How do I measure my hydraulic hose? AF1.8, DN2.0 and the caliper

Most incorrect orders we encounter are due to one thing: the hose has not been measured, or not accurately enough. That's a shame, because it only takes five minutes. This article explains what the codes on your hose mean, how to measure correctly, and what to do if there's nothing written on it.

 

What's written on the hose

Factory hoses have a print. It might be faded sometimes, but usually still readable if you wipe the hose clean. The code you're looking for looks like this:

• AF1.8 or DN1.8: internal diameter 1.8 mm.
• AF2.0 or DN2.0: internal diameter 2.0 mm.

DN stands for diamètre nominal, the nominal internal diameter. AF is an abbreviation used by some manufacturers for the same thing. The number after it is what counts.

The internal diameter determines how much oil can pass through and thus how fast the roof moves. Therefore, this is not a dimension you can choose freely: a line with a deviating internal diameter will cause the roof to move too slowly or unevenly.

 

What's not written, but still matters

The print indicates the internal diameter. However, for a repair coupling, the external diameter is also crucial, as the coupling clamps onto it. Our couplings are made for an external diameter of 4.5 mm, with an allowed tolerance between 4.45 and 4.55 mm.

That's a margin of one-tenth of a millimeter. It cannot be judged by eye, nor with a ruler or folding rule.

 

You need a caliper

A precision caliper is the only tool that suffices here. Digital versions can be purchased for a few tens of euros and measure accurately down to hundredths of a millimeter. Here's how to measure:

1. Clean the hose at the spot where you measure. Dirt and oil residues will give an inflated value.
2. Measure on a straight section, not in a bend and not close to a fitting. In those places, the hose is deformed.
3. Place the jaws of the caliper around the hose and close them without squeezing. Rubber is compressible: if you squeeze, you'll measure too small. This is the most common measurement error.
4. Read and note.
5. Measure in three places along the hose and compare. If the values differ, the hose is deformed or damaged. This is a reason to replace the entire line instead of repairing it.

Do you get a reading between 4.45 and 4.55 mm? Then the coupling will fit. Do you get a reading outside that range? Then you need a custom-made hose.

 

There's nothing on my hose

This happens, especially with older cars or with hoses that have been replaced before. In that case, measure the outer diameter as described above. If it's correct, there's a good chance the inner diameter is 1.8 or 2.0 mm and the coupling will fit. You'll only know for sure if you cut the hose and measure the inside, but that's an irreversible action.

In that case: send us a photo of the hose, the fitting, and the location of the damage. We see enough of them to often be able to say something meaningful at first glance. That will cost you five minutes and prevent an incorrect order.

 

Measuring for a custom hose

If you order a complete custom hose, you need other information:

• The length. Measure the existing hose from fitting to fitting, along the path it takes, not stretched taut in a straight line. The hose must be able to move with the roof. Too short is fatal; a few centimeters of play is normal and desired.
• Both connectors. The fittings at the ends differ per model and sometimes per side. Take clear photos of both ends, from the side and from the front.
• The route. Photograph where the hose runs. Sharp bends and attachment points also help determine which version works best.

For unusual or exotic connectors, we have a special version.

 

The 19 mm rule

If you are repairing with a coupling instead of replacing an entire hose, a minimum applies: there must be at least 19 mm of hose remaining from the metal connector. This is the length the coupling needs to clamp properly. If you cut closer to the fitting, there isn't enough material and the connection won't hold.

So, measure before you cut. The coupling can only be used once: once it's crimped, you cannot start over.

 

Measurement errors we see most often

• Measuring with a ruler or folding rule: insufficient accuracy, a difference of 0.5 mm is not visible.
• Squeezing the caliper: too low a value, coupling won't fit.
• Measuring in a bend: deformed hose, unreliable value.
• Measuring close to the fitting: the hose is expanded there.
• Measuring over dirt or oil: too high a value.
• Measuring in one spot: deformation elsewhere goes unnoticed.
• Measuring the length stretched taut: the hose will be ordered too short.

 

In short

1. Look for the print: AF1.8, DN1.8, AF2.0 or DN2.0.
2. Measure the outer diameter with a caliper, in three places, without squeezing.
3. Between 4.45 and 4.55 mm? Then the repair kit will fit.
4. Outside that range, or is your model on the exceptions list? Then a custom-made hose.
5. Are you repairing? Leave at least 19 mm from the connector.
6. In doubt? Send a photo before ordering.

Not sure which solution suits your situation? Then read repair kit or new custom hose.

 

Are you unsure about your measurement? Send us a photo of the print and the damage, and we will check with you before you order.