# Overview & Safety — Vintage 2-Stroke Motorcycle Rebuild

## Bike Context
- **Make/Model:** Yamaha YL1 (Twin Jet 100)
- **Years produced:** 1964–1968
- **Engine:** 97cc air-cooled 2-stroke parallel twin
- **Induction:** Piston-port (no reed valves)
- **Oiling:** Autolube oil injection (crankshaft-driven pump)
- **Electrical:** 6-volt magneto/points ignition
- **Transmission:** 4-speed constant mesh
- **This engine serial:** L1-47603

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## Rebuild Philosophy

### Go slow, document everything
A vintage restoration cannot be rushed. Parts are either no longer made or extremely difficult to source. A mistake that destroys a casting or strips a thread may end the project. Assume every fastener is corroded and treat it accordingly before applying torque.

### Understand before you disassemble
Read the service manual **fully** for any system before touching it. On a 2-stroke twin, the engine, carburetion, ignition, and oiling systems are tightly interdependent. A misfire may be a carburetion issue, a sealing issue, a timing issue, or all three simultaneously.

### Restore, don't replace, unless necessary
Where OEM or NOS (new-old-stock) parts exist, prefer them. Aftermarket parts for small-bore vintage Japanese bikes vary widely in quality. When substituting, document what was substituted and why.

### Match both sides
The YL1 is a parallel twin. Carbs, ignition timing, port height, and ring end-gap must be matched between cylinders or the bike will never run smoothly. Always measure and set both sides identically.

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## Personal Safety

### Chemical hazards
| Hazard | Precaution |
|---|---|
| Gasoline | No open flame; ventilate; nitrile gloves |
| Carburetor cleaner / acetone | Respirator; gloves; no skin contact |
| Parts-washing solvent | Splash goggles; gloves |
| 2-stroke exhaust (running/tuning) | Outdoors or forced ventilation only |
| Battery acid (if lead-acid) | Splash goggles; keep baking soda nearby |
| Rust converters / rust remover (phosphoric acid) | Gloves; eye protection |

### Mechanical hazards
- Never work under a bike on a single stand — use proper paddock stands front and rear, or a sturdy centerstand with wheel chocks.
- Hot aluminum and iron components retain heat long after the engine stops. Allow 30+ minutes of cool-down before disassembly after a run.
- Compressed air should not be directed at skin. When blowing out ports or passages, use low pressure (< 30 PSI) and eye protection.
- When splitting cases, use a proper case-splitter tool or a soft-faced (rubber/rawhide) mallet. Steel hammers against aluminum will crack castings.

### Fire safety
- Keep a multipurpose (ABC) fire extinguisher within arm's reach whenever fuel is present.
- Never run the engine indoors without exhaust ventilation.
- Keep shop rags away from exhaust pipes and manifolds.

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## Shop Environment

### Cleanliness
- 2-stroke engines are highly sensitive to contamination. Even small particles of grit in a running fit will score cylinder walls or bearings.
- Clean the exterior of any assembly thoroughly before opening it.
- Work on a clean, dedicated surface (white shop towels are useful for finding small parts).
- Use a lint-free rag. Paper towels leave fiber.

### Organization
- Use egg-crate trays, labeled zip-lock bags, or a dedicated hardware tray to store fasteners by location.
- Label bags with the system, sub-assembly, and orientation (e.g., "LH crankcase cover — M6 × 20mm bolts").
- Do not mix fasteners from different assemblies.

### Lighting
- Adequate lighting is non-negotiable. Many critical dimensions (port timing windows, seal lips, bearing surfaces) are invisible under poor light.
- A flexible LED task light and a small inspection mirror are essential.

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## Tools — Minimum Set for This Project

### Measuring
- Digital vernier caliper (0–150mm, 0.01mm resolution)
- Telescoping gauge set (for bore measurement)
- Digital micrometer (0–25mm)
- Dial indicator + magnetic base (for runout checks)
- Feeler gauge set (metric)

### Engine
- Impact driver (manual, not air) for stuck JIS screws
- JIS screwdrivers (Phillips lookalikes — do not use standard Phillips; they will strip)
- Flywheel/rotor puller — YL1-specific or universal with correct thread adapter
- Case-splitting tool or soft mallet
- Piston ring compressor
- Small circlip pliers (internal and external)
- Bearing driver/puller set

### General
- Metric combination wrench set (6–22mm)
- Metric socket set (6–19mm, deep and shallow)
- Torque wrench (0–25 Nm, beam or click type)
- Heat gun or propane torch (bearing/bushing installation)
- Penetrating oil (PB Blaster, Kroil, or AeroKroil)

### Electrical
- Digital multimeter
- Timing light (or LED timing tool)
- Point file or 600-grit wet-dry sandpaper for point surfaces

### Note on JIS vs. Phillips
The YL1 uses **JIS (Japanese Industrial Standard)** cross-head screws throughout. JIS screws have a slightly different geometry than Phillips. Using a Phillips driver on JIS screws **will round them**. Use JIS drivers (Vessel brand recommended).

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## Reference Standard
- Service manual: `01_Documentation/2Cyl2StrMan.pdf`
- Engine serial: L1-47603 (confirms production date and specification revision)