Maruha Original 87mm Stroker Crankshaft · BP 1.8L Engine · NA8C / NB8C

The crankshaft is the backbone of the engine. Every performance decision you make — cams, pistons, head work — depends on the crank being stable, true, and in perfect condition. Maruha's 87mm crankshaft does everything their 85mm unit does, then goes further: a genuine 2mm stroke increase that raises displacement, torque, and power across the rev range. This is where a Miata engine rebuild becomes something more than a restoration.

The OEM Mazda crankshaft is discontinued.

Mazda part number BP06-11-300D was officially discontinued as of January 2021. There is no new OEM crank available anywhere in the Mazda supply chain. For NA8 and NB8 owners, this Maruha unit is now the only new crankshaft you can buy — and the 87mm version gives you the opportunity to make the most of a rebuild you're already committing to.

What the OEM crank got wrong — and what Maruha fixed

The stock BP crankshaft leaves only 6mm of nose engagement for the pulley boss after the timing gear is installed. Under the constant opposing tension of the alternator and power steering belts, that 6mm wears. The pulley wobbles. The front seal weeps. The crank nose deteriorates — and because the crank itself is the worn part, a new pulley boss alone can't fix it.

Maruha extended the front nose to deliver 14mm of pulley boss engagement — more than double OEM. Combined with over 1kg of weight saved through counterweight redesign, the result is a crankshaft that is both more reliable and more responsive than anything Mazda ever shipped from the factory.

Technical Specifications:

Stroke: 87mm — +2mm over OEM BP-ZE (85mm stock) for genuine displacement increase
Cast construction — Maruha's deliberate choice to keep the price accessible without compromising function
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 failure mode
Main journal: 50mm · Pin journal: 45mm — accepts OEM-spec bearing shells and Maruha Power Metal (F112)
Journal overlap: +4mm — sufficient structural rigidity maintained at increased stroke


Displacement combinations possible with the 87mm crank (from Maruha's engine spec table):

87mm crank + OEM/Mazda pistons (83.0mm) + Maruha 1.8mm metal gasket (83.5mm bore) → 1,905cc · 9.3:1 C/R
87mm crank + Maruha/Mahle pistons (83.5mm) + Maruha 1.8mm metal gasket (83.5mm bore) → 1,905cc · 10.8:1 C/R
87mm crank + Maruha/Mahle pistons (85.5mm) + Maruha 1.8mm metal gasket (86mm bore) → 1,998cc · 11.7:1 C/R
87mm crank + Maruha/Mahle pistons (85.5mm) + 2.0mm metal gasket (86mm bore) → 1,998cc · 11.5:1 C/R


Managing the +1mm piston protrusion at TDC

When the 87mm crank is used with standard pistons and connecting rods, the piston top sits 1mm higher at top dead centre than with the OEM 85mm crank. Maruha offers two clean solutions:

Option A — Thicker head gasket: Maruha's own 1.8mm bead-type metal head gasket (available in 83.5mm, 84.5mm, or 86mm bore sizes) absorbs the 1mm protrusion. Allows use of OEM or standard pistons.
Option B — Short pin height piston: Maruha × Mahle 85.5mm forged piston with -1mm pin height, purpose-built for the 87mm crank. Allows use of standard-thickness head gaskets and gives the option to bore the block simultaneously.


Who this is for

The 87mm crank is for the builder who is already committing to a full engine rebuild and wants to extract genuine additional displacement and torque from the BP engine — not just restore it to factory condition. The supporting parts ecosystem is complete: Maruha makes the matching pistons, gaskets, cams, and bearings. Everything needed to build around this crank is available through Chikara in a single order.

If you're pulling the engine anyway, this is the moment to make it count.

Fitment: NA8C Miata 1994–1997 · NB8C Miata 1998–2005 · BP-ZE 1.8L engine · Full engine rebuild required. Displacement changes may require re-registration in some jurisdictions — consult local regulations.
Made in Japan. Authorized Maruha import by Chikara Motorsports — North America's only official Maruha dealer. Contact us for a full build consultation before ordering.