Hose clamps: size, band and how tight to go

“I changed the clamp, tightened it well and the hose still leaks.” I take the clamp off and there is a clean cut in the hose. It was tightened so hard that the band bit into the rubber.

Size: the range, not the diameter

Every worm-drive clamp works over a clamping range, a minimum and a maximum. Measure the outside diameter of the hose already fitted on the hose barb, not of the loose hose, and choose the clamp that has it around the middle of its range. At the end of the travel the screw has only a few threads engaged and the tail of the band stays long. At the start of the travel the clamp is almost fully open and does not tighten evenly.

An example with numbers: a fitted hose of 26 mm sits right in the middle of the 20-32 mm range, while in a 25-40 mm clamp it would be almost at the start of the travel. If you only have a flexible tape measure, measure the circumference and divide it by 3.14 to get the diameter.

The ranges on the shelf come in steps, so check the data sheet of the model. In Fischer's data sheet the SGS runs from 8-12 mm up to 120-140 mm; the Unifix W2 from 8-12 mm up to 170-190 mm; the Unifix W4 from 10-16 mm to 80-100 mm. To choose, we need the diameter of the fitted hose, not the one printed on the hose.

The band: wide or narrow

A wide band spreads the pressure over a larger surface: on soft rubber hoses and sleeves it does less damage and holds better. Reinforced versions, such as the reinforced hose clamp by Unifix, take higher tightening without deforming.

The numbers, from the data sheets: standard clamps have a 9 or 12 mm band (the Unifix W2 and W4 come in 9 and 12 mm; the Fischer SGS is 9.0 × 0.6 mm). The reinforced one goes up to 21, 24 or 26 mm in width and 1, 1.5 or 1.8 mm in thickness, and is tightened with a 10, 13 or 17 mm spanner. It is the choice for large hoses and for sleeves.

Look at the edges too: a band with rounded edges does not cut the rubber, one with sharp edges does. In the SGS data sheet Fischer writes that the rolled edges protect the hose from damage.

W2 and W4: the material

On clamps made to DIN 3017 the letter W tells you what steel the band, housing and screw are made of. From the standard and the manufacturers' data sheets:

Code	Band and housing	Screw
W1	carbon steel	class 8.8
W2	stainless 1.4016 (AISI 430)	zinc-plated steel
W3	stainless 1.4016 (AISI 430)	stainless A2-70
W4	stainless 1.4301 (A2, AISI 304)	stainless A2-70
W5	stainless 1.4401, 1.4436 or 1.4571	stainless

W2: stainless band and housing, but it is an AISI 430, a chromium steel, and the screw is zinc-plated steel. The data sheet of the Fischer SGS gives exactly these materials: CQ15 screw to DIN EN 10263, band and housing in AISI 430. It is fine in many cases, but outdoors or with water the screw sooner or later rusts. The Unifix W2 clamps also have a zinc-plated screw.

W4: all stainless, band and screw. The Unifix W4 is made of A2 stainless (AISI 304), with a 7 mm hexagon-head screw with slot, to DIN 3017. It is the one to use outdoors, on plants with water, in farming and in damp rooms. A clamp with a rusted screw can no longer be reopened or retightened.

Seawater is a case of its own. For it the standard provides the W5, with 316-type stainless (1.4401, 1.4436, 1.4571), hard to magnetise. If the hose is on a boat or in a marine environment, tell me: A2 is not the steel the standard indicates.

Fitting: three things that make the difference

• Push the hose all the way onto the hose barb and put the clamp behind the ridge, not on top of it or in front of it: underneath there must be the hose barb to support the rubber.
• Do not overtighten. Tighten until the hose is held and the rubber just begins to compress. With a power driver it is very easy to overdo it: better to finish by hand with the screwdriver or the socket wrench. On the Unifix W2 and W4 the screw is a 7 mm hexagon with a slot (the W2 also has the PH2 cross recess), so both a wrench and a screwdriver will do.
• Where there is pressure or vibration, two clamps side by side with the screws staggered work better than a single one tightened to the maximum. Staggered means with the screws at different points on the hose, for example one on top and one underneath.

After the first cycles of heat and cold a rubber hose settles, and the clamp may end up a little loose. Check the tightening again after a few days of operation.

When a clamp is not what you need

In the same area of the shelf there are nylon cable ties and pipe-fixing clips. They are for bundling cables or fixing a pipe to the wall, not for holding a pressurised hose on a fitting. It sounds obvious, but I see quite a few come back used exactly like that.

• Nylon cable ties: the Unifix 0513 is nylon 6.6, from 2.5 to 9 mm wide and from 100 to 760 mm long, in packs of 100, to CEI EN 62275 and self-extinguishing UL 94 V2. Nylon does not conduct, which is why it is used in panels and cable bundles. You cut it to open it; outdoors you need the UV-resistant ones, and the data sheet says so.
• Pipe-fixing clips: the Fischer FF anchors pipes and cables from 16 to 63 mm to the wall, with a fixing base of 25 × 15 × 20 mm (40 pieces per pack). It supports the pipe, it does not clamp it onto a hose barb.
• Stove flue pipes: to fix them to the wall there are the wall-fixing clamps with wall plug for Ø 8 and 10 cm pipes in matt black.

And if the hose is hardened, cracked or swollen near the fitting, no clamp will save it: the hose has to be replaced.

What to choose for what

Job	What to choose	Why
Flexible hose on a hose barb, dry environment	W2 hose clamp, 9 or 12 mm band	Stainless band and zinc-plated screw: indoors it is enough
Water, outdoors, farming, damp rooms	W4 hose clamp in A2 stainless	The screw is stainless too, and in the W2 it is the part that gives way first
Large hoses and sleeves	Reinforced Unifix, 21 to 26 mm band	Wider and thicker band: the pressure is spread over more surface
Seawater	W5, 316-type stainless	It is the code the standard indicates for seawater
Cables and wiring	Nylon 6.6 cable tie	It does not conduct and is cut flush
Pipe fixed to the wall	Pipe-fixing clip	It anchors the pipe, it does not clamp it

Questions I get asked

W2 or W4 for a hose in a cellar?

If the cellar is dry, W2 is enough. If there is damp, condensation or an occasional leak, take the W4: the screw is what changes, and the screw is the first part to stop working.

Can a hose clamp be reused?

It reopens and refits, but only if the band is not marked or bent and the screw turns freely. If the screw is rusted or the band has taken the shape of the hose, replace it.

My hose falls between two ranges: which one do I take?

The one that keeps it closest to the middle of the range. If it sits right at the limit of one range, take the next one, so that your diameter falls nearer the centre.

What I need to know

• The outside diameter of the fitted hose, or the clamping range.
• What flows in the hose and whether it is under pressure.
• Indoors or outdoors, water, marine or aggressive environment: this gives the W code you need.
• Soft rubber, reinforced or rigid hose.
• The spanner you have available: 7 mm hexagon, or 10, 13 or 17 mm for the reinforced ones.
• Quantity and whether you need an assortment of sizes.

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We stock these

A few items from the range we keep. If you cannot find the size or the model you need, just ask: what you see online is only part of what we can source for you.

Browse the whole category: Hose Clamps (Worm-Drive)