Why the Motor-to-Transmission Coupler Matters
After removing the gas engine from your Mazda Miata, you need to connect the electric motor to the original transmission. The component that does this is the coupler — a precision-machined adapter that transfers the motor's torque to the transmission input shaft.
A poorly made coupler causes vibration, bearing wear, and noise. A correctly designed one transmits power smoothly and silently for the life of the conversion.
Coupler Design Options
| Type | How It Works | Pros | Cons |
|---|---|---|---|
| Lovejoy / Jaw Coupling | Two metal hubs with a polyurethane spider insert between them | Forgives misalignment, dampens vibration, easy to source, affordable | Spider wears over time; limited torque capacity on small sizes |
| Solid Adapter Plate | Single rigid plate bolted to motor face, with a pilot bore for the trans shaft | Simple design, no wearing parts | Requires near-perfect alignment; transmits all vibration |
| Spline Coupling | Motor shaft and trans shaft connected via splined collar | High torque capacity, compact | Expensive to machine, requires precise spline matching |
For most Miata conversions, a Lovejoy-style jaw coupling with a polyurethane spider is the practical choice. It provides enough compliance for real-world alignment tolerances while handling the instant torque of an electric motor.
Critical Measurements
Before designing the coupler, collect these dimensions:
| Measurement | Where to Measure | Typical Miata Value |
|---|---|---|
| Motor shaft diameter | Output shaft of your chosen motor | 19–24 mm (varies by motor) |
| Motor bolt pattern | Face of motor, near shaft opening | Typically 4 or 6 bolts on a PCD |
| Trans input shaft diameter | Miata 5-speed or 6-speed input shaft tip | ~20 mm (pilot end) |
| Trans input shaft spline | Spline count and diameter on input shaft | Miata 5MT: 20 spline (pre-99) or 10 spline (99+) |
| Bellhousing bolt pattern | Transmission face where bellhousing mounts | Must match your adapter plate |
| Distance from motor face to trans shaft tip | Measured when both are loosely positioned | Determines coupler thickness |
Fabrication Steps
1. Design the Adapter Plate
Machine a flat aluminum plate (6061-T6 recommended) that bolts to the motor face. Include a center bore for the Lovejoy hub and clearance holes for the motor bolts. Typical thickness: 10–15 mm.
2. Machine the Hub
The hub is a steel cylinder bored to fit the Miata transmission input shaft. One end receives the Lovejoy spider; the other end bolts to the adapter plate. Heat-treating is not required for street-driven Miatas — mild steel (1018) is sufficient.
3. Select the Spider Insert
Polyurethane spiders are rated by durometer (hardness). For a Miata EV:
- 92A durometer — good balance of torque capacity and vibration damping
- 98A durometer — stiffer, higher torque capacity, less damping
- Standard Lovejoy sizes: L035, L050, L070 (choose based on torque output of your motor)
4. Assemble
Bolt the plate to the motor. Insert the spider. Slide the transmission into position, guiding the input shaft into the hub bore. Torque all bolts to spec and check free rotation before installing the driveline.
FAQ
Can I use the Miata's original flywheel and clutch?
Yes, if you keep the original transmission. The clutch pedal and hydraulic system remain unchanged. The coupler replaces the flywheel-to-crankshaft connection. Your EV motor bolts to the adapter plate, and the clutch operates normally.
Do I need a machine shop or can I DIY the coupler?
A machine shop is strongly recommended. The tolerances required (< 0.1 mm on shaft bores, < 0.05 mm on bolt patterns) are beyond what most DIY tools can achieve. A CNC-machined coupler costs $100–300 and is worth every dollar.
What motor works with the Miata 5-speed transmission?
Common choices include Hyper9 (9-inch motor, ~80 kW), AC-50 (~20 kW), or a Tesla Small Drive Unit (~220 kW). The coupler design is specific to each motor's shaft diameter and bolt pattern. CMVTE offers motors and drivetrain components compatible with Miata conversions.
How do I align the motor and transmission?
Use a dial indicator on the coupler hub to measure runout. Target: less than 0.1 mm total indicated runout (TIR). Shim the motor mounts as needed. Proper alignment prevents vibration and bearing wear.
Does the coupler need balancing?
Yes — have the assembled coupler (plate + hub + spider) balanced at the machine shop. Unbalanced couplers cause vibration at higher RPM, which is especially noticeable in a silent EV.
Get Components for Your Miata Coupler
Browse CMVTE's Motors and Drivetrain Components for parts compatible with your Miata EV conversion.
Need help selecting a motor that fits your Miata transmission? Contact our team with your target power, budget, and transmission type — we will recommend compatible components.
Related Miata conversion guides:
