The most versatile and successful passenger car engine of all time
We explode and explore the iconic Volkswagen flat-four.
The Chevrolet V8 might have been built in larger numbers, but it never achieved the
world domination of this air-cooled flat-four. Beetle sales numbered 21.5 million between 1938 and 2003 – and that doesn’t include the Type 2 and T25 vans, Karmann Ghia, Porsche 914 and 912, Variant models or [A] ROCKER COVERS These pressed-steel items are one of the few conventional bits of an extremely far-sighted design, but even they have a neat trick. See the W-shaped bits of black wire attached to each cylinder head? They clip over the covers, doing away with fiddly and often-overtightened fixing bolts.
[B] OIL PUMP The square item is the baseplate. The pump body is just below, with the pump gears below that. The whole assembly fixes onto the lower front portion of the crankcase, were it's driven by the nose of the camshaft. The oil strainer (below the row of bearings) fits in the round hole in the base of the crankcase, above which the oil pickup is still fitted. The sump plate (below it) fixes underneath. [C] CYLINDER HEADS The cylinder heads on this engine are the twin-port type introduced in 1967 for the Type 3 and in 1971 for the Beetle and other models. One inlet port per cylinder rather than the previous ‘siamesed’ design improved gas-flow. However, twin-port heads have the reputation of cracking more easily between the valve seats and spark plug holes. [D] CRANKCASE Cast in a light and relatively stiff alloy of aluminium and magnesium. The crankshaft runs above the camshaft and the flywheel mounts at the left-hand end as we view it. On the right of the upper half is the mounting for the dynamo or alternator. Look opposite to the lower half and the holes for the fuel pump and distributor can be seen. Go left from there to spot the place the oil cooler fixes on. [E] BARRELS AND PUSHROD TUBES The cast-iron cylinder barrels are clamped between the crankcase and cylinder heads by long studs. The pushrod tubes run beneath. Between each pair of barrels is a deflector plate that sends air from the cooling fan around the base of each barrel. They’re the only pieces of the array of cooling system ‘tinware’ that you need to fit during the engine build. [F] BEARINGS AND SUNDRIES From the left we have the flywheel gland nut, the rear crank oil seal, shims to set end-float, the rear main bearing, the centre main bearing (two halves), the first front main bearing and the narrower second one, the crankshaft timing wheel, the brass distributor drive gear and the oil thrower. The short shaft below the lower pushrod tubes is the distributor drive. Bottom left are distributor shaft shims, crankshaft circlips and a blanking plate that plugs the hole in the crankcase at the flywheel end of the camshaft.
[G] PISTONS The piston diameter looks large for the engine… and it is. The 1600cc version is oversquare like all Volkswagen flat-fours. Bore grew more than stroke as the design evolved from the original one-litre version, as fitting expanded barrels was a great deal cheaper and easier than increasing the stroke with a new crankshaft and a roomier crankcase.
[H] CON-RODS The sturdy forged steel con-rods are one factor in the engine’s famed ability to soldier on in the face of poor servicing and abuse. The big-end bearing surfaces are notably large. The gudgeon pins float in phosphor-bronze bushes, which are also generously over-specified for the engine's cubic capacity and power output.
[I] VALVES AND SPRINGS The engine has a conventional arrangement of single valve springs, valve caps and collets grouped around long-stemmed valves with inlets noticeably larger than exhausts. They’re lubricated from above by the rocker arms.
[J] ROCKER SHAFT The rocker arms pivot on a hollow rocker shaft that locates on two posts on the cylinderhead, not visible from this angle. Valve clearances are adjusted conventionally via slotted screws with locking nuts.
[K] PUSHRODS AND FOLLOWERS
The camshaft followers are a simple buckettype design that allows oil to pass into the hollow pushrods. The pushrods push up against the rockers every time the camshaft goes round and also deliver oil to the valvegear. The oil eventually finds its way back down again, making decent seals at the top and bottom of each pushrod tube important.
[L] CRANKSHAFT The hefty forged steel crankshaft is supremely tough in normal use – but there is a price to pay. The alloy crankcase casting is somewhat softer, so over time the crank batters the case and creates excessive end-float. Line-boring the empty case allows the fitting of oversize main bearings to make everything tight once more. The end-float is set at between 0.007in and 0.014in using circular shims between the rear main bearing and flywheel (shown in the line of bearings and crank sundries on the opposite page).
[M] CAMSHAFT The camshaft has less to do than in some engines, though it does drive the oil pump fitted directly in front of it. The cam lobes are tall and steep. A more modern design might extract more power with greater overlap, but here the emphasis is on low-speed torque and low-revving, un-burstable service.
[N] BEARING SHELLS The larger set at the bottom of the page are the big end shells. The smaller set above are for the camshaft. Unlike a typical in-line engine where the camshaft is poked into a gallery from the front, the Volkswagen’s split crankcase means the camshaft can be treated like a smaller crankshaft, allowing replaceable shells to be used on all of its three bearing surfaces. n
Brazilian spin-offs. The engines remain in production for the classic scene almost 80 years after launch. They’re also built for drag racing, where the design has been stretched from the 985cc and 24bhp of the original to 2400cc and 700bhp. The engine seen here is a 1600cc twin-port in standard trim.