There are two basic types of cylinder heads used on today-s automobiles: the Overhead Valve (OHV) and the Overhead Camshaft (OHC). The latter can also be broken down into two subgroups: the Single Overhead Camshaft (SOHC) and the Dual Overhead Camshaft (DOHC). Generally, if there is only a single camshaft on a head, it is just referred to as an OHC head. Also, an engine with an OHV cylinder head is also known as a pushrod engine.
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. When the valve contacts the seat, it does so on precision machined surfaces, which seals 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 has been opened by the valve train or camshaft. 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 heads are disassembled, thoroughly clean all of the components.
- Install the cylinder heads using new gaskets.
- Install the timing sprockets and the belt.
Cylinder Head Overhaul
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 work space and, if necessary, arrange the parts to their respective positions.
- Apply a small amount of new engine oil to the valve stem and insert the valve into the cylinder head. If possible, install the valve in the original location.
- Install the inner and outer valve spring seats.
- Coat the new valve stem seal with new engine oil. If installing an intake valve seal, a special seal installation tool or a small deep socket should be used. Place the seal onto the valve and over the valve guide. Place the tool on top of the seal and lightly tap it with a hammer until the seal snaps over the guide. The lower edge of the seal should be 0.091 inch (2.3mm) from the outer valve spring seat. If installing an exhaust valve seal, install it by hand over the valve guide.
- Position the inner and outer valve springs and the retainer on the cylinder head. Be sure that the outer valve spring is installed with the narrowly spaced coils closest to the cylinder head.
- Compress the valve spring using the C-clamp style valve spring compressor.
- Install the valve locks.
- Slowly release the spring compressor, being careful not to dislodge the valve locks.
- Repeat Steps 1 through 7 until all of the valves have been installed.
- Install the camshaft and new camshaft seal.
- If removed during disassembly, install the rocker arm shaft support and lifter assembly, and the rocker arm shaft and rocker arm assemblies.
If removed during disassembly, install the exhaust manifold.
- If not already removed, remove the exhaust manifold from the cylinder head.
- If not already removed, remove the rocker arm shaft and rocker arm assemblies, and the rocker arm shaft support and lifter assembly.
- Remove the camshaft.
Position the cylinder head to allow use of a C-clamp style valve spring compressor tool.
NOTEIt is preferred to position the cylinder head gasket surface facing you with the valve springs facing the opposite direction and the head laying horizontally.
Compress the valve spring using the C-clamp style valve spring compressor.
NOTEDue to engine varnish, the retainer may stick to the valve locks. A gentle tap with a hammer may help to break it loose.
- Remove the valve locks. A small magnetic tool or screwdriver will aid in removal.
- Release the compressor tool and remove the spring retainer and springs.
- Withdraw the valve from the cylinder head.
Remove the valve seal.
NOTESpecial valve seal removal tools are available. Regular or needlenose type pliers, if used with care, will work just as well.
Remove the inner and outer valve spring seats. A small magnetic tool or screwdriver will aid in removal.
NOTECylinder heads that have seen a lot of miles and/or abuse may have mushroomed the valve lock grove and/or tip, causing difficulty in removal of the valve. If this has happened, use a metal file to carefully remove the high spots around the lock grooves and/or tip. Only file it enough to allow removal.
- Remove the valve from the cylinder head.
Repeat Steps 5 through 11 until all of the valves have been removed.
There are several things to check on the cylinder head: valve guides, seats, cylinder head surface flatness, cracks and physical damage.
Now that all of the cylinder head components are clean, it-s time to inspect them for wear and/or damage. To accurately inspect them, you will need some specialized tools:
If you do not have access to the proper tools, you may want to bring the components to a shop that does.Cracks And 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.Cylinder Head 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 in. (0.076mm) within a 6.0 in. (15.2cm) span, or 0.006 in. (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.
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.Valve Seats
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 in. or 0.051mm).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 which 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 an 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 which were reamed. Oversizes are generally 0.003 to 0.030 in. (0.076 to 0.762mm), with 0.015 in. (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.
WARNINGDo not get the valves out of order after they have been lapped. They must be put back with the same valve seat with which they were lapped.
Removal & Installation
The factory specifies that the cylinder head bolts ARE NOT to be reused. Obtain the proper replacement parts before beginning this procedure. Check carefully that all bolts are removed before attempting to remove a cylinder head. A tab, part of the head, contains 1 lightly tightened head bolt that is external to the valve cover. Do not overlook this "hidden" bolt or the head will be damaged.
