rear brake drums: a few questions

["Ross Eddington" <reddingt@pobox.une.edu.au>]

A few questions about the rear brake drums, namely about the 
differences between the cast iron and later finned alluminium ones 
(67.5-70.)

(1) Do these have a scrap thickness: IE: how to tell if they are no 
longer safe?

(2) do the later alluminium ones have any performance advantages, or 
conversely, are they worse in anyones opinions.

(3) are they interchangeable?

thanks for any advice, opinions or comments
-- 
Ross Eddington

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