Body Tube Metric Conversion Guide
BT numbers are names, not millimeter measurements. This metric conversion guide uses the eRockets body-tube catalog as its single dimensional source. Figures below describe its listed standard paper tubes, not a global BT tolerance or an automatic fit with another maker’s product. The metric values are rounded conversions of the listed inch dimensions.
Read OD, ID and wall separately
OD is the outside diameter of a tube; ID is its inside diameter. Wall thickness is the supplier’s nominal catalog value. For a nose-cone shoulder or internal coupler, compare the insert’s outside diameter to the airframe ID. For an outer sleeve, compare its ID to the airframe OD. Nominal labels and rounded millimeters alone cannot establish fit.
Standard thin-wall BT examples in metric
The supplier table lists the following inch values. Metric columns are approximate, rounded to 0.1 mm; they are not independent product specifications.
| Tube | OD (in) | ID (in) | Wall (in, listed) | OD (mm, approx.) | ID (mm, approx.) |
|---|---|---|---|---|---|
| BT-5 | 0.541 | 0.515 | 0.013 | 13.7 | 13.1 |
| BT-20 | 0.736 | 0.710 | 0.013 | 18.7 | 18.0 |
| BT-50 | 0.976 | 0.950 | 0.013 | 24.8 | 24.1 |
| BT-55 | 1.325 | 1.283 | 0.021 | 33.7 | 32.6 |
| BT-60 | 1.640 | 1.600 | 0.021 | 41.7 | 40.6 |
| BT-70 | 2.217 | 2.175 | 0.021 | 56.3 | 55.2 |
| BT-80 | 2.600 | 2.558 | 0.021 | 66.0 | 65.0 |
Why a rounded equivalent can mislead
A nominal “18 mm” designation does not prove interchangeability with BT-20, whose listed ID and OD differ from that round number. Heavy-wall and other variants also change the exterior fit: eRockets lists BT-80H/BTH-80 at 2.635-inch OD rather than the standard BT-80’s 2.600-inch OD. The catalog notes a legacy BT-60 dimensional variant; use the exact listing and batch you are buying. A motor mount tube and its centering rings require their own specifications, independent of the airframe’s BT label.
Using the figures in CAD and purchasing
- Identify the exact tube SKU and supplier; get its current ID, OD and matching-part dimensions.
- Convert dimensions for the drawing, but do not assign a universal printer or adhesive clearance from this table.
- Measure the actual tube and fabricated part, make a short prototype shoulder or ring where practical, and dry-fit before permanent assembly.
- For a motor-mount or structural part, follow the kit and component makers’ specifications and evaluate the full rocket under the applicable NAR model-rocket or high-power code.
For the site’s maintained supplier-dimension reference, consult the corrected body-tube size chart. Do not substitute a nominal metric tube solely because its rounded number looks close.
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