We have developed what we believe is the premier aftermarket BBK for the E39 M5. Comprised of electro nickel plated versions of the AP Racing 9660 six-piston caliper in the front and the AP Racing 9449 four-piston caliper in the rear, the kit employs AP Racing "J-hook" rotors, 372mm in the front and 368mm in the rear, custom Spiegler stainless steel brake lines, FEA tested Partee Racing custom brake brackets made from 7075-T6 aluminum and hard anodized, and Ferodo 2500 brake pads. The kit fits nicely under most if not all aftermarket 18" wheels, including the popular Apex forged models, but like most BBK's does not fit under the stock E39 M5 wheels.
The AP calipers here are the best in the business: The state of the art Radi-CAL design that minimizes weight and maximizes cooling, variegated stainless steel pistons, high temperature seals, anti-knockback springs, and all internal fluid passageways (no external tubing).  The calipers do NOT have dust boots, but our experience is that with periodic cleaning of the type that any self-respecting E39 M5 owner would perform, there is no material dust buildup on the calipers. And dust boots do nothing more than melt on track days and weak havoc.
If you want the best commercially available BBK for the E39 M5, this is it.
