En­gine Au­topsy

The most ver­sa­tile and suc­cess­ful pas­sen­ger car en­gine of all time

Practical Classics (UK) - - CONTENTS -

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.


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.

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