Remember the 107 first hit the streets in late-1970 and early-1971. Presumably Aluminium was too difficult/expensive. Weight saving then had not become the issue it is now. On the 129, first issued in mid- to late-1989, but signed off as a design by 1986, aluminium was used specifically to address the problem posed by the weight of the top on its predecessor. Of course, the panoramic top available from 1985 is incredibly heavy! On the new 230 many of the body panels are aluminium.

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JJ Rodger
2013 G350 Bluetec
1999 SL 500
1993 E300 diesel T
1990 190