Maruha Original 85mm Crankshaft · BP 1.8L Engine · NA8C / NB8C
The crankshaft is the backbone of any engine. Every cam profile, every piston spec, every valve timing decision you make — all of it depends on the crankshaft being true, stable, and in perfect condition. Maruha describes it this way: "A good engine that revs freely — that feeling comes from the crankshaft above all else." You cannot build a reliable performance engine on a compromised crank.
Which brings us to the problem every NA8 and NB8 owner now faces.
The OEM Mazda crankshaft is discontinued.
Mazda part number BP06-11-300D — the OEM BP crankshaft for NA8C and NB8C — was officially discontinued as of January 2021. There is no new stock anywhere in the Mazda supply chain. If your crank has worn journals, a damaged front tip, or simply high mileage that makes reuse a gamble, there is no longer a factory solution. Used cranks are increasingly scarce and expensive. This is the replacement.
Why the OEM crank was already a compromise
The stock BP crankshaft has a known design weakness at the front tip. After the timing gear is installed, only 6mm of crank nose remains to engage the pulley boss. That 6mm — under constant tension from the alternator belt and power steering belt pulling in opposite directions — eventually develops wear between the crank nose and pulley boss. The pulley begins to wobble. The front seal leaks. And because the crank itself is worn, a new pulley boss doesn't fix it.
Maruha redesigned the front tip of their crankshaft to provide 14mm of pulley boss engagement — more than double the OEM dimension. The result is a stable, long-life front end that the OEM design never managed to deliver.
Technical Specifications:
Stroke: 85mm — identical to OEM BP-ZE specification, correct for standard rebuilds
Cast construction — Maruha's deliberate choice to keep the price accessible without compromising function or durability
Over 1kg lighter than OEM — counterweight redesign improves throttle response and engine rev feel
Front nose extended: 14mm pulley boss engagement vs. OEM's 6mm — eliminates the OEM wear point permanently
Main journal: 50mm · Pin journal: 45mm — direct compatibility with OEM-spec bearing shells and Maruha Power Metal (F112 material)
Journal overlap: +5mm — correct structural rigidity throughout
Who this is for
The 85mm crank is the correct choice for any NA8 or NB8 engine rebuild where you want OEM stroke — whether that's a reliability-focused refresh, a cam and head build, or a full engine overhaul. It replaces a part that Mazda can no longer supply, fixes the front tip weakness that the OEM never corrected, and does it at a price that makes sense given what's at stake.
If your build involves cam upgrades, solid lifters, or ITBs, the crankshaft is the foundation everything else sits on. Don't rebuild on an unknown used crank when a new Maruha unit is available.
Fitment: NA8C Miata 1994–1997 · NB8C Miata 1998–2005 · BP-ZE 1.8L engine · Full engine rebuild required.
Made in Japan. Authorized Maruha import by Chikara Motorsports — North America's only official Maruha dealer. Contact us for build consultation before ordering.
