Maruha 2.1L Stroker Kit for Mazda MX-5 NA NB 1.8
Maruha’s custom stroker kit uses oversize pistons and a stroked crankshaft to achieve an insane 2.1L displacement from any 1.8 BP engine. This kit comes from one of the most respected Japanese manufacturers of Mazda MX-5 parts, Maruha. There are two ways of increasing displacement in an engine, you can increase the piston diameter, or you can increase the piston stroke. Maruha’s stroker kit increases both of these things, taking the pistons to a huge 85.5 (the biggest you can reliably fit in a BP block) and increasing the stroke to 89mm. The stroke is how far down the piston goes on the downward stroke and how far up the piston comes on its upward stroke.
Crankshaft
The billet crankshaft is beautifully machined from 43040 Chromoly Steel and has a whole host of features not seen in the stock crankshaft. Firstly, Maruha have added 3mm to the crankpin diameter which increases the surface area between crankpin and journal bearing, further increasing durability. The crankshaft features two chamfered oiling holes and, when used with Maruhas upgraded main and rod bearings, ensures a consistent and healthy flow of oil into the crankpin. Maruha has also taken the time to improve on the crank nose of the engine. They’ve achieved this by enlarging the crank nose to increase the durability of the pulley boss, preventing accelerated wear in extreme driving conditions. On top of all this, the crankshaft is finely balanced at the final stage of manufacturing to ensure the most seamless install possible.
The main event, however, is the massive 89mm stroke that the crankshaft is designed around. This increases the distance of the rod bearings from the crankshaft centre and allows the piston to travel both further up and further down in the chamber, increasing displacement to 2.1L.
Connecting Rods
Maruha have used their own forged connecting rods which have been specifically designed for use in the stroker kit. The H-beam rod weighs in at only 460g and is 80g lighter than stock Mazda MX-5 rods. For use in the stroker kit, the rod has been shortened by 2mm to ensure the piston doesn’t travel too far upwards and hit the head. The big end on Maruha’s rod has been enlarged by 3mm to reduce loads on the crankpin journal. Alongside this, oversize ARP bolts have been used to secure the cap to the rod for all the high revving goodness you can throw at them.
Pistons
To complement the custom crank and rods, Maruha has commissioned a custom piston spec for their stoker kit from Mahle, a well-known manufacturer of Formula 1 pistons. These pistons feature a unique piston skirt shape which allows for the use of Maruha modified oil squirters to enhance oiling and piston cooling. Measuring at 85.5mm, they are the biggest pistons you can realistically fit in a BP block and use a unique GRAFAL coating to reduce drag, friction and piston noise.
Compression ratio: 11.6:1 with standard height head.
Rod and Main Bearings
Designed specifically for the stroker kit, Maruha’s own rod and main bearings allow for more oil flow to the crankpin journal increasing overall durability.
Maruha Modified Oil Squirters
Due to the unique design of the MAHLE piston, there is a chance of running into interference issues with the stock oil squirter. Maruha have solved this by creating a modified version of the OEM oil squirter that clears the large MAHLE pistons.
Kit includes
- 89mm stroker crank
- 2mm shorter Maruha connecting rods
- Main bearings
- Connecting rod bearings
- 85.5mm high comp Mahle custom forged pistons
- 4x Maruha oil squirters
