The term rail can be confusing, since there is no actual “rail”. The rail refers to the rocker arm itself at the portion where the arm sits on the valve tip. Up until 1965 Ford used non-rail rocker arms and from 1966 up to the 70’s (maybe early 80’s too, I’m not sure) they used a rail rockers. Rail rocker arms have a parallel rail along the tip where the rocker sits on top of the valves. The valve tips are longer and the rail keeps the rocket arms and push rods aligned as they go up and down. Non-rail rockers are flat at the tip and the vale stems are very short. The reason why there are two different types is that 1965 and before heads had cast in push rod guides that kept the rods in alignment with the rockers. Ford got cheap in 1966 and stopped casting the guides into the heads so the part where the push rods slide into the heads is round and the rods fit in loosely (as opposed to the 65 heads where the rods fit into a tight slot). To compensate Ford designed the rail rocker arms. It’s a fairly decent set up for a street car but not as reliable in the long run, especially in performance applications. I had rail rockers on my car but I switched to 65 heads and non-rail rockers, since I was rebuilding my engine anyway. Hope this helps. Another good source is the book “how to rebuild your small bock Ford”. It has some pictures of both.
These were origanally called 'self-aligning' rockers since the modifed tip did indeed keep the push rod in alignment. The advantage of this design is the push rods don't rub aganist the head and wear down thereby weakening the rod. I doubt 'cheap' had much to do with it seeing as it probably cost more to make the rocker with the 'rails' than without.
They work very well up to 6500 or so.
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