Thank you to every builder who has run an HM-01 and taken the time to share feedback with us. Many of you told us what could be easier, from mounting the flywheels to finding the right battery for the ignition. We listened, worked with RETROL, and decided to update the engine.
Since September 2026, every RETROL HM-01 we ship is the updated version. Here's what changed:
- Double-taper flywheel locking sleeves: the flywheels are easier to install and stay more securely in place.
- AA-powered CDI: the lithium batteries for the original CDI were hard to find for many of you. The new CDI runs on a single 1.5V AA battery, the same kind you use in a TV remote. A battery holder is included.
- Retuned compression ratio: the engine starts more easily.
- Refined manual: several small clarifications in the instructions.
What's Different: Before vs. After September 2026
| Before Sept 2026 | Since Sept 2026 | |
|---|---|---|
| Flywheel mounting | Original fixing method | Double-taper locking sleeves |
| Compression ratio | Original | Retuned for easier starting |
| CDI version | Gen 1 | Gen 2 |
| CDI power | 6–12V (e.g. a 2S LiPo, or two 18650 cells in the holder) | One 1.5V AA battery |
| Battery holder | Sold separately (18650 holder) | AA holder included with the CDI |
| Wiring | Same for both versions | |
What You Need to Start the Engine
The HM-01 engine is sold on its own, without ignition or a fuel tank. To get it running, you'll need:
- RETROL HM-01 EngineKIT or RTR version.
- CDI Ignition with Spark Plug Starter Kit (Gen 2)Includes the CDI, ME-8 spark plug and AA battery holder.
- 1 × 1.5V AA batteryNot included. Any standard AA battery works.
- Base, Fuel Tank & Oil Pipe SetSupplies fuel to the engine and gives it a stable base.
- FuelUse the fuel type specified in the instruction manual.
- Optional: Engine Cart KitFor a classic stationary-engine display. You can also get the engine and cart together as the HM-01 Full Set with Engine Cart.
Already Own a Gen 1 CDI?
You can keep using it with any HM-01. You'll just need its matching power supply:
- Battery Holder with JST Connector (18650)
- 18650 3.7V Lithium Battery (2pcs/set)One set has both batteries the holder needs.
Wiring the Ignition
- Red high-voltage wire → spark plug. Push the cap firmly onto the ME-8 spark plug.
- Black ground wire → engine housing. Attach it to a bare metal point on the engine.
- Sensor wire → ignition trigger. Plug the CDI's sensor lead into the Hall sensor connector.
- Power → JST plug. Connect the battery holder to the CDI last.
How the HM-01 Ignition Works
- Power: The battery powers the CDI (Capacitor Discharge Ignition), which stores energy in a capacitor.
- Timing: A small magnet on the camshaft passes the Hall sensor once every engine cycle. The sensor detects it and signals the CDI that it's time to fire.
- Spark: The CDI instantly releases its stored energy as a high-voltage pulse through the red wire to the spark plug.
- Return path: The current flows back through the engine body to the black ground wire. That's why a clean metal-to-metal ground connection matters.
If any link in this chain is weak, such as a low battery, a loose sensor plug or a poor ground, the spark becomes weak or stops.
Gen 2 CDI (1.5V AA) — Video & Wiring Diagram
Gen 1 CDI (6–12V Lithium) — Video & Wiring Diagram
For builders using the original CDI. This video shows the previous version.
FAQ
How do I know which RETROL HM-01 CDI I have?
If an AA battery holder came with your ignition kit, it's Gen 2. If you bought your ignition kit before September 2026 and it had no holder, it's Gen 1.
Is the AA battery included with the HM-01 Gen 2 CDI?
No. The holder is included, but please use your own standard 1.5V AA battery.
Can I use the AA battery holder with an HM-01 Gen 1 CDI?
No. The Gen 1 CDI needs a 6–12V supply. Please use the 18650 battery holder with one 2pcs set of 18650 batteries, or a 2S LiPo.
Are both CDI versions compatible with all RETROL HM-01 engines?
Yes. Gen 1 and Gen 2 CDIs both work with HM-01 engines made before and after September 2026. The wiring is the same; just use the power supply that matches your CDI.
Questions? Contact our support team. We're happy to help you get your HM-01 running.
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