Identifying the Valves
Keep the valves in order, so that you know which valve (intake and exhaust) goes in which combustion chamber. If the valve faces are not full of carbon, you may number them, front to rear, with a permanent felt tip marker.
Removing the Valves and Springs
Using an appropriate valve spring compressor, compress the valve springs and lift out the keepers with needlenose pliers. Then, slowly release the compressor, and remove the valve, spring and spring retainer. A valve stem seal is used beneath the keepers which can be discarded. Check the keeper seating surfaces on the valve stem for burrs which may scratch the valve guide during installation of the valve. Remove any burrs with a fine file.
This section assumes that the cylinder head is removed for this operation. However, if it is desired to remove the valve springs with the head installed, it will be necessary to screw a compressed air adaptor into the subject spark plug hole and maintain a pressure of 85 psi to keep the valve from dropping down.
Inspect the exhaust valves closely. More often than not, the cause of low compression is a burned exhaust valve. The classic burned valve is cracked on the valve face from the edge of the seat to the stem the way you could cut a pie. Remove all carbon, gum and varnish from the valve stem with a hardwood chisel, or with a wire brush and solvent (i.e. carburetor cleaner, lacquer thinner).Hot-Tanking the Cylinder Head
Take the head(s) to an engine rebuilding or machine shop and have it (them) hot-tanked to remove grease, corrosion, carbon deposits and scale.
Make sure that the hot tanking solution is designed to clean aluminum, not to dissolve it.
After hot-tanking, inspect the combustion chambers (around the spark plug hole) and the exhaust ports for cracks. Also, check the plug threads, manifold studs, and rocker arm studs for damage and looseness.Degreasing the Remaining Cylinder Head Parts
Using solvent (i.e. Gunk® or Zep® carburetor cleaner), clean the rockers, rocker shafts, valve springs, spring retainers, keepers and the pushrods. You may also use solvent to clean the cylinder head although it will not clean as well as hot-tanking. Also clean the sheet metal shrouding at this time. Do not clean the pushrod tubes in solvent.Cleaning the Cylinder Head
Chip carbon away from the combustion chambers and exhaust ports using a chisel made of hardwood. Remove the remaining deposits with a stiff wire brush. You may also use a power brush (drill with wire attachment if you use a very light touch). Remember that you are working with a relatively soft metal (aluminum), and you do not want to grind into the metal. If you have access to a machine shop that works on aluminum heads, ask them about glass-beading the cylinder head.Checking the Valve Stem-to-Guide Clearance
Clean the valve stem with lacquer thinner or carburetor cleaner to remove all gum and varnish. Clean the valve guides using solvent and an expanding wire-type valve guide cleaner or brass bristle brush. Mount a dial indicator to the head so that the gauge pin is at a 90° angle to the valve stem, up against the edge of the valve head. Insert the valve by hand so that the stem end is flush with the end of the guide. Move the valve off its seat, and measure the clearance by rocking the stem back and forth to actuate the dial indicator. Check the figure against specifications. Maximum rock should not exceed the wear limit.
To check whether excessive rock is due to worn valve stems or guides (or both), one of two methods may be used. If a new valve is available, you may recheck the valve rock. If rock is still excessive the guide is at fault. Or, you may measure the old valve stem with a micrometer, and determine if it has passed its wear limit.
In any case, most VW and Porsche mechanics will replace the exhaust valve and guides anyway, since they often wear out inside of 50,000 miles.
VW does not make available oversize valve stems to clean up excessive valve rock. Therefore, if excessive clearance is evident, replace the guides.Knurling the Valve Guides
Knurling is a process whereby metal is displaced and raised, thereby reducing clearance. It is a procedure used in engines where the guides are shrunk in making replacement a costly procedure. Although this operation can be performed on VW and Porsche engines, it is not recommended, since the exhaust guides will eventually need replacement anyway.Replacing the Valve Guides
The valve guides are a press fit into the head.
If your replacement valve guides do not have a collar at the top, measure the distance the old guides protrude above the head.
