Constant Tension Hose Clamp Size Chart and How to Measure Correctly

A constant tension hose clamp size chart is only useful if you read it backwards: start from the hose outside diameter you measured on the fitting, then pick the clamp whose range puts that number near the middle, not at either end. For OneTouch 4CT clamps, aim for roughly 25 to 75 percent of the published range, because the spring washer stack needs screw and band travel left to do its job.

Most charts list clamps by part number. This page flips that: the two lookup tables below are ordered by measured hose OD in 1/8 in and 1/4 in steps, with millimetres alongside, and they show where your OD falls inside each standard and heavy-duty clamp range as a percentage. After the chart comes a short measuring lab written only for constant tension clamps, a rule for when the wider HD band is the better call, five worked examples, and the sizing mistakes we see most often on 4CT orders.

If you want the part-number view with screw material and a full buying worksheet, that lives in our guide on how to choose a constant tension hose clamp. For general measuring tools (calipers, pi tape) and the size conventions shared by every clamp family, use how to measure hose clamp size and the hose clamp size chart in mm and inches.

How to read this constant tension hose clamp size chart

OneTouch lists 41 constant torque clamps in its constant tension clamp category. They split into two builds:

• Standard 4CT (11 sizes): 9/16 in band, 301 stainless band and housing, 410 stainless screw, 5/16 in hex. The pages we checked list a 70 in-lb rating. Ranges run from 9/16 in up to 3-1/2 in (14 to 89 mm).
• HD 4CT…L and 4CT…S (15 sizes, each in two screw versions): 5/8 in band, 301 stainless band and housing, 3/8 in hex, 92 in-lb rating on the pages we checked. The L version has a zinc-plated carbon steel screw and the S version a 410 stainless screw. Ranges run from 1 in up to 8-5/8 in (25 to 219 mm).

Every range in this constant tension hose clamp size chart comes from the product pages as published in October 2026. The percentage in brackets is where your measured OD sits inside that clamp’s range: 0 percent is the smallest diameter the clamp can close to, 100 percent is the largest it opens to. A reading of 47 percent means the clamp will finish close to the middle of its travel, which is what you want.

• Best standard 4CT and Best HD 4CT are the sizes closest to 50 percent in each build.
• Near closed end (under 25 percent) or near open end (over 75 percent) is a warning, not a ban. Read the section on mid-range sizing before you accept it.
• Also in range, but off-centre lists every other clamp that technically fits, so you can see what a supplier might substitute and why it is a weaker choice.
• If your OD falls between two rows, read both; either best fit will be inside the target band.

Constant torque clamp sizes from 5/8 in to 3-1/2 in OD

This is the overlap zone. Below 1 in only the standard build exists; from 1 in to 3-1/2 in you can choose between the 9/16 in and 5/8 in bands, and several standard clamps overlap each other heavily (4CT0016 and 4CT0020 start at the same diameter, for example).

Between 1-1/4 in and 3 in there are usually three or four standard clamps that “fit”, and only one of them lands near the middle. Also, ranges that share an end point are not interchangeable: at 2-1/4 in the 4CT0028 is fully open (100 percent) while the 4CT0036 sits at 47 percent. Only one leaves the spring stack room to work.

HD constant torque clamp sizes from 3-3/4 in to 8-1/2 in OD

Above 3-1/2 in OneTouch lists only the 5/8 in band HD series. Each HD range spans 7/8 in and the next size starts 1/2 in higher, so most diameters fall inside two clamps. The rule is simple at this end of the chart: pick the size that puts you between roughly 30 and 60 percent, and treat the neighbour as the off-centre option.

Need a size that is not here, such as a clamp below 9/16 in or above 8-5/8 in? Constant tension designs at those sizes are not in the OneTouch listing today. Send the dimensions and we will tell you honestly whether we can quote a match.

Why constant tension clamps must sit mid-range

An ordinary worm gear clamp is a rigid loop once you stop turning the screw. A constant tension clamp adds a stack of spring washers between the screw head and the housing. OneTouch’s own 4CT0010 page describes it as a stack of spring washers in the screw housing that holds sealing pressure steady as the hose grows and shrinks. These are cone-shaped Belleville washers, which behave as stiff, short-travel springs under axial load. The background on why that matters for heat-cycled hoses is in our explainer on what a constant tension hose clamp is; here we only care about what it means for size.

The stack can only compensate if two kinds of travel are still available after installation:

• Spring travel. When the hose and fitting heat up and swell, the band is pushed outward and the washers compress further. When the system cools and the rubber has taken a permanent set (engineers call this compression set), the washers extend and pull the band back in. A stack that is already crushed flat has nothing left to give on the hot side.
• Band travel. As the washers extend on the cold side, they draw the screw, and with it the band, a little further through the housing. Near the closed end of a range there may be very little slotted band left to draw. Near the open end the screw engages only a few slots, the tail barely clears the housing and the joint is more sensitive to vibration.

