Backhoe Loader (JCB) and Excavator Fan Belt: Size, Number, Tension and Failure Signs

A JCB fan belt should give about 10 mm (3/8 inch) when you press it with your thumb halfway between two pulleys. That is the figure JCB’s own operator handbooks give for backhoe loaders with an adjustable alternator, and a belt that gives much more than that is slipping.

The fan belt is the rubber belt at the front of the engine. It takes power from the crankshaft and turns the cooling fan and the alternator, and on many engines the water pump as well. When it slips or snaps, the engine runs hot and the battery stops charging at the same time. This guide covers the belt on backhoe loaders (which most people in India call a JCB, whatever the brand) and on excavators: what kind of belt it is, how to read the number printed on it, how tight it should be, how often to check it and the signs that it needs changing. It gives no prices, because a belt’s price changes with its size and the seller.

What does the fan belt do on a JCB?

It drives the parts of the engine that cannot drive themselves. A pulley bolted to the front of the crankshaft turns the belt, and the belt turns the pulleys of the other parts. On a backhoe loader those are usually the cooling fan and the alternator. JCB’s 3CX and 4CX quick reference guide calls it the “fan/alternator drive belt” for that reason.

The fan sits in front of the radiator and pulls air through it. Wikipedia’s article on the fan clutch describes the common layout: the fan is fixed to the front of the water pump, which is driven by a belt from the crankshaft. On such an engine one belt keeps three things going: the air through the radiator, the coolant round the engine and the charge into the battery.

Machines with air conditioning often carry a second, separate belt for the AC compressor. The same JCB guide lists it as its own weekly check.

A fan belt, a drive belt and an alternator belt are often the same belt, called by whichever part the speaker is worried about. Check where it runs before you order one.

Does every machine use a belt to turn the fan?

No. On backhoe loaders such as the JCB 3CX and 4CX the belt turns the fan, which is why JCB names it the fan/alternator belt. Many larger machines turn it with a small hydraulic motor, which lets the fan run only as fast as the heat needs and, on some, run backwards for a few seconds to blow dust out of the radiator. On those machines the belt still turns the alternator, but a slow fan points to the hydraulic drive, not the belt.

Here is how some machines listed in India describe their fan drive, as written in each machine’s own feature description:

If your machine is not in this list, open the engine cover with the engine off and follow the fan back. A pulley behind the fan with a belt running over it means a belt drive. Hydraulic hoses running to a small motor behind the fan mean a hydraulic drive.

Is a JCB fan belt a V-belt or a ribbed belt?

It depends on the engine. Older backhoe loader engines use a V-belt: a single belt with a cross-section shaped like a V cut short at the bottom, running in a V-shaped groove in each pulley. Under load it wedges deeper into the groove, which is why it grips. Many newer engines use a ribbed belt, also called a poly-V or multi-V belt: a flat, thin belt with several small V-shaped ribs side by side along its length.

You can tell them apart in a few seconds. A V-belt is thick and narrow, roughly as tall as it is wide. A ribbed belt is thin and wide, with a row of fine ridges on its inner face and a smooth back.

The rib range and the way a ribbed belt can run over a pulley on its smooth back come from the Wikipedia article on mechanical belts, which also notes that a ribbed belt bends more easily than a V-belt for the same driving surface, so it wastes less energy. Wikipedia’s article on the serpentine belt adds the catch: lose that one belt and the water pump, the alternator and anything else on it stop together, so the engine overheats quickly.

How do you read the number on a fan belt?

A fan belt carries two different numbers, and people mix them up. One is the size code printed or moulded on the back of the belt by its maker. The other is the machine maker’s part number, which appears in the parts book and on genuine packaging. The size code tells you what the belt is. The part number tells you what the machine maker fitted.

On a ribbed belt the size code is usually written rib count, profile, length. Wikipedia’s example is 6PK1880: 6 ribs, the PK rib profile, 1,880 mm long. The same belt may be marked in inches in older American style, as 6K740 or 740K6: 74 inches long, 6 ribs.

On a V-belt the code gives the section and the length. Sections have letters. In the classical series, Z is 10 mm wide at the top, A is 13 mm, B is 17 mm and C is 22 mm. The narrow series (SPZ, SPA, SPB, SPC) has the same top widths but deeper belts. Some belts sold for engines are marked with the top width in mm and the length, such as 13 x 1150, which usually means a belt 13 mm wide at the top and about 1,150 mm long.

Figure 1: Reading a ribbed belt’s size code

• Yellow, 6: the number of ribs. Count them on the old belt to check.
• Blue, PK: the rib profile, which sets the spacing of the ribs. It must match the grooves in the pulleys.
• Green, 1880: the length in millimetres.

Length is the number that trips people up. A belt can be measured along its inside, its outside or a line in between (the datum or pitch line), and the three figures differ by a few centimetres. Two belts with different numbers can be the same belt measured different ways, and two belts with the same number from different makers can differ. Match the code exactly as written, or take the old belt along.

People search for things like “Hyundai 210 fan belt number” or “3DX fan belt size”. Spec sheets do not list the belt: the specifications of the Hyundai R210 Smart Plus, SANY SY210C-9, Tata Hitachi Zaxis 220LC Ultra and JCB 3DX list the engine but not the belt. The number you need is on the old belt, in the machine’s parts book, or with the dealer, who will look it up from the machine’s serial number.

How tight should a JCB fan belt be?

