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?
While it is true there is a chance you can reuse a good condition used crank shaft, it is difficult for the general user to judge whether this reconditioned crank shaft will last another 100000km - 200000km without issues. Perhaps a skilled engine building shop can tell you an absolute answer. However, if you ask any veteran engine builder, "should you use a used crank shaft or a brand new one?", the answer to that is quite obvious. Maruha's new BP crank shaft was developed to address the issue of aging and discontinued OE crank shafts at an affordable price and with the least issues in the long run. Also for the reason of cost effectiveness, it is a cast design instead of forged.
Weaknesses of the OE Mazda BP crankshaft
The original OE crankshaft has a very short tip. This short tip has an oil pump, timing gear, and crank pulley attached to it, and locked with a center bolt. The issue comes after the timing gear is installed on the tip, there is only 6mm left to install the crank pulley. This pulley also pulls all the accessories of the car (power steering pump, water pump, alternator, and A/C compressor). With so much weight and tension pulling on only 6mm of margin, almost every OE crank shaft with a decent amount of mileage will have excess wear and tear on the tip that cannot be reconditioned easily or at a feasible cost.
While you can replace the crank pulley, you cannot repair the worn down crank tip easily. When this crank pulley wobbles while spinning due to the excess wear on the crank shaft tip, you will have a crank pulley (aka harmonic balancer) that can no longer protect your engine components efficiently. The most crucial crankshaft-to-camshaft timing will be misaligned, ultimately leading to numerous performance and drivability issues. Complete engine failure could be next!
Maruha's original BP crank shaft has a 14mm tip extension compared to the 6mm OE tip.
This allows for a reliable seating of the crank pulley. A stable long lifespan can be expected from this crank shaft from this simple yet effective upgrade.
Lightened crank shaft.
By shaving off some weight during the design process, the Maruha original BP crank shaft has reduced weight by 1kg or more compared to Mazda OE. This lightweight savings at the crank means less mass rotating resulting in an improved response from the engine.
85mm vs 87mm stroke
The genuine OE crank shaft has a stroke of 85mm. If you would like to stay within OE specs, this option is for you.
Maruha has also developed a crankshaft with a stroke of 43.5mm x 2 = 87mm. With this option, you will increase engine displacement. This will give you more torque and power.
Again, both options are lighter than OE, so you can expect a livelier engine response regardless of which crank you choose.
How to use 87mm stroke crankshaft
The 87mm stroke has a 1mm larger turning radius than OE. When combined with OE pistons and rods, it will stick out 1mm more. In this case, a 1mm thicker head gasket will be required to compensate for the difference. This overall 2mm thick 86mm inner diameter head gasket is available from Maruha for 19800 yen.
If you have plans for or have already built an oversized piston and rods setup, all that is required is to match up with a suitable head gasket thickness (also available from Maruha). This is by far the most cost effective way to use this new 87mm crankshaft.
Final words
The original purpose of the new Maruha BP crankshaft is to address the concern of the aging and discontinued OE crankshaft. While doing so, Maruha has made improvements over the OE by extending the crank tip and making it 1kg lighter. The result is an extremely reliable design with improved response and ease of use. Lastly, at an approximate MSRP cost of 120000 yen ($1200USD), it is considered very good value.
Here are two examples of stroker builds for comparison:
OEM displacement is 1840cc (1.8L)
1883cc OE piston naturally aspirated 1.9L stroker build
Maruha 87mm crank shaft
Original 83mm Mazda pistons and rods
1mm thicker head gasket (Maruha part# MAR0000006)
Maruha new Fcams or Race cams
Maruha cam gears
Maruha solid lifters / cap shims (not necessary for NA8 using HLA lifters)
Maruha Valve springs (optional)
ECU tuning
1998cc Maruha 85.5mm piston naturally aspirated 2.0L stroker build
Maruha 87mm crank shaft
Maruha Mahle 85.5mm new pistons and rods (no need for thicker head gasket)
Maruha new Fcams or Race cams
Maruha cam gears
Maruha solid lifters / cap shims (not necessary for NA8 using HLA lifters)
Maruha Valve springs (optional)
ECU tuning