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.
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.

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