| Quick Specifications | |
|---|---|
| Production Years | 1973–1977 |
| Engine Type | Two-stroke Parallel Twin |
| Gearbox Oil | SAE 10W-30 (SE) / SAE 80W (0.6 L) |
| Spark Plug | NGK B8HS (0.6–0.7 mm gap) |
| Battery | 6N6-3B (6V) |
| Tire Pressure (F/R) | 1.6 bar / 1.8 bar |
At MotoSpecsLab, we recognize the Yamaha RD 125 AS3 not just as a vintage motorcycle, but as a pivotal engineering milestone. A unique historical highlight of this machine is that it was one of the first production 125cc motorcycles to utilize a horizontally split crankcase. This design, pioneered on Yamaha’s factory racing machines, provided unprecedented structural rigidity and simplified maintenance compared to the vertically split cases common in that era.
Technical Engineering Overview
The RD 125 AS3 (often referred to as the D or DX in specific markets) bridged the gap between the raw power of the early YAS1 and the refined reed-valve RDs. The engine features a 7-port induction system that allows for efficient scavenging, delivering a surprisingly wide powerband for a small-displacement twin. The DX variant specifically introduced the hydraulic front disc brake, a significant upgrade over the earlier twin-leading-shoe drum setups.
Maintenance & Service Specifications
To maintain the high-revving performance of this 1970s icon, strict adherence to factory tolerances is required.
1. Lubrication System
The gearbox requires 0.6 Liters of lubricant. While vintage manuals suggest SAE 10W-30 Type SE, modern equivalents like a dedicated SAE 80W gear oil (specifically for wet clutches) provide superior shear stability for the 5-speed transmission. Ensure the Autolube pump is synchronized to prevent lean-run conditions.
2. Ignition & Electrical
The 6V electrical system is powered by a 6N6-3B battery. For optimal combustion, use NGK B8HS spark plugs. The electrode gap must be precisely set between 0.6 mm and 0.7 mm. Given the age of these units, checking the contact breaker points for pitting is a critical monthly task.
3. Chassis & Tires
Handling precision on the RD 125 is highly sensitive to tire pressure. The factory-specified 18-inch wheels should be fitted with:
- Front: 2.75-18 (1.6 bar / 23 psi)
- Rear: 3.00-18 (1.8 bar / 26 psi)
Editor’s Verdict
The Yamaha RD 125 AS3 remains a masterclass in two-stroke engineering. Its lightweight frame and twin-cylinder symphony offer a visceral riding experience that modern entry-level bikes cannot replicate. For collectors, the AS3 represents the purest form of Yamaha’s ‘Race Developed’ philosophy.
Frequently Asked Questions
A: Yes, provided it is rated for wet clutches (JASO MA). Avoid friction-modified automotive oils which can cause clutch slippage.
Q: What is the significance of the ‘AS3’ designation?
A: The AS3 was the internal factory code that evolved into the ‘RD’ (Race Developed) branding, signifying the transition to more advanced porting and braking systems.