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SAE J829: Fuel Tank Filler Cap and Cap Retainer Standard – Dimensional Specifications and Design Compatibility
Overview and Scope
SAE J829, revised in April 2019, defines dimensions for three types of fuel tank filler caps—vented, non-vented, and pressure-vacuum—and their corresponding retainers. Developed primarily for passenger car and truck applications, this standard ensures dimensional interchangeability while preventing incompatible combinations. For instance, it precludes use of a vented or non-vented cap with a pressure-vacuum retainer, but allows pressure-vacuum caps on non-vented retainers. The standard also references SAE J285 for nozzle spouts, SAE J1140 for filler pipe clearance, and ISO 13331 for vapor recovery systems.
Key Dimensional Requirements and Cap Types
The standard specifies caps for three nominal filler pipe sizes: 44.5 mm (1.75 in), 50.8 mm (2.00 in), and 57.2 mm (2.25 in). The 44.5 mm size is not recommended for passenger cars and trucks. Table 1 shows selected dimensions for the 50.8 mm size, illustrating the differences that enforce compatibility.
| Cap Type | Dimension A (mm) | Dimension B (mm) | Dimension C (mm) | Deg X (±4°) |
|---|---|---|---|---|
| Non-Vented | 34.8–35.6 | 41.7–42.9 | 11.9–13.0 | 140 |
| Vented | 34.8–35.6 | 41.7–42.9 | 6.6–7.6 | 140 |
| Pressure-Vacuum | 34.8–35.6 | 41.7–42.9 | 6.6–7.6 | 140 |
🔍 Design Insight: The gasket sealing surface must be flat within 0.25 mm TIR over the range 46.3–57.9 mm. The nozzle latching lip must be located 4.0–11.0 mm from that surface, extend at least 100° around the inner circumference, and be oriented symmetrically at least 35° on each side of the 6-o’clock reference plane (Figure 3 of the standard).
Design Insights and Compatibility
Dimensions are carefully chosen to prevent cross‑use of incompatible cap and retainer combinations. For example, a vented cap has a shorter C dimension (6.6–7.6 mm) compared to a non‑vented cap (11.9–13.0 mm). This ensures that a vented cap cannot seal on a pressure‑vacuum retainer, and vice versa. The standard does, however, permit a pressure‑vacuum cap on a non‑vented retainer.
⚠️ Common Mistake: Using a vented cap on a pressure‑vacuum retainer is prevented by design if dimensions are strictly followed. However, failure to maintain the required tolerances—especially the flatness of the gasket sealing surface—can compromise sealing and vapor recovery performance.
External clearance to the fuel nozzle and allowable leakage during refueling with vapor recovery systems are covered by SAE J1140, which should be consulted in conjunction with this standard.
Frequently Asked Questions
What are the main cap types defined in SAE J829?
SAE J829 defines three cap types: vented, non-vented, and pressure-vacuum. Each has distinct dimensional features that ensure proper application and prevent cross‑use.
Can a pressure‑vacuum cap be used on a vented retainer?
No. The dimensions are designed so that a pressure‑vacuum cap will not seal on a vented cap retainer. This prevents misapplication and maintains system integrity.
What is the required angular coverage of the nozzle latching lip?
The latching lip must extend at least 100° around the inside circumference of the filler pipe, with a minimum of 35° on either side of the reference plane (typically oriented at 6 o’clock).
Why is the 44.5 mm filler pipe size not recommended for passenger cars?
The 44.5 mm (1.75 in) size is not recommended for passenger cars and trucks per SAE J1140. It may be used in other applications such as marine or industrial, but for automotive use, the 50.8 mm and 57.2 mm sizes are standard.
🛠️ For detailed information, refer to the full SAE J829 standard.