Several different methods may be used to remove worn valve guides. One method is to press or tap the guides out of the head using a stepped drift. The problem with this method is the risk of cracking the head. Another method, which reduces this risk, is to first drill out the guide about 2 / 3 of the length of the guide so that the walls of the guide at the top are paper thin ( 1 / 32 in. or so). This relieves most of the tension from the cylinder head guide bore, but still provides a solid base at the bottom of the guide to drift out the guide from the top. A third method of removing guides is to tap threads into the guide and pull it out from the top. After tapping the guide, place an old wrist pin (or some other type of sleeve) over the guide, so that the wrist pin rests squarely on the boss on the cylinder head around the guide. Then, take a long bolt (about 4 or 5 inches long with threads running all the way up to the bolt head) and thread a nut about halfway up the bolt. Place a washer on top of the wrist pin (see illustration) and thread the bolt into the valve guide until the nut contacts the washer and wrist pin. Finally, screw the nut down against the washer and wrist pin to pull out the guide.
If you are installing the guides without the aid of a press, using only hand tools, it will help to place the new valve guides in the freezer for an hour or so, and the clean, bare cylinder head in the oven at 350-400° F for 1 / 2 hour to 45 minutes. Controlling the temperature of the metals in this manner will slightly shrink the valve guides and slightly expand the guide bore in the cylinder head, allowing easier installation and lessening the risk of cracking the head in the process.
Most replacement valve guides, other than those manufactured by VW, have a collar at the top which provides a positive stop to seat the guides in the head. However, VW guides have no such collar. Therefore, on these guides, you will have to determine the height above the cylinder head boss that the guide must extend (about 1 / 4 in.). Then, obtain a stack of washers, their inner diameter slightly larger than the outer diameter of the guide at the top of the guide. If the guide should extend 1 / 4 in., use a 1 / 4 in. thick stack of washers around the guide.
To install the valve guides in the head, use a collared drift, or a special valve guide installation tool of the proper outer diameter (see illustration).
If the replacement guide is collared, drive in the guide until it seats against the boss on the cylinder head. If the guide is not collared, drive in the guide until the installation tool butts against the stack of washers (approx. 1 / 4 in. thick) on the head.
If you do not heat the head to aid installation, use penetrating lubricant in the guide bore, instead.Resurfacing (Grinding) the Valve Face
Using a valve grinding machine, have the valves resurfaced according to specifications. The valve stem tip should also be squared and resurfaced, by placing the stem in the V-block of the grinder, and turning it while pressing lightly against the grinding wheel.
After grinding, the minimum valve head margin must be 0.50mm (.020 in.). The valve head margin is the straight surface on the edge of the valve head, parallel with the valve stem.Replacing Valve Seat Inserts
This operation is not normally performed on VW and Porsche engines due to its expense and special shrink fit of the insert in the head. Usually, if the seat is destroyed, the head is also in bad shape (i.e. cracked, or hammered from a broken valve or piston). Some high-performance engine builders will replace the inserts to accommodate larger diameter valve heads. Otherwise, the operation will usually cost more than replacement of the head. Also, a replacement insert, if not installed correctly, could come out of the head, damaging the engine.Resurfacing the Valve Seats
Most valve seats can be reconditioned by resurfacing. This is done with a reamer or grinder. First, a pilot is installed in the valve guide (a worn valve guide will allow the pilot to wobble, causing an inaccurate seat cut). When using a reamer, apply steady pressure while rotating clockwise. The seat should clean up in about four complete turns, taking care to remove only as much metal as necessary.
Never rotate a reamer counterclockwise.
When using a grinder, lift the cutting stone on and off the seat at approximately two cycles per second, until all flaws are removed.
It takes three separate cuts to recondition a VW or Porsche valve seat. After each cut, check the position of the valve seat using Prussian blue dye. First, you cut the center of the seat using a 45° cutter. Then, you cut the bottom of the seat with a 75° cutter and narrow the top of the seat with a 15° stone. Equally as important as the width of the seat is its location in relation to the valve. Using a caliper, measure the distance between the center of the valve face on both sides of a valve. Then, place the caliper on the valve seat, and check that the pointers of the caliper locate in the center of the seat.Checking Valve Seat Concentricity
In order for the valve to seat perfectly in its seat, providing a gas tight seal, the valve seat must be concentric with the valve guide. To check concentricity, coat the valve face with Prussian blue dye and install the valve in its guide. Applying light pressure, rotate the valve 1 / 4 turn in its valve seat. If the entire valve seat face becomes coated, and the valve is known to be concentric, the seat is concentric.Lapping the Valves
With accurately refaced valve seat inserts and new valves, it is not usually necessary to lap the valves. Valve lapping alone is not recommended for use as a resurfacing procedure.
