One of the most important procedures in servicing your air-cooled Porsche engine is to check and, if necessary, adjust the valve clearances. In the 993 the task is handled automatically, by special oil-pressurised rocker arms that eliminate unwanted backlash in the valvetrain. In all previous versions, however, it means that at roughly 12,000mile intervals you will need to remove the four camshaft covers and, using a feeler blade, measure the air gap, at a precise point in the engine’s operating cycle, between the top of each valve stem and the hardened pad on the end of the adjacent rocker arm.
To the uninitiated, or perhaps to those
more accustomed to lower-maintenance ‘modern’ engines with hydraulic cam followers, it probably appears to be a long-winded, low-tech and even confusing process. You need, for instance, slowly (and accurately) to turn the crankshaft in its normal direction of rotation, or DOR, through a set sequence in order to place the six individual cams on each shaft in the correct checking position for each of the three pairs of valves per cylinder bank. But like many things 911, when you get right down to it the process is both surprisingly simple and logical – and immensely satisfying. You will need adequate – and safe – access to the engine from beneath the rear of the vehicle (assuming that it’s still in the car, of course). You will also require a set of new camshaftcover gaskets, together with the special aluminium sealing washers and self-locking nuts. Try to make do with the original components and you will invariably suffer from annoying oil leaks. But that apart – and, if required, the time-honoured manipulation of screwdriver, spanner and appropriate feeler blade, which can take a
bit of getting used to – the fact is that it is the proverbial piece of cake.
Well-known 911 specialist Nick Fulljames kindly showed us the process on an engine nearing the end of a full rebuild, and suggested also running through the basics of another apparently black art – correctly setting the valve timing. Certainly that requires a little more in the way of specialised equipment, in the form of a dial gauge to measure the total valve lift for cylinder one (left-hand bank) and then cylinder four (right-hand bank) at the prescribed crankshaft angles, and it is not something that as a DIYER you will necessarily ever want – or need – to do. As with so many of these how-to stories, though, it is often as useful to have an idea of what others are doing on your behalf – and with your money – as it is to be able to undertake the work yourself. PW
With two banks of three cylinders each, it’s all too easy to tie yourself up in knots when checking and adjusting the aircooled engines’ valve clearances, but follow the same long-established routine used by the professionals and you shouldn’t go far wrong. Key is to rotate the crankshaft pulley in its normal direction, such that each piston in turn is placed at top dead centre on the compression stroke, and thus with both valves fully closed. Only then can you gain a true picture of the air gap between the base circle of each cam lobe and the relevant rocker arm
The most efficient way to check all 12 valve clearances is to rotate pulley clockwise in sequence shown. Digits only just visible here – 1, 6 and 2 on outer ‘circle’, then 4, 3 and 5 on the inner – relate to standard firing order. Line them up in turn with the matching mark at 12 o’clock on the engine, and you should be spot-on. With crank in the required position it should be possible to feel slight up-anddown play in the end of the rocker arm, or to slide it sideways on its shaft. If not, it may mean that clearances are way too tight – or that you haven’t set crank correctly. Adjustment process is the same as for any other engine – albeit much easier with a specially shaped small feeler blades on a ‘hook’. Check fit of blade again after you have tightened the adjuster’s locknut. Use new camshaftcover gaskets, plus sealing washers and nuts. Refitting the old parts is always false economy. Upper (inlet) and lower (exhaust) valve covers are slightly different, and so too the gaskets. This engine was at the end of a rebuild, so all components were spotless, but usually all you will need to do is give the covers a quick rinse. Check for damage, though: scratches or gouges on joint faces, cracks, and perhaps even distortion from previous overtightening. Don’t forget the washers. Porsche torque figure for valve-cover nuts is 8Nm, but winding them on with a speed brace and giving them a final modest tweak is sufficient. Ribbed, black cover is heavier-duty item for the exhaust valves. Note that the gaskets are fitted ‘dry’, with no sealant. If you need that to stop oil leaks, then you need to look for other problems that might lie behind them
With the pin removed the crankshaft will now be free to rock backwards and forwards until you can align TDC mark on the pulley with the one on the engine. The camshaft won’t move, though: it will be effectively ‘locked’ by the tension of the valve springs. The locating pin should now slide into the appropriate hole. If not, choose the next best: that will be more than accurate enough. Replace the ‘Belleville’ spring washer to prevent the pin falling out, and then the large nut on the end of the camshaft. You will need one of these special crow’s foot spanners to tighten (and first to loosen) the camshaft end-nut to the required figure of 120Nm. There is no workable alternative, but they are available from tool manufacturers and many specialist Porsche suppliers. Also needed, of course, is a socket to hold the end of the camshaft steady while you tighten that nut. That, too, is an aircooled 911 ‘special’, but again readily available. Don’t be tempted to cut corners on this task. If either of the two large nuts ever loosens it will be game over
Valve timing is set by adjusting crankshaft’s position relative to fully open inlet valves for cylinders one and four – and those points, for the leftand then right-hand cylinder banks, respectively, are established by using a dial gauge against the valve-spring retaining collar. Each camshaft chainwheel is ultimately secured by a large nut and washer, but its position is first determined by a tiny threaded pin passing through any one of 17 different holes in the sprocket. If it is necessary to adjust the crank angle in relation to maximum valve lift, first you have to extract the pin with a suitably threaded rod
Felt-pen mark on this engine’s right-hand camshaft sprocket shows that despite being trial-assembled by the book, in practice the valve timing was out by no fewer than two ‘holes’ – there can be few better examples to show why the system is necessary, and thus provided. Crank movement needed to line up second hole was relatively small. Interestingly, 993 engines have no pin to lock sprocket to camshaft, relying instead on the clamping force provided by the nut alone. It works well enough, but can still slip, especially if the engine is driven hard, so regular checking is absolutely essential