The M20 six-cylinder runs its camshaft off a toothed rubber belt, and that belt is a wear item with a clock on it. It is cheap, it is hidden behind the radiator and three accessory belts, and it gets ignored for a hundred thousand miles because the car runs fine right up until the belt does not. Replacing it is the highest-value afternoon you can spend under the hood of one of these cars. It is also a real job: budget a full day the first time, because everything in front of the belt has to come off to reach it.
This guide covers the M20 "eta" and "i" engines in the 325e, 325es, 325i, and 325is. If you have a four-cylinder 318, you have a timing chain and this is not your job.
Read This First: Safety
Work on a cold engine, and respect the cooling and air-conditioning systems. This job starts by draining the coolant, so let the engine sit overnight. Opening a hot system sprays scalding coolant. If your car has air conditioning, the compressor gets unbolted and moved aside, but never unscrew a refrigerant hose from it. Trapped refrigerant under pressure can blind you or frostbite your skin. You only move the compressor, you do not open it.
One more, and it is the one that matters most. Once the belt is off, do not turn the camshaft or the crankshaft. They are set to specific marks, and rotating either one with the belt removed, or running the engine even slightly out of time, can let a valve and a piston meet. The M20 is widely regarded as a non-interference engine, so a belt that simply snaps usually means a tow rather than bent valves. That is cold comfort and the exact reason these belts get neglected. Treat the timing as sacred anyway, because a mistake on assembly is a different and more expensive failure than a clean break.
Before You Start
This is a six-cylinder job only. The M20 in the 325e/es/i/is uses the belt. The four-cylinder 318i and 318is use a maintenance-free timing chain, and the M3 uses a chain as well. If you are not sure which you have, count the spark plug wires: six means you are in the right place.
Do the whole front of the engine while you are in there. You have already paid the labor to get this deep, so the smart money replaces everything the belt touches at the same time. That means a new belt tensioner pulley, a new water pump (it sits directly behind the tensioner and is a known leaker), a new thermostat, fresh accessory belts, and the camshaft seal and front O-ring while they are exposed. None of it will ever be this easy to reach again.
Buy the right tensioner and belt. BMW revised the tensioner pulley and belt over the M20's life, and the early versions are gone. Only fit parts carrying the "Z 127" marking. If you are replacing an old-style belt, replace the tensioner pulley with a Z 127 unit too, they are a matched pair.
Check for the sprocket recall. Six-cylinder engines built between July and September 1986 left the factory with a camshaft sprocket that can crack. A sprocket stamped "w + P 286" must be replaced. If the work was done, there is a blue paint dot on the thermostat housing and a punched "14" tag on the driver's door pillar. If you cannot confirm it, have the sprocket inspected before you trust the new belt to it.
Know your crank hub before you start. Early cars have a two-piece crankshaft hub that must come off to slide the belt on, and its center bolt is torqued to around 410 Nm (297 ft-lb), which takes a long breaker bar and a way to hold the engine still. Later one-piece hubs leave enough clearance to slide the belt over without removing the hub. Figure out which you have so you are not surprised.
What You're Replacing
You are renewing the belt that times the engine, plus the parts around it that are foolish to leave old. The exploded view shows how the belt wraps the crankshaft, intermediate shaft, and camshaft sprockets and rides against the tensioner.
| Part | Notes |
|---|---|
| Camshaft drive belt (timing belt) | The toothed rubber belt. Must be marked Z 127. Never reuse a belt that has been run, it overstretches. |
| Belt tensioner pulley | Spring-loaded pulley that tensions the belt. Replace it with the belt; must be Z 127. Replace if it is noisy, rough, or has play. |
| Water pump (coolant pump) | Sits right behind the tensioner. A classic E30 leak point and the reason to do this now. |
| Thermostat | Cheap, exposed, and old. Replace it while the system is drained. |
| Camshaft seal and front O-ring | Right there once the sprocket is off. A small leak here drips onto the new belt and ruins it. |
| Accessory drive belts (alternator, A/C, power steering) | Have to come off anyway. Fit new ones. |
| Coolant and gasket set | The system is drained; refill with fresh coolant and use new gaskets where called for. |
What You'll Need
| Part | OEM part no. | Notes |
|---|---|---|
| Camshaft drive belt (Z 127) | 11 31 1 711 081 | The timing belt itself. Must be marked Z 127. |
| Belt tensioner pulley (Z 127) | 11 31 1 711 154 | Replace with the belt. Spring 11 31 1 267 746 and pin 11 31 1 280 960 if worn. |
| Water pump and gasket | varies by supplier | A known E30 leak point. Buy a spare gasket. |
| Thermostat and gasket | varies (80 or 88 deg C) | Cheap and exposed while the system is drained. |
| Camshaft seal and front O-ring | 11 12 1 284 154 (seal), 11 12 1 265 087 (O-ring) | Cheap insurance against an oil-soaked new belt. |
| Accessory belts (3) | 11 51 1 711 091, 64 55 1 722 991, 32 42 1 706 597 | Alternator/water pump, air conditioning, power steering. |
| Coolant | n/a | Enough to refill the system. |
The belt (11 31 1 711 081) fits every M20 E30. The tensioner pulley was revised over the M20's run, so fit only a Z 127 unit, and confirm the tensioner, water pump, and thermostat numbers against RealOEM or FCP Euro for your exact engine before ordering.
