There are two basic types of cylinder heads used on these vehicles. They are both an Overhead Camshaft (OHC) design, that can also be broken down into two subgroups: the Single Overhead Camshaft (SOHC) and the Dual Overhead Camshaft (DOHC) cylinder head. This abbreviation describes how many camshafts are found in each cylinder head. Although the 1996 Audi A4 model's engine is referred to as a SOHC engine, the engine has two camshafts because it is a V6 design engine that uses two cylinder heads, one for each bank of 3 cylinders.
Most cylinder heads these days are made of an aluminum alloy due to its light weight, durability and heat transfer qualities. However, cast iron was the material of choice in the past, and is still used on many vehicles today.
Whether made from aluminum or iron, all cylinder heads have valves and seats. Some use two valves per cylinder, while the more hi-tech engines will utilize a multi-valve configuration using 3, 4 and even 5 valves per cylinder as with the 1.8L engine and the 2.8L 5V DOHC V6 engines. When the valve contacts the seat, it does so on precision-machined surfaces, which seals the combustion chamber and allows the valve to dissipate the heat from the combustion chamber.
All cylinder heads have a valve guide for each valve. The guide centers the valve to the seat and allows it to move up and down within it. The clearance between the valve and guide can be critical. Too much clearance and the engine may consume oil, lose vacuum and/or damage the seat. Too little, and the valve can stick in the guide causing the engine to run poorly if at all, and possibly causing severe damage.
The last component all cylinder heads have are valve springs. The spring holds the valve against its seat. It also returns the valve to this position when the valve train or camshaft has opened the valve. The spring is fastened to the valve by a retainer and valve locks (sometimes called keepers). Aluminum heads will also have a valve spring shim to keep the spring from wearing away the aluminum.
An ideal method of rebuilding the cylinder head would involve replacing all of the valves, guides, seats, springs, etc. with new ones. However, depending on how the engine was maintained, often this is not necessary.
A major cause of valve, guide and seat wear is an improperly tuned engine. An engine that is running too rich, will often wash the lubricating oil out of the guide with gasoline, causing it to wear rapidly. Conversely, an engine which is running too lean will place higher combustion temperatures on the valves and seats allowing them to wear or even burn.
Springs fall victim to the driving habits of the individual. A driver who often runs the engine rpm to the redline will wear out or break the springs faster then one that stays well below it.
Unfortunately, mileage takes it toll on all of the parts. Generally, the valves, guides, springs and seats in a cylinder head can be machined and re-used, saving you money. However, if a valve is burnt, it may be wise to replace all of the valves, since they were all operating in the same environment. The same goes for any other component on the cylinder head. Think of it as an insurance policy against future problems related to that component.
Unfortunately, the only way to find out which components need replacing, is to disassemble and carefully check each piece. After the cylinder head(s) are disassembled, thoroughly clean all of the components.
Cylinder Head Overhaul
The cylinder head assembly is a precision component that should be handled with extreme care. The following points MUST be adhered to.
- Clean the area around the cylinder head-to-block mating surface thoroughly before removal
- Place the engine at Top Dead Center (TDC) on the compression stroke for cylinder No. 1
- Matchmark the following timing marks and components:
- Camshaft sprocket(s).
- Crankshaft sprocket and/or crankshaft pulley and/or flywheel.
- If equipped, the intermediate and/or balance shaft sprocket(s) and shaft(s).
- If equipped, the distributor rotor location and if removed, the distributor drive gear.
Match-marking the timing marks on the applicable engine components ensures proper assembly and retains proper balance of the engine.
Cleanliness and organization are an absolute must! For additional information, please refer to the following topic(s): Engine Reconditioning.Assembly
The first step for any assembly job is to have a clean area in which to work. Next, thoroughly clean all of the parts and components that are to be assembled. Finally, place all of the components onto a suitable workspace and, if necessary, arrange the parts to their respective positions.Cup Type Camshaft Followers
To install the springs, retainers and valve locks on heads which have these components recessed into the camshaft follower's bore, you will need a small screwdriver-type tool, some clean white grease and a lot of patience. You will also need the C-clamp style spring compressor and the OHC tool used to disassemble the head.
- Lightly lubricate the valve stems and insert all of the valves into the cylinder head. If possible, maintain their original locations.
- If equipped, install any valve spring shims that were removed.
- If equipped, install the new valve seals, keeping the following in mind:
- Place the valve spring and retainer over the stem.
- Position the spring compressor and the OHC tool, and then compress the spring.
- Using a small screwdriver as a spatula, fill the valve stem side of the lock with white grease. Use the excess grease on the screwdriver to fasten the lock to the driver.
- Carefully install the valve lock, which is stuck to the end of the screwdriver, to the valve stem then press on it with the screwdriver until the grease squeezes out. The valve lock should now be stuck to the stem.
- Repeat Steps 6 and 7 for the remaining valve lock.
