Maruha New BP 85mm and 87mm Crank Shaft Explained


The crank shaft is the backbone of an entire BP engine build. After all, it is this piece that converts reciprocating motion into rotational motion in your MX-5. A small, imprecise change in this piece will cause a chain reaction to the rest of the components in your BP build (i.e. cam timing, piston timing). If your engine has gone through it's fair share of mileage, chances are you already know the OE crankshaft is worn out (i.e. oil seeping from the crankcase).

The R&D department at Maruha Motors Japan has thought about the aging crankshaft in MX-5s for close to a decade now. Countless years of research was spent to find the most cost effective and reliable production process. The result is two variations of the new Maruha BP crank shaft.


As of January 2021, product number BP06-11-300D Mazda OE crankshaft is discontinued!


For those who wish to stay within stock limits, there is an 85mm OE spec crank shaft. For those who would like to add more displacement to their engine, there is an 87mm Maruha spec crank shaft.

Why not just reuse an existing crank shaft? Can't I just pull a used crank shaft from my local auto wrecker?