4130 Aircraft Tubing Size Guide
Aircraft 4130 tubing is made in a regular series of diameters and walls. The series was laid out so that tubes fit inside one another, which is what makes sleeved splices and repairs possible.
Condition N: The Aircraft Mandate
Before discussing size, aircraft builders must understand condition. For certificated aircraft, sizing is irrelevant if the condition is wrong. The tubing must meet AMS-T-6736, which explicitly requires Condition N (Normalized). As-drawn mechanical tubing (ASTM A519) does not meet this requirement, regardless of whether the physical dimensions are correct.
Fuselage Truss Sizing Logic
In a classic Warren truss welded steel fuselage, sizes change throughout the airframe based on the load path:
- Engine Mount & Forward Cabin: Heaviest tubing (e.g., 0.750" to 1.000" OD, 0.049" to 0.065" wall) to support the engine weight, torque, and landing gear loads.
- Tailcone / Aft Fuselage: Progressively lighter tubing (e.g., 0.500" to 0.625" OD, 0.035" wall) as loads decrease toward the tail, critical for maintaining the aircraft's center of gravity.
| Aircraft Structural Area | Typical 4130 Tube Dimensions |
|---|---|
| Engine Mount | 0.750" - 1.000" OD x 0.058" - 0.065" Wall |
| Forward Cabin Truss | 0.750" - 1.000" OD x 0.049" - 0.065" Wall |
| Aft Fuselage | 0.500" - 0.750" OD x 0.035" - 0.049" Wall |
| Control Surface Frames | 0.375" - 0.500" OD x 0.035" Wall |
| Required Condition | Normalized (AMS-T-6736) |
Telescoping Sizes
Aircraft tube diameters rise in steps of 1/16" in the small sizes and 1/8" in the larger ones. A tube with a 0.058" wall has a bore 0.116" smaller than its outside diameter, so the next size down in 1/8" steps slides inside it with a few thousandths of clearance.
| Outer tube | Bore | Inner tube that fits |
|---|---|---|
| 0.875" × 0.058" | 0.759" | 0.750" OD |
| 1.000" × 0.058" | 0.884" | 0.875" OD |
| 1.125" × 0.058" | 1.009" | 1.000" OD |
| 1.250" × 0.058" | 1.134" | 1.125" OD |
This is the basis of the inner and outer sleeve splices described in aircraft repair guidance. For a repair, the sleeve material and wall must be at least equal to the original tube, and the method has to follow approved data for the aircraft.
Remember that tube has tolerances: check the actual fit before cutting sleeves. Order by OD and wall in the normalized condition to the specification in your design data. See 4130 aircraft tubing and the 1.000" OD table.
Frequently Asked Questions
Can I substitute a slightly thicker wall if the exact aircraft size is unavailable?
In experimental aviation, builders sometimes substitute the next wall thickness up (e.g., 0.049 instead of 0.035) if a size is unavailable. While structurally safe, this adds weight. In certificated aircraft repair, substitutions must be approved by the design authority or IA in accordance with AC 43.13-1B.
Which wall thickness lets aircraft tubes telescope?
A 0.058" wall lets the next 1/8" smaller diameter slide inside.
What fits inside 1.000" × 0.058" tube?
A 0.875" OD tube, with a small clearance.
Order AMS-T-6736 Aircraft Tubing
Submit your aircraft cut list for Normalized AMS-T-6736 supply with full MTC documentation.
- List of OD & Wall sizes
- Standard: AMS-T-6736 Condition N
- Footage per size
- MTC 3.1 required