Prior to lapping, invert the cylinder head, lightly lubricate the valve stem and install the valves in their respective guides. Coat the valve seats with fine Carborundum® grinding compound, and attach the lapping tool suction cup (moistened for adhesion) to the valve head. Then, rotate the tool between your palms, changing direction and lifting the tool often to prevent grooving. Lap the valve until a smooth, polished seat is evident. Finally, remove the tool and thoroughly wash away all traces of grinding compound. Make sure that no compound accumulates in the guides as rapid wear would result.Checking the Valve Springs
Place the spring on a flat surface next to a square. Measure the height of the spring and compare that value to that of the other 7 springs. All springs should be the same height. Rotate the spring against the edge of the square to measure distortion. Replace any spring that varies (in both height and distortion) more than 1 / 16 in.
If you have access to a valve spring tester, you may use the following specifications to check the springs under a load (which is the only specification VW gives).
If any doubt exists as to the condition of the springs, and a spring tester is not available, replace them, they're cheap.Installing the Valves
Lubricate the valve stems with white grease (molybdenum-disulphide), and install the valves in their respective guides. Lubricate and install the valve stem seals.
Position the valve springs on the head. The spring is positioned with the closely coiled end facing the head.
Check the valve stem keys (keepers) for burrs or scoring. The keys should be machined so that the valve may still rotate with the keys held together. Finally, install the spring retainers, compress the springs (using a valve spring compressor), and insert the keys using needlenose pliers or a special tool designed for this purpose.
You can retain the keys with wheel bearing grease during installation.Inspecting the Rocker Shafts and Rocker Arms
Remove the rocker arms, springs and washers from the rocker shaft.
Lay out the parts in the order they are removed.
Inspect the rocker arms for pitting or wear on the valve stem contact point, and check for excessive rocker arm bushing wear where the arm rides on the shaft. If the shaft is grooved noticeably, replace it. Use the following chart to check the rocker arm inner diameter and the rocker shaft outer diameter.
Minor scoring may be removed with an emery cloth. If the valve stem contact point of the rocker arm is worn, grind it smooth, removing as little metal as necessary. If it is noticed at this point that the valve stem is worn concave where it contacts the rocker arm, and it is not desired to disassemble the valve from the head, a cap (see illustration) may be installed over the stem prior to installing the rocker shaft assembly.Inspecting the Pushrods and Pushrod Tubes
After soaking the pushrods in solvent, clean out the oil passages using fine wire, then blow through them to make sure there are no obstructions. Roll each pushrod over a piece of clean, flat glass. Check for run-out. If a distinct, clicking sound is heard as the pushrod rolls, the rod is bent, necessitating replacement. All pushrods must be of equal length.
Inspect the pushrod tubes for cracks or other damage to the tube that would let oil out and dirt into the engine. The tubes are particularly susceptible to damage at the stretchable bellows. Also, the tubes must be maintained at length "a" (see illustration) which is 190-191mm or 7.4-7.52 in. If a tube is too short, it may be carefully stretched, taking care to avoid cracking. However, if the bellows are damaged or if a gritty, rusty sound occurs when stretching the tube, replace it. Always use new seals. When installing tubes rotate the tubes so that the seams face upwards.
If, on an assembled engine, it is desired to replace a damaged or leaky pushrod tube without pulling the engine, it may be accomplished using a "quick-change" pushrod tube available from several different specialty manufacturers. The special two-piece aluminum replacement tube is installed after removing the valve cover, rocker arm assembly and pushrod of the subject cylinder. The old tube is then pried loose with a screwdriver. Using new seals, the replacement tube is positioned between the head and crankcase, and expanded into place, via a pair of threaded, locking nuts.