Can I use the stock lugs that came with my '82 300CD for 15 inch Pentas? Or do I have to source different lugs? Thanks.

If the lug bolt seat contour matches that of the wheel seating surface and the thread projection for the bundts and the R9's is the same they should be fine. Otherwise, you will need to buy new lug bolts with those characteristics. Hopefully, the R9's are hub-centered wheels like all genuine MB wheels.

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Fred Hoelzle