- Relieve the spring pressure slowly and insure that neither valve lock becomes dislodged by the retainer.
- Remove the spring compressor tool.
- Repeat Steps 2 through 10 until all of the springs have been installed.
- Install the followers, camshaft(s) and any other components that were removed for disassembly.
There are several things to check on the cylinder head: valve guides, seats, cylinder head surface flatness, cracks and physical damage.Checking The Cylinder Head For Warpage (Flatness)
After you have cleaned the gasket surface of the cylinder head of any old gasket material, check the head for flatness.
Place a straightedge across the gasket surface. Using feeler gauges, determine the clearance at the center of the straightedge and across the cylinder head at several points. Check along the centerline and diagonally on the head surface. If the warpage exceeds 0.003 inch (0.076mm) within a 6.0 inch (15.2cm) span, or 0.006 inch (0.152mm) over the total length of the head, the cylinder head must be resurfaced. After resurfacing the heads of a V-type engine, the intake manifold flange surface should be checked, and if necessary, milled proportionally to allow for the change in its mounting position.Cracks & Physical Damage
Generally, cracks are limited to the combustion chamber, however, it is not uncommon for the head to crack in a spark plug hole, port, outside of the head or in the valve spring/rocker arm area. The first area to inspect is always the hottest: the exhaust seat/port area.
A visual inspection should be performed, but just because you don't see a crack does not mean it is not there. Some more reliable methods for inspecting for cracks include Magnaflux®, a magnetic process or Zyglo®, a dye penetrant. Magnaflux® is used only on ferrous metal (cast iron) heads. Zyglo® uses a spray on fluorescent mixture along with a black light to reveal the cracks. It is strongly recommended to have your cylinder head checked professionally for cracks, especially if the engine was known to have overheated and/or leaked or consumed coolant. Contact a local shop for availability and pricing of these services.
Physical damage is usually very evident. For example, a broken mounting ear from dropping the head or a bent or broken stud and/or bolt. All of these defects should be fixed or, if unrepairable, the head should be replaced.Valve Guides
Now that you know the valves are good, you can use them to check the guides, although a new valve, if available, is preferred.
- Before you measure anything, look at the guides carefully and inspect them for any cracks, chips or breakage. Also if the guide is a removable style (as in most aluminum heads), check them for any looseness or evidence of movement. All of the guides should appear to be at the same height from the spring seat. If any seem lower (or higher) from another, the guide has moved.
- Mount a dial indicator onto the spring side of the cylinder head. Lightly oil the valve stem and insert it into the cylinder head. Position the dial indicator against the valve stem near the tip and zero the gauge.
- Grasp the valve stem and wiggle towards and away from the dial indicator and observe the readings. Mount the dial indicator 90° from the initial point and zero the gauge and again take a reading. Compare the two readings for a out of round condition.
- Check the readings against the specifications given. An Inside Diameter (I.D.) gauge designed for valve guides will give you an accurate valve guide bore measurement. If the I.D. gauge is used, compare the readings with the specifications given. Any guides that fail these inspections should be replaced or machined.
A visual inspection of the valve seats should show a slightly worn and pitted surface where the valve face contacts the seat. Inspect the seat carefully for severe pitting or cracks. Also, a seat that is badly worn will be recessed into the cylinder head. A severely worn or recessed seat may need to be replaced. All cracked seats must be replaced. A seat concentricity gauge, if available, should be used to check the seat run-out. If run-out exceeds specifications the seat must be machined (if no specification is given use 0.002 inch or 0.051mm).Disassembly
Whether it is a single- or dual-overhead camshaft cylinder head, the disassembly procedure is relatively unchanged. One aspect to pay attention to is careful labeling of the parts on the dual camshaft cylinder head. There will be an intake camshaft and followers as well as an exhaust camshaft and followers and they must be labeled as such. In some cases, the components are identical and could easily be installed incorrectly. Do NOT MIX THEM UP! Determining which is which is very simple; the intake camshaft and components are on the same side of the head as was the intake manifold. Conversely, the exhaust camshaft and components are on the same side of the head as was the exhaust manifold.
Inspect the camshaft(s) and followers as described earlier in this section.Cup Type Camshaft Followers
The cylinder heads used in the models covered by this manual use hydraulic lifters that rest on top of the cup type camshaft followers and have the valve spring, retainer and locks recessed within the follower's bore. You will need a C-clamp style valve spring compressor tool, an OHC spring removal tool (or equivalent) and a small magnet to disassemble the head.
- If not already removed, remove the camshaft(s) and/or followers. Mark their positions for assembly.
- Position the cylinder head to allow use of a C-clamp style valve spring compressor tool.
- With the OHC spring removal adapter tool positioned inside of the follower bore, compress the valve spring using the C-clamp style valve spring compressor.
- Remove the valve locks. A small magnetic tool or screwdriver will aid in removal.
- Release the compressor tool and remove the spring assembly.
- Withdraw the valve from the cylinder head.