| Tool | For |
|---|---|
| 32 mm wrench, thin (fan clutch) | The fan clutch nut. It is left-hand thread, see Step 1 |
| Long 12 mm socket | Reaching the bolts behind the lower pulley and damper |
| Breaker bar and heavy 1/2 in drive | The crank hub bolt is enormous if you have the two-piece hub |
| Torque wrench (small range, Nm) | The tensioner and cover bolts are low torque and matter |
| Crankshaft holding tool or helper | Keeping the engine from turning while you break the hub bolt |
| Penetrating oil | The fan clutch nut and anything else seized |
| Drain pan, fresh coolant | Catching and replacing the coolant |
| Paint pen or marker | Marking your own timing reference before you start |
| Zip bags and a marker | Labeling bolts and hoses so they go back where they came from |
Step 1: Drain the Coolant and Get the Radiator Out
With the engine cold, drain the radiator and the engine block, then remove the cooling fan and the radiator. You can technically do this belt without pulling the radiator, but everyone who has done it both ways pulls the radiator, it turns a knuckle-busting job into a reachable one.
The fan clutch is where first-timers get stuck. The big nut behind the fan is left-hand threaded. Standing in front of the car looking back, you loosen it by turning the 32 mm wrench clockwise. Hold the pulley still with a screwdriver wedged between its bolts and it comes free. Force it the normal direction and you will only tighten it.
Step 2: Strip the Front of the Engine
Pull the distributor cap and rotor and set them aside, labeling the plug wires so they go back in order. Take off the three accessory belts (alternator, air conditioning, power steering). Remove the upper timing belt cover (two bolts) and the engine hoisting bracket behind it, then the upper alternator bracket.
Label everything. The single best habit on this job is bagging and tagging each bolt as it comes out, because they are different lengths and you will not remember in six hours.
Step 3: Find Top Dead Center and Mark It
This is the reference the whole job hangs on. Put a socket on the center crankshaft (vibration damper) bolt and rotate the engine clockwise until the No. 1 cylinder is at Top Dead Center on the compression stroke. You will know you are there when two marks line up at once:
- The "OT" mark on the vibration damper aligns with the notch on the lower timing cover.
- The arrow on the camshaft sprocket aligns with the cast mark on the cylinder head.
When both agree, you are at TDC. Make your own paint marks across each pair while you are here, sprocket-to-head and damper-to-cover, so you have an unmistakable reference to come back to. Depending on the model you may need to unclip a TDC sensor (325e/es) or unbolt the reference sensor (325i/is) to get clearance.
Step 4: Remove the Lower Pulley, Damper, and Old Belt
Remove the lower timing cover bolt and cover. Then remove the six bolts holding the lower drive pulley and vibration damper to the front of the crankshaft and pull both off, prying gently if needed.
CAUTION: Only the early two-piece crankshaft hub has to come off to free the belt, and its center bolt is torqued to roughly 410 Nm (297 ft-lb). Hold the engine still with a proper holding tool before you lean on that bolt. If you have the later one-piece hub, leave it alone, the belt slides over it.
Now relieve the belt. Loosen the tensioner pulley's upper bolt and the lower nut at the base of its stud, then use a long screwdriver to rotate the tensioner pulley counterclockwise (toward the passenger side) as far as it will go and snug the upper bolt to hold it there. Peel the old belt off the sprockets and tensioner by hand.
CAUTION: Do not pry the belt off with tools, and once it is off, do not let the camshaft or crankshaft rotate. They are sitting on your timing marks and they need to stay there.
Step 5: Replace the Tensioner, Water Pump, and Seals
With the belt off, inspect the tensioner pulley: if it catches, feels rough, is noisy, or has any side-to-side or in-and-out play, it is done. Replace it with a Z 127 unit, and honestly, replace it regardless while you are this deep.
This is the moment for the water pump and thermostat. The pump is directly behind the tensioner, and the tensioner spring pin actually seats into a notch on the pump, so it is right in your hands. A weeping pump on a fresh belt is a heartbreak, so fit a new one now. If the camshaft seal or the front O-ring shows any sign of seeping, replace them too, because oil on the new belt will destroy it.