On a backhoe loader with an adjustable alternator, tight enough that it gives about 10 mm (3/8 inch) at the middle of its longest free run. The JCB 4CX operator handbook and the JCB 2CX operator handbook (publication 9801/8800) both give that figure and the same method:

1. Park on firm level ground, apply the parking brake, put the transmission in neutral, raise and block the loader arms, lower the backhoe and stop the engine.
2. Disconnect the battery so the engine cannot be started while your hands are near the belt.
3. Loosen the alternator’s two pivot bolts and the two bolts on its adjustment link.
4. Move the alternator outward until the belt gives 10 mm at the middle.
5. Tighten the adjustment link bolts first, then the pivot bolts.
6. If you fitted a new belt, check the tension again after the first 20 hours of work. New belts stretch and bed into the grooves.

Too loose and the belt slips: the fan turns slowly, the alternator undercharges and the belt glazes. Too tight is also a fault. Wikipedia’s article on mechanical belts puts it this way: the right tension is the lowest that does not slip under full load, because more tension adds stress to the belt and the bearings. An over-tight belt wears out the alternator and water pump bearings, as the guide to the JCB self motor and alternator explains.

If your engine has an automatic tensioner, a spring-loaded arm with a pulley pressing on the belt, there is nothing to set. The spring holds the tension. A belt that is loose on such an engine is worn or stretched, or the tensioner itself is weak. Many tensioners carry marks on the casting that show when the belt has stretched past its limit, and the operator manual for your machine shows where they are. Do not lever the tensioner tighter or pack it out.

How often should you check the fan belt?

JCB’s quick reference guide for the 3CX and 4CX puts “fan/alternator drive belt condition” and “A/C compressor belt condition” under the weekly checks, at every 50 hours. Its daily list does not include the belt. Both operator handbooks also list fan belt tension and condition on their service schedules, and the 2CX handbook adds a line for hot weather: check the fan belt regularly.

For most Indian sites that means a look every week, and a quicker one in May and June, when the cooling system is working hardest and a slack belt shows up first as a climbing temperature gauge. The fuller list of what to check at each interval is in the service schedule guide.

To check it, stop the engine and take out the key. Then:

• Press the belt at the middle of its longest run and feel how far it gives.
• Twist it a little and look at the inner face and the sides with a torch.
• Look at the pulleys for oil, rust or a chipped edge.
• On an automatic tensioner, check the arm sits within its marks and does not shake when the engine is running (watch from a safe distance, hands clear).

What are the signs of a bad fan belt?

The most common sign is a squeal from the front of the engine, loudest at start-up or when you rev it suddenly. That is the belt slipping on a pulley. Other signs build up slowly, and the table sorts them by what you see or hear.

The last row but one is the one to remember. A temperature gauge climbing and a battery charging light coming on at the same time almost always means the belt, because it is the one part both systems share. The steps for a machine that is running hot are in the guide to excavator engine overheating, and what each dashboard light means is in the guide to JCB warning lights.

A slipping fan belt can also look like a radiator fault. A radiator packed with dust and a slow fan give the same high reading, so check the belt and clean the cooling pack before paying for radiator work.

What should you match when you buy a new fan belt?

Take the old belt with you if you can, and match these points:

• The type: V-belt or ribbed. They are not interchangeable.
• The section (A, B, SPA and so on) for a V-belt, or the rib count and profile (6PK, 8PK) for a ribbed belt.
• The length, written the same way as on the old belt.
• Whether the V-belt is plain or cogged. A cogged belt has notches cut across its underside so it bends round small pulleys more easily.

Before fitting it, spin each pulley by hand with the belt off. A pulley that grinds, wobbles or sticks will ruin the new belt in days. An alternator pulley with play points to worn alternator bearings, and the battery will tell you about it soon after.

Never prise a new belt over the edge of a pulley with a screwdriver. It cuts the cords inside the belt, and the belt fails early with no warning. Slacken the alternator or tensioner and slip it on.

What decides the price of a fan belt?

Size comes first, because a longer belt, a wider section or more ribs all mean more material. A belt bought as a kit with a new tensioner and idler pulley costs more again, but on an engine with an automatic tensioner and many hours behind it, the kit is often the better buy.

This guide gives no figure, because belt prices change with size, maker and seller. Ask the dealer or a parts shop for a quote with the belt’s size code or the machine’s part number in hand.

If an older machine keeps throwing belts and overheating, compare the cost of continued repairs with a newer machine. Browse backhoe loaders such as the JCB 3DX and JCB 4DX, or excavators such as the Tata Hitachi EX 210 Infra, and speak to a dealer. Other wearing parts are covered in the parts and running costs guides.

Where the facts in this guide come from

The tension figure and the adjustment steps come from the JCB 4CX operator handbook and the JCB 2CX operator handbook (publication 9801/8800), which give the same 10 mm figure and the same 20-hour recheck. The weekly check comes from JCB’s 3CX and 4CX quick reference guide. Belt types, the size code example, the V-belt section widths and the advice on tension come from Wikipedia’s articles on mechanical belts, the serpentine belt and the fan clutch. Fan drive types come from each machine’s own feature description, read in October 2026.

The 10 mm figure applies to machines with an adjustable alternator. Machines with an automatic tensioner, and other makes, follow their own operator manuals, which should always be checked first.

Prices, specifications and features are indicative, vary by variant, location and date, and should always be confirmed with the official OEM or authorised dealer before any purchase decision. DesiMachines is not liable for decisions taken on the basis of information that may have changed after publication.