- Before servicing the vehicle, refer to the Precautions section.
- Properly relieve the fuel system pressure.
- Drain the coolant.
Remove or disconnect the following:
Negative battery cable
- Clean all parts well.
- Inspect the cylinder head for damage, cracks and leakage of water and oil. If necessary, replace the head. Check the head gasket surface for burrs and nicks. If the head is cracked, it must be replaced.
Install the exhaust manifold on the cylinder head.
- Position a new head gasket and the cylinder head on the block. Examine the head bolt washers. Note that the washers have a chamfer or bevel on one side. The beveled side should face "up" when installed. Examine the new replacement head bolts. There are different lengths. The head bolts in positions 4, 7, 9 and 12 are 5.00 inches (127mm) long and the rest are 4.17 inches (106mm) long. Be sure the new cylinder head bolts are installed in the correct positions.
Tighten the new head bolts in the following sequence:
- First pass: cylinder head bolts to 22 ft. lbs. (29 Nm).
- Second pass: cylinder head bolts to 43 ft. lbs. (59 Nm).
- Third pass: Loosen all of the cylinder head bolts completely.
- Fourth pass: cylinder head bolts to 7 ft. lbs. (10 Nm).
- Fifth pass: intake manifold bolts and nuts to 2.9 ft. lbs. (4 Nm).
- Sixth pass: intake manifold bolts and nuts to 13 ft. lbs. (18 Nm).
- Seventh pass: intake manifold bolts and nuts to 12-14 ft. lbs. (16-20 Nm).
- Eight pass: Loosen all of the intake manifold bolts and nuts completely.
- Ninth pass: cylinder head bolts to 22 ft. lbs. (29 Nm).
- Tenth pass: cylinder head bolts to 40-47 ft. lbs. (54-64 Nm).
- Eleventh pass: cylinder head sub-bolts to 6.7-8.7 ft. lbs. (9-12 Nm).
- Twelfth pass: intake manifold bolts and nuts to 2.9 ft. lbs. (4 Nm).
- Thirteenth pass: intake manifold bolts and nuts to 6.5 ft. lbs. (9 Nm).
Fourteenth pass: intake manifold bolts and nuts to 6-7 ft. lbs. (8-10 Nm).
- Fill the cooling system. An oil and filter change is recommended.
- Start the vehicle and check for leaks. Check the ignition timing and adjust as required.
The factory specifies that the cylinder head bolts ARE NOT to be reused. Obtain the proper replacement parts before beginning this procedure. Check carefully that all bolts are removed before attempting to remove a cylinder head. A tab, part of the head, contains one head bolt that is external to the rocker arm cover. Do not overlook this "hidden" bolt or the head will be damaged.
- Remove the lower intake manifold.
- Remove the timing belt.
- Remove the distributor.
- Disconnect the A/C compressor electrical connectors and position the wiring harness aside.
- Remove the spark plugs.
- Raise and safely support the front of the vehicle securely on jackstands.
- Remove the engine and transaxle inner splash shield.
- If removing the left cylinder head, remove the radiator.
- Remove the bolt from the transaxle dipstick tube and position the tube aside.
- Remove the three exhaust manifold heat shield bolts and remove the shield.
- Remove the three left exhaust manifold-to-right exhaust manifold nuts.
- If removing the left cylinder head, remove the two exhaust manifold-to-inlet pipe nuts.
- Lower the vehicle.
If removing the left cylinder head, remove the three left exhaust manifold heat shield bolts, position the engine oil dipstick tube aside, and remove the heat shield.
WARNINGDo not rotate the crankshaft or camshafts with the timing belt removed. Damage to the valves and/or pistons may result.
- Remove both camshaft sprockets. A special tool is recommended to hold the camshaft sprockets in position while removing the sprocket bolts, however, if the special tool is not available, use the following procedure. Wrap a rag around the outer edge of the sprocket. Grip the rag and sprocket with a large pair of water pump pliers, being careful not to damage the sprocket. Remove the sprocket bolt.
- Remove the four timing belt rear cover bolts from the cylinder heads and remove the cover.
- Remove the two right exhaust manifold-to-engine lifting eye bracket bolts and remove the bracket.
