On the orders we supplied from March to September 2026, buyers asked for ±0.35 to ±0.7 mm on the OD of closed-end protection tubes from 6 to 20 mm, ±1 mm at 24 mm, and ±1 mm on length. Multi-bore insulators were ±0.2 to ±0.5 mm on the OD. The general standard, DIN 40680, is tighter than that on small diameters and much looser on length: ±1% above 1000 mm, which is ±10.3 mm on a 1030 mm tube.
The figures below are the tolerances written on those contracts and purchase orders, grouped by size. They are what buyers specified and received, not a promise for every order: the tolerance for your order is the one in its quotation.
Closed-end protection tubes
| OD (mm) | OD tolerance on orders | ID tolerance on orders | DIN 40680 medium, same OD |
|---|---|---|---|
| 6 | ±0.35 to ±0.5 | ±0.35, ±0.4 or +1/−0 | ±0.20 |
| 7–8 | ±0.35 to ±0.5 | ±0.35 or +1/−0 | ±0.25 |
| 10 | ±0.4 to ±0.6 | ±0.4 to ±0.7, or +1/−0 | ±0.30 |
| 12 | ±0.5 | ±0.5 or +1/−0 | ±0.35 |
| 14 | ±0.5 | +1/−0 | ±0.40 |
| 15–16 | ±0.5 to ±0.6 | ±0.5 to ±0.6, or +1/−0 | ±0.40 |
| 20 | ±0.7 | ±0.7 | ±0.45 |
| 24 | ±1 | ±1 or +1/−0 | ±0.50 |
Length was ±1 mm on every line that gave a length tolerance, for tubes 300 to 2030 mm long. DIN 40680 medium would allow ±7.60 mm on a 530 mm tube and ±1% of length above 1000 mm. Tubes are cut to the ordered length on a wet cutting bench (see our factory).
On diameter it goes the other way: most buyers wrote wider figures than DIN 40680 medium, for example ±0.4 to ±0.6 mm on a 10 mm tube where medium is ±0.30 mm.
Why many IDs are +1/−0
Many ID tolerances on these orders were one-sided: +1/−0 mm, measured at the open end. A bore that comes out a little large does no harm; one that comes out small won't take the insulator.
Some orders skip the ID tolerance and name the part that has to reach the bottom of the tube instead, such as an insulator of a stated diameter. If fit is what matters to you, that is the clearest way to write it.
Multi-bore insulators
| On 2026 orders | |
|---|---|
| Sizes | 2, 4 and 5 bores, OD 3 to 8.5 mm, 480 to 1680 mm long |
| OD | ±0.2 to ±0.5 mm |
| Bores | ±0.15 to ±0.3 mm where given (±0.5 mm on the 4 mm centre bore of a 5-bore insulator) |
| Length | ±1 mm |
Some buyers give no bore tolerance at all. They name the wire that has to pass through instead: 0.6, 0.8 or 0.9 mm on our orders. For a thermocouple insulator that is often the more useful spec. Our standard bore sizes are on the insulators page.
Large through tubes
Above 30 mm, DIN 40680 puts extruded and cast parts in the coarse class.
| OD (mm) | OD | ID | Length | DIN 40680 coarse, same OD |
|---|---|---|---|---|
| 34 | ±2 | ±1.5 | ±1 | ±2.0 |
| 50 | ±2 | ±2 | ±2 | ±2.5 |
| 65 | ±2 | ±2 | ±2 | ±3.0 |
Straightness
None of these round-tube orders stated a straightness figure. DIN 40680 medium allows a bow of 3.00 mm on a part 500 to 600 mm long, 5.00 mm at 900 to 1000 mm and 0.5% of length beyond that: 7.15 mm on a 1430 mm tube. If the tube slides into a close-fitting sheath, put a straightness figure on the drawing and we'll confirm it in the quotation.
How we measure
Outside and inside diameters are checked with 0.02 mm vernier calipers. We don't publish our own inspection limits yet.
What to put on your order
- OD and ID, each with a tolerance. Use +x/−0 on the ID if something has to fit inside.
- A length tolerance. ±1 mm was normal on this year's orders.
- The insert or wire that must pass, for protection tubes and multi-bore insulators.
- Straightness, if the tube is long or goes into a close-fitting sheath.
- The Al₂O₃ grade. See C 799, C 610 and C 530 if your drawing gives a class instead.
Sizes we make and stock are in protection tube sizes.
Sources
- Morgan Haldenwanger price list, valid from 1 January 2026: standard sizes and DIN 40680 tolerance table
- Our own figures: the factory's standard-size list, 2026 orders and test report, as recorded in our product catalog.