Cast Iron VS. Aluminim

I run the early cast-iron tappet blocks in my '64 motor.  When I see these at the swap meets, I always try to buy them if the price is good.  The aluminum blocks wear out faster, and usually the iron ones are straight and tight.  The only difference I really see (besides the obvious weight difference) is a taller "lip" where the cork seals sit. LEFT: Cast Iron(painted silver)  RIGHT: Aluminum

You can see the taller lip, Photo below . . .

When Sportster tappets wear, the motor sounds noisy
as the roller followers rattle and become loose in their bores.
 Cam timing may be affected or varied.
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I've always assumed the aluminum tappet blocks were originally produced for the XLR race motors, and H-D began including these on consumer model engines for cost effectiveness in 1962.