ORDER OF BUSINESS
As with most maintenance and repair procedures, there are some basic ground rules and techniques. Follow them as closely as circumstances permit and you won’t – or shouldn’t, anyway – go far wrong.
You will, for instance, need to rotate the crankshaft through several complete revolutions. Porsche practice is to use a 19mm ring spanner on the crankshaft pulley bolt (or 17mm for the Carrera 3.2). Seen from this end of the engine, the crankshaft’s normal direction of rotation is clockwise, and the firing order is 1-6-2-4-3-5. Both this sequence and the numbering of the cylinders remain the same right the way through to the latest watercooled flat-sixes, and obviously ‘work’ regardless of which way round in the chassis the engine is mounted. Again looking from the pulley end of the unit, cylinders one, two and three are in the left-hand bank (with number one nearest you), and cylinders four, five and six in the right-hand bank (with six furthest away from you).
In order to gain an accurate picture of the operating clearance in the valvetrain it is necessary to rotate the camshafts, by turning the crankshaft as required, so that each pair of valves is fully closed. This situation occurs twice for every complete revolution of the crankshaft (the camshafts revolve at half engine speed), but what you are looking for is the point at which each relevant piston is at top dead centre, or TDC, on the compression stroke. At around the point at which the spark plug would normally be igniting the charge of fuel and air in the combustion chamber, basically.
The simplest and the most reliable way of finding this for first cylinder one and then, with minimal effort and confusion, the remaining five, is to rotate the crankshaft so that the ‘Z1’ mark on the pulley (see photo opposite) is aligned with the nearby notch on the crankcase, and then both to feel for the required lateral movement in the two rocker arms for cylinder one (which will prove that the valve stems are ultimately in contact with the base circle of the cams), and at the same time to make sure that the valves for cylinder four are ‘rocking’. One of those two valves will be at the point of closing, in other words, and the other just opening. If all of these conditions are not met, rotate the crankshaft through a further 360 degrees and try again. The beauty of doing it this way is that all you have to do thereafter is rotate the crankshaft to the positions in the numbered sequence shown on the pulley in these photos – which will amount to two complete revolutions – and the relevant pairs of cams will be placed in the correct position for checking the clearances.
Valve clearances in all of these engines should always be checked and set with the unit at ambient temperature – which means letting it stand idle for at least six to eight hours. In all cases, from the earliest 2.0-litre right through to the very last 3.6, and regardless of the motor’s specification or camshaft profile, the required clearance figure is 0.1mm for both inlet and exhaust valves. This seemingly arbitrary figure, doubtless calculated after much experimentation, is such that even after the engine has warmed up and expanded there remains enough clearance in the mechanism to allow each of the 12 valves to close fully at the required time. If they didn’t there would, fairly obviously, be a loss of combustionchamber pressure and thus efficiency, and perhaps even the danger of any affected valve and/or its seat being burned by the heat of the combustion process.
It is possible to use a conventional fan-type set of feeler blades to set your 911’s valve clearances, but for speed and convenience professionals like Nick Fulljames use a single stubby blade of the required thickness, secured in a specially shaped compact holder. Nick, who with many years’ experience prefers to set his clearances so that the blade is quite a firm sliding fit between the valve stem and the adjuster, uses the blades themselves by the dozen. Try Design 911 (www.design911.co.uk) for yours, or any of the other big, on-line engine-parts retailers.
How often you check your valve clearances is up to you. Porsche stipulates every 12,000miles for road cars, but there is no harmin doing it more often that that. And competition, trackday or even hard road use will warrantmore frequent investigation; perhaps even before every event. The nature of the workmeans that you are also likely to spot any problems that might be developing in the relevant or adjacent areas, and that can only be to your benefit.
Our thanks to Nick Fulljames for his help in the preparation of this feature.
It’s not just the valve clearances that are essential to any engine’s health. So, too, is the timing of the valves’ opening and closing. Get it badly wrong, such as when a drivechain (or rubber belt) jumps a tooth on its sprockets, and there is a danger that one or more valves will be hit by an adjacent piston. Game over. But even a minor discrepancy can have a significant effect on power, performance and fuel economy.