- If equipped, remove the valve seal.
- If equipped, remove the valve spring shim. A small magnetic tool or screwdriver will aid in removal.
- Repeat Steps 3 through 8 until all of the valves have been removed.
Cylinder Head Flatness
Certain cracks can be repaired in both cast iron and aluminum heads. For cast iron, a tapered threaded insert is installed along the length of the crack. Aluminum can also use the tapered inserts; however, welding is the preferred method. Some physical damage can be repaired through brazing or welding. Contact a machine shop to get expert advice for your particular dilemma.Refinishing & Repairing
Many of the procedures given for refinishing and repairing the cylinder head components must be performed by a machine shop. Certain steps, if the inspected part is not worn, can be performed yourself inexpensively. However, you spent a lot of time and effort so far, why risk trying to save a couple bucks if you might have to do it all over again-Cylinder Head
Most refinishing procedures dealing with the cylinder head must be performed by a machine shop. Read the sections below and review your inspection data to determine whether or not machining is necessary.Cylinder Head Surface
If the cylinder head is warped, it must be machined flat. If the warpage is extremely severe, the head may need to be replaced. In some instances, it may be possible to straighten a warped head enough to allow machining. In either case, contact a professional machine shop for service.
Unless the valve guides need machining or replacing, the only service to perform is to thoroughly clean them of any dirt or oil residue.
There are only two types of valve guides used on automobile engines: the replaceable-type (all aluminum heads) and the cast-in integral-type (most cast iron heads). There are four recommended methods for repairing worn guides.
Knurling is a process in which metal is displaced and raised, thereby reducing clearance, giving a true center, and providing oil control. It is the least expensive way of repairing the valve guides. However, it is not necessarily the best, and in some cases, a knurled valve guide will not stand up for more than a short time. It requires a special knurlizer and precision reaming tools to obtain proper clearances. It would not be cost effective to purchase these tools, unless you plan on rebuilding several of the same cylinder head.
Installing a guide insert involves machining the guide to accept a bronze insert. One style is the coil-type that is installed into a threaded guide. Another is the thin-walled insert where the guide is reamed oversize to accept a split-sleeve insert. After the insert is installed, a special tool is then run through the guide to expand the insert, locking it to the guide. The insert is then reamed to the standard size for proper valve clearance.
Reaming for oversize valves restores normal clearances and provides a true valve seat. Most cast-in type guides can be reamed to accept a valve with an oversize stem. The cost factor for this can become quite high, as you will need to purchase the reamer and new, oversize stem valves for all guides that were reamed. Oversizes are generally 0.003 to 0.030 inch (0.076 to 0.762mm), with 0.015 inch (0.381mm) being the most common.
To replace cast-in type valve guides, they must be drilled out, then reamed to accept replacement guides. This must be done on a fixture which will allow centering and leveling off of the original valve seat or guide, otherwise a serious guide-to-seat misalignment may occur making it impossible to properly machine the seat.
Replaceable-type guides are pressed into the cylinder head. A hammer and a stepped drift or punch may be used to install and remove the guides. Before removing the guides, measure the protrusion on the spring side of the head and record it for installation. Use the stepped drift to hammer out the old guide from the combustion chamber side of the head. When installing, determine whether or not the guide also seals a water jacket in the head, and if it does, use the recommended sealing agent. If there is no water jacket, grease the valve guide and its bore. Use the stepped drift, and hammer the new guide into the cylinder head from the spring side of the cylinder head. A stack of washers the same thickness as the measured protrusion may help the installation process.Valve Seats
If the seats are in good condition, the valves can be lapped to the seats, and the cylinder head assembled. See the valves section for instructions on lapping.
If the valve seats are worn, cracked or damaged, they must be serviced by a machine shop. The valve seat must be perfectly centered to the valve guide, which requires very accurate machining.Springs, Retainers & Valve Locks
There is no repair or refinishing possible with the springs, retainers and valve locks. If they are found to be worn or defective, they must be replaced with new (or known good) parts.Valves
Any valves that were not replaced should be refaced and the tips ground flat. Unless you have access to a valve-grinding machine, this should be done by a machine shop. If the valves are in extremely good condition, as well as the valve seats and guides, they may be lapped in without performing machine work.
It is a recommended practice to lap the valves even after machine work has been performed and/or new valves have been purchased. This insures a positive seal between the valve and seat.Lapping The Valves
- Invert the cylinder head.
- Lightly lubricate the valve stems and insert them into the cylinder head in their numbered order.
- Raise the valve from the seat and apply a small amount of fine lapping compound to the seat.
- Moisten the suction head of a hand-lapping tool and attach it to the head of the valve.
- Rotate the tool between the palms of both hands, changing the position of the valve on the valve seat and lifting the tool often to prevent grooving.
- Lap the valve until a smooth, polished circle is evident on the valve and seat.
- Remove the tool and the valve. Wipe away all traces of the grinding compound and store the valve to maintain its lapped location.