This is also why a constant tension hose clamp size chart shows positions, not just yes or no. There is a measurement effect that pushes the final position lower than the chart number. The OD you measure is the free OD of the hose on its fitting. When the clamp bites, it compresses the hose wall, so the band closes past that number. Soft or thick walls compress more. That is why we treat 25 to 75 percent as the acceptable band and 35 to 60 percent as the sweet spot, and why we are more wary of a reading below 25 percent than one above 75.

Measuring lab: how to measure for a constant tension clamp

The general method (calipers or a pi tape) is in the measuring guide linked above. The steps below are the extra checks that matter only because a spring stack is involved.

Measure installed OD on the fitting, at the clamp line

1. Push the hose fully home on the fitting before measuring. A hose that is half on reads smaller than it will be.
2. Mark the clamp line: usually 1/4 in or so in from the hose end, and behind any bead on the fitting.
3. Measure the OD at that mark, not at the free hose next to the fitting, which can be smaller, and not at the cut end, which may be flared.
4. Take two readings at 90 degrees. If they differ noticeably, the hose is oval on the fitting; use the average and note it on your order.
5. Record it in inches and millimetres; rounding a metric hose to the nearest 1/8 in can move you a full row.

Measure cold hose, not warm hose

Rubber and silicone hoses swell when hot, and so do the metal or composite fittings under them. If you measure a coolant hose straight after the engine stops, you read a larger OD and pick a clamp centred too high. When the system cools, the clamp ends up lower in its range than you planned. Measure and install at workshop temperature; the spring stack handles the hot condition.

Thick-wall vs thin-wall silicone hose

Silicone is softer than EPDM coolant hose, and multi-ply silicone used on engines and charge air systems often has a thick wall. Both points change the reading. A thick-wall silicone hose on a given tube can be 1/4 in or more larger in OD than a thin-wall rubber hose on the same tube, so never size from the tube alone. And because silicone compresses more under the band, the clamp closes further than the free OD suggests. For thick silicone, choose the size whose percentage is in the upper half of the sweet spot (around 50 to 65 percent) rather than one that is already low. Our constant tension vs standard clamp comparison explains why silicone joints benefit most from a spring-backed clamp in the first place.

Beaded vs plain fittings

On a beaded tube you take two measurements. The first is the OD behind the bead, where the clamp will sit: that is the number you look up. The second is the OD of the hose over the bead, which the clamp has to pass during installation. Check that this second number is below the maximum of the clamp you chose, or plan to put the clamp on the hose before you push it on the fitting. On a plain (unbeaded) tube there is only one reading, but the clamp line matters more, because you lose the bead’s help against blow-off and the band has to be fully on the tube.

Reading Belleville compression after tightening

Your measurement is confirmed, or proven wrong, by looking at the washer stack once the clamp is tightened. Before tightening, the washers are visibly coned with clear gaps between them. As you tighten, the gaps close. Instruction sheets for Belleville-type constant torque clamps generally describe the target as a stack that is nearly flat, with a hairline cone still visible, and some also ask that the screw tip protrude a short distance beyond the housing. Follow the sheet that comes with the clamp, and use the torque rating on the product page as the upper limit, not the target. Our hose clamp torque chart covers driver settings by band width.

Standard 9/16 in band vs HD 5/8 in band: when the chart says HD

Between 1 in and 3-1/2 in the chart offers both builds. The 1/16 in of extra band width looks small, but the HD clamp also has a larger 3/8 in hex screw and a higher listed rating (92 in-lb against 70 in-lb on the pages we checked). Choose HD in these cases:

• Your OD is above 3-1/2 in. Only HD is listed there.
• The standard best fit is off-centre and the HD best fit is not. Example: at 3-3/8 in the standard 4CT0048 sits at 87 percent, while the HD 4CT350 sits at 71 percent. At 3-1/4 in HD is at 57 percent against 73 for standard.
• Heavy reinforced hose. Thick, wire- or multi-ply-reinforced walls on truck radiators, off-highway machines and charge air joints resist compression; the bigger screw and wider band handle that better.
• Soft silicone over 2 in OD. A wider band spreads the load and lowers the risk of the band edge cutting in.

Stay with standard where the hose is small (under about 1-1/4 in, where HD starts at its closed end), where clearance around the housing is tight, or where your shop drivers are all 5/16 in. Within HD, the L version has a zinc-plated carbon steel screw and the S version a 410 stainless screw; the ranges are identical, so the choice is about exposure, not size. Pick S anywhere the clamp gets washed, splashed or salted. For a deeper look at grades on this family, see 304 vs 316 constant tension hose clamps.