Step 6: Fit the New Belt
First, confirm one more time that both timing marks are still aligned, sprocket-to-head and damper-to-cover. Then install the new belt starting at the crankshaft sprocket and work it on counterclockwise, slipping it over the intermediate shaft sprocket, the camshaft sprocket, and the tensioner pulley.
Do not force it on with a screwdriver or pry bar. The forum trick that works: wrap the belt fully around the lower sprocket first and leave it half-off the upper one, which gives you the slack to walk it onto the cam sprocket by hand. If it fights you, the belt is still loose on the crank sprocket, reseat it there.
Step 7: Tension the Belt and Verify the Timing
With the belt seated, slowly loosen the tensioner's upper bolt and let the spring pull the pulley against the belt. The spring sets the tension, you do not set it by hand, so just let it do its job.
Now the step the shade-tree writeups skip, and the one that saves the engine. Put the socket back on the crankshaft bolt and rotate the engine clockwise through two complete revolutions, a full 720 degrees, by hand until the timing marks come back around. Both marks must line up exactly again.
CAUTION: If the marks do not come back perfectly aligned, the belt jumped a tooth. Release the tensioner, peel the belt off the camshaft gear, and reseat it. You are allowed to nudge the cam gear by hand one or two teeth either way to dial it in. Do not start the engine until both marks are dead-on after the two rotations.
Once the timing is confirmed, torque the tensioner mounting bolts, upper first then lower, to 22 Nm (16 ft-lb).
Step 8: Reassemble and Set the Torques
Work back out in reverse. The numbers that matter:
| Fastener | Torque |
|---|---|
| Tensioner mounting bolts (upper then lower) | 22 Nm (16 ft-lb) |
| Vibration damper bolts (the six) | 22 Nm (16 ft-lb) |
| Timing cover M6 nuts and bolts (small) | 9 to 11 Nm (7 to 8 ft-lb) |
| Timing cover M8 nuts and bolts (large) | 22 Nm (16 ft-lb) |
| Reference sensor mounting bolt | 7 Nm (5 ft-lb) |
| Crankshaft hub center bolt (two-piece hub) | 410 Nm (297 ft-lb) |
Refit the covers, the brackets, the sensors, the distributor cap and rotor with the wires in their marked order, and the three accessory belts. Adjust the accessory belt tension. Refill the cooling system with fresh coolant and bleed it. Last, do what the factory asks and what your future self will thank you for: stick a label in the engine bay with the date and the odometer reading, so the next person knows when the belt went in.
Step 9: First Start and Final Check
Start the engine and let it reach temperature while you watch the cooling system bleed and check for leaks at the water pump and hoses. It should idle smoothly. A rough idle, a backfire, or a no-start almost always means the timing is off by a tooth, so shut it down and recheck the marks before you go chasing anything else. Confirm the temperature gauge settles where it should and that there are no drips, then take it easy on the first drive.
Troubleshooting
The new belt will not go onto the camshaft sprocket. It is too tight at the crank, not too short. Reseat it fully on the crankshaft sprocket first, wrap it around the tensioner side, and walk it onto the cam sprocket last. Never pry it on.
After two rotations the marks do not line up. The belt is a tooth off. This is exactly why you turn it by hand and check before starting. Release the tensioner, lift the belt off the cam gear, and reseat it, nudging the cam gear a tooth as needed. Do not start it until both marks are perfect.
It will not start, or it backfires. Timing. Either the belt is off a tooth or the distributor and plug wires went back wrong. Recheck the marks at TDC, then confirm the rotor points at the No. 1 plug wire and that the wires follow firing order.
The fan clutch nut will not budge. It is left-hand thread. From the front of the car, clockwise loosens it. If you have been cranking it counterclockwise you have been tightening it the whole time.
Coolant is weeping after the job. Almost always the water pump gasket or a hose clamp, not the pump casting. This is why you scrape every trace of the old gasket off the block and use a fresh one. Retorque the pump and check the clamps.
A Note on Doing It Right
A timing belt is the cheapest insurance on the car and the most expensive thing to get wrong. The belt, tensioner, and water pump together cost less than one tow and a bad afternoon. Take the extra ten minutes to make your own paint marks, to turn the engine two full revolutions by hand, and to confirm the timing twice before the key ever goes in. Rushed, this job bites. Done patiently, it is one of the most satisfying jobs on an M20 and it buys you years.
Wrap-Up
You renewed the one belt that keeps the valves and pistons out of each other's way, replaced the water pump and tensioner that would have been the next thing to strand you, and you set it all to the factory marks and torques. Log the date and mileage, keep the cooling system honest, and this engine will spin happily to the next interval. For the rest of the chassis, see our BMW E30 Torque Specifications guide.
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