- Disconnect the heater core hose from the coolant crossover tube.
- Remove the four bolts from the coolant crossover tube and remove the tube and lower radiator hose.
If removing the left cylinder head:
Remove the four A/C compressor bolts and position it aside, securing it with wire.
If removing the right cylinder head:
Disconnect the vacuum hoses and electrical connectors from the bracket on the head and position them aside.
- Remove the rocker arm cover of the head that is to be removed.
- Remove the rocker arm shafts and rocker arms.
Remove the rocker arm shaft supports and lifters.
NOTEPlease note that there is a "tab" on one end of each cylinder head. This tab contains a head bolt. Be sure that this is the first bolt in the removal sequence. This bolt is easily forgotten or overlooked and the tab can be broken off if the cylinder head is moved prior to the bolt being removed. Also, note that the head bolts are not to be reused.
- Loosen the head bolts in sequence, in two steps. Remove the head bolt washers and discard the head bolts.
- Remove the right cylinder head and exhaust manifold assembly, and the left cylinder head, and discard the gaskets.
- If removing the right exhaust manifold from the right cylinder head, please refer to Exhaust Manifold, Removal and Installation for the proper loosening sequence.
- The installation is the reverse of removal, but please note the following important steps.
- The cylinder heads should be cleaned and inspected before installation. Please refer to Engine Reconditioning, Cylinder Head for the procedure.
- Use all new gaskets.
Note that the head bolt washers have a chamfer or bevel on one side. The beveled side should face up when installed. Examine the new replacement head bolts. There are different lengths. The head bolts in positions 4, 7, 9 and 12 are 5.00 inches (127mm) long and the rest are 4.17 inches (106mm) long. Be sure the new cylinder head bolts are installed in the correct positions. When installing the cylinder heads, the cylinder head bolts and the lower intake manifold bolts and nuts must be tightened in the following order:
- Tighten all head bolts, in sequence, to 21 ft. lbs. (29 Nm).
- Tighten all head bolts, in sequence, to 44 ft. lbs. (59 Nm).
- Loosen all of the bolts completely, following the loosening sequence.
- Tighten all head bolts, in sequence to 89 inch lbs. (10 Nm).
- Install the coolant crossover tube. Tighten the thermostat housing mounting bolts to 18 ft. lbs. (24 Nm). Tighten the crossover tube bracket bolts to 15 ft. lbs. (21 Nm).
- Position the new lower intake manifold gaskets on the cylinder heads.
- Position the lower intake manifold on the engine.
- Install the bolts and nuts, and tighten them in sequence to 35 inch lbs. (4 Nm).
- Continue tightening the lower intake manifold bolts and nuts, in sequence, to 13 ft. lbs. (18 Nm).
- Retighten the lower intake manifold bolts and nuts, in sequence, to 13 ft. lbs. (18 Nm).
- Loosen all of the lower intake manifold bolts and nuts in the reverse order of the tightening sequence.
- Tighten all head bolts, in sequence, to 21 ft. lbs. (29 Nm).
- Tighten all head bolts, in sequence, to 44 ft. lbs. (59 Nm).
- Install the -tab- head bolt, and tighten it to 89 inch lbs. (10 Nm).
- Tighten the lower intake manifold bolts and nuts, in sequence, to 35 inch lbs. (4 Nm).
- Continue tightening the lower intake manifold bolts and nuts, in sequence, to 80 inch lbs. (9 Nm).
- Retighten the lower intake manifold bolts and nuts, in sequence, to 80 inch lbs. (9 Nm).
- When installing the exhaust manifolds, refer to Exhaust Manifold, Removal and Installation for the torque and tightening sequence.
- When installing the A/C compressor mounting bracket bolts, tighten them to 32-43 ft. lbs. (43-58 Nm).
- When installing the A/C compressor bolts, tighten them to 13-22 ft. lbs. (22-29 Nm).
- When installing the exhaust inlet pipe, tighten the nuts to 44-52 ft. lbs. (60-70 Nm).
- When installing the rear timing belt cover, tighten the bolts to 26-43 inch lbs. (3-5 Nm).
- When installing the fuel rail bolts, tighten them to 8-11 ft. lbs. (11-15 Nm).
- After installation, fill the cooling system. An oil and filter change is recommended.
- Run the engine and check for leaks.
Check the ignition timing and adjust, if needed.