What Size Belt Does My Swamp Cooler Need?

Your cooler needs the belt code printed on the old belt, and if that has worn off, the length listed for its model. Champion's OEM parts list a 4L-450 for the 3000 series, a 4L-560 for the 4000 DD and 4001, a 4L-690 for the 5000 DD/DM and a 4L-850 for the 95 series. The table below gathers those and the MasterCool, window and commercial belts into one place, and the formula further down sizes a drive when there is no code and no model plate at all.

How to read the code on the belt

Most residential cooler belts are light-duty 4L belts. Gates states the rule on its belt ID chart: the letters give the section and the digits give the outside circumference to the nearest tenth of an inch. A 4L450 is 1/2 in wide across the top and 45.0 in around the outside. Gates also prints its own catalog numbers on some belts, and its chart gives the translation: a Gates 2450 is a 4L450.

A-section belts look identical and are named differently. Gates names its classical A and B belts by inside circumference, so for the same belt the A number comes out lower. PartMatch's conversion table puts the gap at about 2 in: a 4L460 (46.0 in outside) is roughly an A44, a 4L590 roughly an A57.

This is the confusion behind most wrong belts. Someone reads "45" off a 4L450, asks for a 45 inch belt and is handed an A45, which is about 47 in around the outside. Two inches of extra belt means the motor has to slide about 1 in further to take up the slack, which many mounts do not have. The rule: a 4L number divided by 10 is outside inches; an A number plus about 2 is outside inches.

3L

3/8 in top width, outside length in tenths of an inch; found on some small units

4L

1/2 in top width, 5/16 in deep, outside length in tenths of an inch; the usual residential cooler belt

A

1/2 in top width, same profile as 4L, named by inside length in inches (about 2 in less than outside)

B

21/32 in top width, named by inside length; used on large commercial cabinets, will not fit a 4L/A pulley

Belt sizes by model

These are Champion's own part numbers and descriptions as listed by Parts Town. Champion's catalog covers its standard coolers and the MasterCool ADA/ASA models, so for MasterCool owners these are the OEM belts too. The model text is copied from the part descriptions, including where the listing truncates it.

Three patterns stand out once the list is in one place.

The number in the model name tells you little. The jump from a 3000 to a 4000 is 11 in of belt, because the 4000/4001 uses a 10 in blower pulley (Champion part 110275) where the 3000 uses a 7 in one (110274). A "45 inch belt for a 4000" is a 3000 belt.

Side-draft and downdraft versions differ. The ADA (down discharge) and ASA (side discharge) MasterCools of the same size take belts 3 to 5 in apart: 4L-640 against 4L-670 for the 70 series, 4L-520 or 4L-530 against 4L-570 for the 50 series. If you are not sure which you have, the downdraft vs side-draft guide shows how to tell.

Generations differ by an inch. Champion lists both a 4L-530 (ADA50/5012/51) and a 4L-520 (ADA51/5112) for 50-series downdrafts. Read the full model on the data plate, including any 12 suffix, before ordering; the same suffix also decides the media depth in the pad size chart.

4L or A: which to buy

Either works on a residential cooler if the length is right. PartMatch gives the two sections the same top width (1/2 in), the same depth (5/16 in) and the same 40 degree groove angle, and lists residential furnace blowers and HVAC fan drives among the jobs where swapping one for the other is reliable. The difference is the duty: 4L belts are built for fractional horsepower drives, under 1 hp, and A belts for roughly 1 to 10 hp. A 1/3 or 1/2 hp cooler motor is exactly what a 4L is made for, and an A belt simply has margin to spare.

So the practical choice comes down to the length conversion. For a 4L-560, buy a 4L560 or an A54. Do not mix sections the other way: a B belt in a 4L/A groove rides high and a 4L belt in a B groove bottoms out and slips. The universal replacement parts guide covers matching pulley grooves when you change a pulley too.

No code, no model: size it from the drive

A belt laid flat and measured with a tape is a poor guide, because it is the worn part. The pulleys and the shaft spacing are what fix the length, and you can measure those. Gates' drive design manual gives the standard relationship between belt length, center distance and pulley diameters; rearranged for length, it is:

C is the distance between the motor shaft center and the blower shaft center, D is the blower pulley diameter and d is the motor pulley diameter, all in inches. Gates writes it with pitch diameters to give pitch length. Using the pulleys' outside diameters instead gives a figure close to the outside length that a 4L number encodes, which is all you need to choose between two stock sizes.

1. Kill power and open the motor side. Follow the safety steps in belt and pulley adjustment.
2. Set the motor at mid-travel. Slide it to the middle of its mount slots, so the new belt leaves room to tighten as it seats in.
3. Measure both pulleys across the rim. For an adjustable motor pulley, measure where the belt rode, marked by the shiny band on the flanges.
4. Measure shaft center to shaft center. Measure edge to edge of the two shafts and add half of each shaft diameter.
5. Work out the length and round. Pick the nearest 4L size. If you are exactly between two, take the shorter one when the motor has room to slide toward the blower to fit it, otherwise the longer.

The formula also explains two things owners notice. Each inch you slide the motor changes the belt path by about 2 in, because both spans grow, so a belt one size too long eats half an inch of mount travel. And closing an adjustable motor pulley from 3.0 to 3.3 in effective diameter adds about 0.5 in to the belt path (1.57 x 0.3), which is why speeding up the blower also tightens the belt. Re-tension at the motor mount after any pulley change, never by the pulley itself.

When a new motor or pulley changes the answer

The OEM lengths assume OEM pulleys. Many coolers have had a universal motor fitted at some point, often with a different adjustable pulley, and the old belt code then describes a drive that no longer exists. If a replacement motor has a larger pulley or mounts further from the blower, size the belt from the drive with the formula, not from the table. The replacement motor guide explains why keeping the same pulley pitch diameter keeps both belt length and blower speed where they were.

A bigger blower pulley or a closed-up motor pulley speeds the blower up as well as changing belt length. Fan power rises with roughly the cube of speed, so check motor amps against the nameplate after any change, as described in the swamp cooler motor guide.

Where this chart stops applying

The table covers the models named in Champion's OEM part descriptions and nothing else. Phoenix-built coolers (Frigiking, Aerocool) and Essick, Breezair and Bonaire units are not in it; for those, read the old belt or size from the drive. Direct-drive coolers, including most portables, have no belt at all; if one of those squeals, start with swamp cooler noises.

Commercial cabinets are different again. Champion's spec sheet for its AS/AD 100 and 150 lists no single belt per model; it selects a pulley-belt kit by motor horsepower, shaft bore and the duct's static pressure, with separate kits above and below set pressures. On those, quote the kit number from the cooler, not a belt length. The commercial cooling guide covers how those systems are specified.

After the new belt is on

A new belt seats into the grooves over its first few hours, so recheck tension after a day of running and set it to the 1/2 to 3/4 in deflection described in belt and pulley adjustment. A belt that wears out within a season on the right length points to the pulleys or bearings rather than the belt; the blower wheel and bearings guide covers what to check. Belt inspection is one line on the spring startup checklist, and the right time to buy the spare.