There have been a variety of engine camshaft gear designs and materials used over the years for the Model A and B. Shown below are a series of pics showing some of the variations.
The camshaft gear has 50 helical cut teeth, and rotates at half the engine speed. The crankshaft gear has 25 helical cut teeth and rotates at engine speed.
The original Ford engine camshaft gear was a phenolic construction made up of laminations of fabric. These are often found with Ford script and the logos or identification of companies such as Formica, Celoron, Micarta, and others. The main feature of these gears is quiet running when used with a good crankshaft gear.
These genuine Ford gears were produced both with and without aluminum hub centers, as shown above. The fabric and phenolic gears having the aluminum hub centers often fail at the interface of the gear to the hub, loosening up and producing a knocking noise when running.
Pictured above is a replacement crankshaft gear. The original crankshaft gears were similarly made and worked well if not corroded. When corroded, any crankshaft gear will have the tendency to prematurely wear out the camshaft gear.
Thus in later years it also became popular to use machined cast and billet aluminum camshaft timing gears because they were deemed more robust than the fabric gears. The metal camshaft gears are also credited with being noisier than original fabric gears.
Shown above is an Alcoa aluminum camshaft gear. Below is a no-brand aluminum gear from the 1970s.
Shown above is an original Ford machined die-cast aluminum camshaft gear which was used in the German G28T engines. This style of gear was also produced in Canada at some point.
For the ultimate gear-head, Bill Stipe produces this steel hub and billet aluminum camshaft gear which is adjustable in order to vary the cam and valve timing relative to crankshaft and piston position. The two-piece gear has a built-in vernier to dial in the exact angle changes desired.
There have also been a couple of sources of machined bronze gears in recent years. Shown above is a billet bronze gear made by Ron Kelley. In addition, Dan McEachern makes both machined cast bronze and cast aluminum camshaft timing gears.
Shown above and below is an example of a steel camshaft gear which came out of a Riley racing engine.
Shown below is an example of another iron/steel camshaft gear. This NOS one was for Gleaner Baldwin combine engines for driving the side-mounted engine governor.
Check out this great YouTube video from Ray White showing how to replace the gear!