One more note on markings. The OneTouch pages give an SAE number for each clamp, but the HD numbers follow a different pattern from the standard ones (the 4CT175L page lists SAE 28, the 4CT300S page lists 312). Order constant torque clamps by diameter range or part number, never by an SAE number copied from a worm gear clamp.

Five worked examples: measurement to chart row to part

Hose wall thicknesses below are typical figures used to make the arithmetic visible; they are illustrative. Always use your own measured OD.

1. Truck radiator hose, 2-1/8 in neck

A reinforced EPDM radiator hose on a 2-1/8 in outlet measures 2-5/8 in (66.7 mm) at the clamp line, cold. On the chart, the standard 4CT0040 lands at 60 percent and the HD 4CT300 at 43 percent. Both are acceptable. Because this is a thick reinforced hose on a heavy truck that sees long heat cycles, the HD row wins, and since radiator areas get pressure-washed, we would pick the stainless-screw HD 4CT300S (2-1/4 to 3-1/8 in).

2. Heater hose, 5/8 in tube

A 5/8 in ID heater hose on a 5/8 in tube measures 1 in (25.4 mm). The standard 4CT0012 sits at 56 percent. The 4CT0010 would be at 88 percent and the HD 4CT175 at 0 percent, so both are poor. Order the 4CT0012 (11/16 to 1-1/4 in). Buyers who “go heavy duty” here end up at the closed end.

3. Genset coolant hose, thick-wall silicone on a beaded tube

A standby generator uses 1-1/2 in ID four-ply silicone on a beaded aluminium tube. Behind the bead, the hose measures 2 in (50.8 mm); over the bead it measures about 2-1/8 in. Chart row 2 in gives the standard 4CT0032 at 47 percent and the HD 4CT250 at 29 percent. Silicone compresses, so the HD option would finish lower still; the standard clamp is the better fit. Its 2-1/2 in maximum clears the 2-1/8 in bead comfortably. Order the 4CT0032 (1-9/16 to 2-1/2 in).

4. Charge air cooler rubber elbow, 3 in tube

A reinforced rubber CAC elbow on a 3 in tube measures 3-1/4 in (82.5 mm). The standard 4CT0048 would sit at 73 percent, at the edge of the acceptable band. The HD 4CT350 sits at 57 percent and has the wider band a boost joint needs, so the chart points to HD 4CT350S (2-3/4 to 3-5/8 in). Two cautions. The OneTouch pages list coolant and heater hoses as the application, so confirm boost duty with us before a fleet roll-out. And NORMA positions its heavy-duty Constant-Torque clamp as a lighter alternative to spring-loaded T-bolts for many charge air joints, not all of them. High-boost joints may still need a T-bolt.

5. Marine raw water hose on a 1-1/4 in barb

Wire-reinforced raw water hose, 1-1/4 in ID, on a bronze seacock barb measures 1-3/4 in (44.4 mm). The chart offers the standard 4CT0028 at 47 percent and the HD 4CT200 at 57 percent. Both fit well. On a boat the decision turns on materials: the band and housing are 301 stainless, the standard screw and the HD S screw are 410 stainless, and none of it is 316. For below-waterline joints, use the HD 4CT200S, double-clamp the barb, and inspect it every season. Our marine hose clamp guide covers where an all-316 worm gear clamp may be the safer pick.

Common constant tension clamp sizing errors

Almost every wrong 4CT order we see traces back to one of these, not to an error in the constant tension hose clamp size chart itself.

• Ordering by hose ID. A “2 inch hose” can need anything from a 2-3/8 in to a 2-3/4 in clamp position depending on wall. The chart is built on OD for a reason.
• Picking the clamp whose maximum equals the OD. It “fits”, but with the band tail barely through the housing and no room to tighten. The 4CT0012 at 1-1/4 in and 4CT0028 at 2-1/4 in are classic examples.
• Confusing two clamps with the same start. 4CT0016 and 4CT0020 both open from 13/16 in, but end at 1-1/2 in and 1-3/4 in. Check the top of the range, not just the bottom.
• Measuring a hot hose. It pushes you one row too large.
• Sizing from the tube when the hose is thick silicone. The wall can add a full chart row or more.
• Forgetting the bead. The clamp that is perfect behind the bead may not open wide enough to pass over it.
• Driving the stack solid with an impact tool. A flat, solid stack is a worm gear clamp with extra parts. Use a torque-limited driver and stop at nearly flat.
• Copying an SAE number off an old worm gear clamp. Constant torque numbering, especially on HD clamps, does not line up. Use the range.
• Treating L and S as different sizes. They share ranges; only the screw changes.

Constant tension clamp sizing questions buyers ask

Send us your measured OD and we will size it

If you are replacing an existing constant torque clamp, look up its part number in our clamp cross-reference first. If you are starting from a hose, send the measured OD at the clamp line, the hose material and wall, and the bead size through our contact page. We will reply with the chart row, the 4CT part we would fit and a quote for your quantity.