speed of the crank.

The timing gears have marks on them that show how they should go together so that the timing of the valve events is correct. If the gears are marked incorrectly, and or installed incorrectly, the engine will not run well or it may not run at all.
There are some variable timing gears on the market that allow fine adjustments for tuning the timing of the cam to the crankshaft.

There are various stock timing gears available.  Ford made fiber, cast iron, and aluminum gears for the cams.  The cam gears made from 1932 1934 had 56 teeth. From 1935 1953 they had 44 teeth.  The earlier engines were press fit gears with an index mark on the gear and the front of the cam. When pressing these gears onto the camshafts, the marks should be lined up with each other.

Aluminum cam gears are preferred for performance engines, especially those with stronger valve springs. Ford did make press on aluminum cam gears.

In addition to the two different tooth counts, there are also two different "hands" to the slant of the helix on the cam gears.  The 1932 1948 style is slanted to make the cam thrust to the rear.  The 1949 1953 style is slanted to thrust to the front.  The crankshaft gear must be different also to match the earlier or later style.

In both cases, there are special provisions for oiling the thrust surfaces.  In the 1932 1948, there is an oil pressure relief valve at the front of the valve chamber. It has a flat that meters oil through a hole above the front cam bearing.  This lubricates the surfaces between the rear of the cam flange and the front of the block. 

In the 1949 1953 version, there is a hole in the front cam journal that takes pressure oil.  This communicates with a passage that allows oil to exit from the front of the cam.  This oil lubricates the thrust surfaces of the distributor drive gear and timing cover.

Why did Ford slant the gears at all? Why not have the gears straight cut, so that there is no end thrust?  The answer is:  straight cut gears are noisier and somewhat weaker because only one tooth is being contacted and driven at any one time.

And why did Ford use fiber gears, considering that they are not as strong? Because fiber gears operate with less noise than the aluminum gears.

You may have noted that I said on four stroke engines the cam ordinarily turns at half the speed of the crank.  An outfit called Schaller made racing cams that turned the cam at one quarter the speed of the crank.  So they had two lobes for each valve lifter.

There are also examples in the radial engines used in aircraft of different cam speeds and various numbers of cam lobes.  Think about the cam speed required if the intake cam has three lobes for a five cylinder, four stroke radial. Gets interesting, doesn't it?

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