This is quite a popular screw in the 126 class cars. It is also used on the exhaust manifold to hold on the aluminum heat shields. The dimensions of the screw are 10 MM hex head, 12 MM length, 6 MM diameter. I sent Mcmaster Carr, Fastenal and Space Air Supply an email. None had it in their on-line catalogs.

Thanks for the leads.

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1989 420 SEL