In most ordinary engines there is no provision for ‘organised’ adjustment, away from the standard factory setting. Put it all together in the way the manufacturer intended, by lining up all the marks provided, and it’s either right or wrong. In these air-cooled units, however, each of the two camshaft sprockets is located on its shaft by a small pin passing through any one of no fewer than 17 holes in the sprocket, and itself secured by the large ‘Belleville’ spring washer under the camshaftsprocket nut. Which means that the sprocket – and thus the camshaft – can be mounted in any one of 17 different positions relative to the crankshaft. (And here with the added complication that with two chains driving two sprockets and thus two camshafts, you have double the chance of getting it wrong.)
The Porsche-approved way of checking the valve timing is to measure inlet-valve lift for cylinders one and four – in this engine 6.2mm – against the crankshaft angle for the relevant two pistons. This is done by means of a zeroed dial gauge set against the spring retainer of the fully closed (inlet) valve for first cylinder one and then cylinder four (for the left- and righthand camshafts, respectively). You then slowly rotate the crankshaft clockwise to TDC, simultaneously checking, via the rotation of the dial-gauge needle (each complete revolution represents exactly 1.0mm), that the valve has opened the required distance. Any necessary adjustment is achieved by removing the securing pin with a special threaded extractor (a suitable screw, basically), and while holding the camshaft in the required position rocking the crankshaft back and forth through just a few degrees until the pin can be refitted in the nearest available hole, and then secured again with the Belleville washer and large nut.
Each of the 12 standard Porsche rocker shafts in these air-cooled flat-sixes is secured inside its bore in the camshaft carrier by a pair of tapered light-alloy plugs, drawn towards each other by a central M6 cap-head screw. It appears to be an almost laughably simple arrangement, but such is the wedging effect of the conical plugs – as well as the minimal lateral thrust experienced by the shafts – that in practice it works perfectly.
The required oil-sealing is provided by a pair of ‘O’-rings set into annular grooves in the rocker shaft, and thus in close contact with the inside of the bore in which the shaft is located. Eventually, though, the rocker-shaft bores can become very slightly enlarged, and such is the inherent difficulty of fitting the shafts without also damaging the ‘O’-rings as they enter the bores that, in the end, minor but persistent leaks become almost unavoidable.
Nick Fulljames is one of a number of 911 engine specialists to have developed a simple but effective solution. His kit retains the standard Porsche rocker arm and shaft, and potentially (although not in practice) even the central screw. The end-plugs, however, are substituted by direct-replacement specials, in high-strength light alloy, which are profiled to incorporate new ‘O’-rings as an integral part of the area that forms the crucial ‘wedge’. A full engine set of 24, together with 24 ‘O’-rings and (for reliability) 12 new screws, currently costs around £200 plus VAT, and could easily be fitted at the same time that you next check your valve clearances. That will add to the labour element and thus to the overall bill, of course, but a good independent should charge for no more than around two hours’ additional work.
Note, incidentally, that there is both a right and a wrong way of fitting the rocker shafts and the conical plugs’ central securing screws – although in both cases the ‘wrong’ way does still work, after a fashion. Essentially you need gently to push each shaft home – using no more than a screwdriver as a lever – until it is laterally centralised not in relation to the ends of the bore in the camshaft housing, but to the rocker arm. Likewise – although this is really relevant only if you fit them during a rebuild, when the timing-chain cases and oil-cooler have been removed – it is only sensible to install the screws (for the rocker arms relating to cylinders one, three, four and six, in other words) from such a direction that they can later be removed without any additional dismantling.
In all cases, whether you use the standard Porsche end-caps and seals, or the Redtek or any other upgrade, the cap-headed securing screws should be tightened to 9Nm. That is practically impossible, though, so Nick, albeit with the benefit of many years of experience, suggests winding them as tight as you comfortably can by hand pressure against a standard-length hex key – and for obvious precautionary reasons checking them for tightness each time you have the camshaft covers off to adjust the valves.