Workshop appointments by arrangement
The engine in detail
A flat-six, drawn in section and in motion. First how it runs, then where it breaks, and finally how we strip it, measure it and build it back up. Scroll — the drawing moves with you.
The drawing shows a flat-six engine, schematic. Chain, oil and bearings follow the same rules at BMW, Mercedes-Benz and Audi.
One tap takes you straight to the right place.
Act I
The boxer layout and its four strokes — so that the failures make sense later.
Straight to Act II — where it breaks01
Each pair of pistons runs exactly opposite each other: both move outwards at the same time and back in again. Their inertia forces largely cancel out — that is why a boxer runs so smoothly.
Every 120° of crank rotation one of the six cylinders fires, in the order 1–6–2–4–3–5. In this section you see cylinder 1 on the right and cylinder 4 on the left; they fire exactly one revolution apart. The crankshaft in the middle turns the pistons' back-and-forth into rotation.
02
Now you turn it. Scrolling turns the crankshaft — two revolutions for one working cycle of cylinder 1.
01Intake
The piston moves inwards with the intake valve open: air and fuel flow in.
02Compression
Both valves are closed and the piston squeezes the mixture.
03Power
Just before top dead centre the plug fires; the pressure drives the piston inwards.
04Exhaust
The exhaust valve is open and the piston pushes the exhaust gas out.
The camshafts turn at half crankshaft speed, so every valve opens once in two revolutions. If valves, rings or head gasket do not seal, this is exactly where pressure escapes — which is what the compression and leak-down tests in the diagnosis measure.
Act II
Seven assemblies, each with its typical failure, the measurement we take and the service that puts it right.
Straight to Act III — how we rebuild it03
The gasket that leaks most often — and under it the camshaft.
The camshaft opens the valves via the rocker arms; the valve cover keeps the oil above them inside the head.
04
Valves, seats and guides — this is where the engine keeps its compression or loses it.
The head holds the combustion chamber, the valves, their seats and guides. As long as the valves close tightly, the engine keeps its compression.
05
Bore, piston and rings — where the oil consumption comes from.
The piston runs in the cylinder bore; its rings seal the combustion chamber from the crankcase and scrape the oil off the wall.
Drawn as an air-cooled flat-six. On the water-cooled 996 and 997 the cylinder looks different — the damage to the bore is the same.
06
The backbone: it carries the crankshaft and keeps everything in line.
The split case carries the crankshaft in its main bearings and keeps the cylinders and crank assembly in line.
07
The most expensive part in the engine, and the first to take offence at oil starvation.
The connecting rods carry the pistons' force to the crankshaft. It does not run on its bearing shells but on a film of oil thinner than a hair.
Worn bearings give the rod play: at every firing stroke it knocks — that is the knock under load. When oil pressure falls the film thins until it breaks; then steel runs on bearing metal.
Knocking under load, oil pressure dropping as it warms, bronze debris on the magnetic plug — do not drive on from here.
08
It keeps the crank and the camshafts in step — once it stretches, it rattles.
The chain drives the camshafts at a ratio of 2 : 1: after two turns of the crankshaft the red marks line up again.
Every link wears a little; together the chain grows longer. The tensioner takes up the slack until it runs out of travel — then the camshafts lag behind and the valves open late.
Many tensioners work on engine oil pressure. In the first seconds after a cold start that pressure is missing and the chain slaps: the rattle.
The principle applies to every chain-driven engine — on the BMW N47 and N57 the chain drive sits on the gearbox side.
A cold-start rattle that lasts longer every week — do not drive far on it. A chain that jumps costs the engine.
BMW N47: rattling on a cold startPrices for the N47 and N57
09
No oil pressure, no bearings — the pump decides everything else.
The pump draws oil from the sump or the oil tank and forces it through the filter and galleries to every bearing, to the camshafts and to the chain tensioner.
Warm oil is thinner and the pressure drops — that is normal. With worn pump rotors it drops too far, and the bearings suffer first.
The pump is drawn schematically; where it sits differs from engine to engine.
Oil pressure light on: switch the engine off at once and do not drive on, not even to the nearest workshop.
Act III
Diagnosis, strip-down, measurement, assembly and test run — in that order.
To the next step10
Only once the picture is clear do we talk about scope and cost.
Fault memory, compression and leak-down tests, borescope inspection of the cylinders, oil and filter checked for wear debris.
Engine and gearbox come out of the car and onto the engine stand.
The strip is complete: crankcase, crank assembly and cylinder heads end up in front of us as cleaned individual parts.
11
Every figure is written down and compared against the manufacturer's specification.
Crankshaft and connecting rods for size and runout, bearing clearances, cylinder bores, flatness of the mating faces, valve guides and valve seats.
| Check point | Spec | Actual |
|---|---|---|
| Main bearing clearance | 0,020 – 0,072 mm | measured on the engine |
| Big-end clearance | 0,028 – 0,072 mm | measured on the engine |
| Runout, centre | ≤ 0,040 mm | measured on the engine |
| Piston-to-bore clearance | 0,02 – 0,05 mm | measured on the engine |
| Bore out-of-round | ≤ 0,01 mm | measured on the engine |
| Valve guide clearance | 0,03 – 0,08 mm | measured on the engine |
| Chain elongation | ≤ 0,6 % | measured on the engine |
| Oil pressure at idle, warm | ≥ 0,5 bar | measured on the engine |
Every spec figure is taken from the manufacturer's documentation, never estimated
12
What goes in is what the measurements call for, not what happens to be on the shelf.
Bores honed, mating faces skimmed, valve seats recut, plus bearings, piston rings, chains, tensioners and gaskets renewed as complete sets. Assembly in clean surroundings, every fastener to torque and angle, the clearances checked a second time before anything is closed up.
Once refitted, oil pressure and leak checks, warm-up and a road test.
You receive the documentation of the work and a clear word on the running-in period, including the first oil change.
Next step
Describe in a few steps what you hear and see, and since when. The more precise the description, the more useful the first assessment.
Describe the faultMake, model, year — and every price for your car is there: rebuild, timing chain, oil service and more.
All pricesEverything done to this engine, each described on its own page.
Fault reported, cause to be found
Rattling on a cold start, swarf in the filter, a crunch going into gear — the more precise the description, the more useful the first assessment.