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SERVICING CYLINDER HEADSService to the cylinder head can involve many different procedures. These procedures vary with the metal used to make the head and the design of the engine. Always refer to the appropriate service information from the manufacturer before starting any work on the head. Crack Repair: Common locations of cracks in a cylinder head include: between the spark plug bore and the valve seat, between the valve seats, around the valve guides, and in the exhaust ports. In most cases, a cracked head should be replaced. However, some cracks can be effectively repaired. Crack repair is normally done by specialty shops. It’s important to keep in mind that most cracks are caused by something other than a defect in the head. The cause of the cracking needs to be identified and corrected. No matter what caused the crack, the crack needs to be repaired if the head is reused. Crack repair is done by the cold process of pinning or the hot process of welding. Furnace Welding Crack Repairs: Furnace welding is considered the best way to repair cracks in a cast-iron head. To furnace weld, the head is first preheated in an oven. This minimizes thermal shock when the flame of a welding torch contacts the head. After the crack has been filled with metal, the head is allowed to slowly cool before it’s used. Flame Spray Welding--Flame spray (powder) welding is also used to repair cast-iron heads. This process uses nickel-based powders and a special torch to fill the crack. Repairing Aluminum Heads--Defects in aluminum heads are commonly found as:
In many cases, these problems result in head replacement. However, some heads are repairable. Tungsten inert gas (TIG) welding is the preferred way to repair aluminum heads (54). Welding aluminum is often considered difficult because it welds differently than iron or steel. When exposed to air, aluminum forms an oxide coating on the surface that helps protect the metal against corrosion. The oxide layer makes welding difficult because it interferes with fusing and weakens the weld. A TIG welder prevents the formation of the oxide by bathing the weld with inert gas (normally argon). Pinning Cracks--Pinning is commonly used to repair small accessible cracks in cast-iron and aluminum heads. Pins are used only when the metal is thick enough to secure the pins. Pinning is done with a drill, tap, and tapered or straight pins. Holes are drilled into the ends of the crack to keep it from spreading, then holes are drilled at various overlapping intervals along the length of the crack. After they are installed, the pins are peened to seal the surface. A sealant is not required when tapered pins are used; however, it must be applied over the repair area when straight pins are used. Resurfacing Cylinder Heads There are three reasons for resurfacing the deck sur face of a cylinder head: 1. To make the surface flat so that the gasket seals properly 2. To raise the compression ratio 3. To square the deck to the main bores Heads that are deformed beyond specifications must be replaced or resurfaced. The refinished surface should not be too smooth. It must be rough enough to provide "bite" but not enough to cause a poor seal and leakage. For proper head gasket seating, the finish should have shallow scratches and small projections that allow for gasket support and sealing of voids. !WARNING! Before operating any surfacing machine, be familiar with and follow all the cautions and warnings given in the machine's operation manual. Also, when operating these machines, wear safety glasses, goggles, or a face shield. Fgr__54 TIG welding an aluminum head. Belt Surfacers -- Belt surfacers resemble belt sanders. These machines are easy to set up and operate. An operator merely places the part to be surfaced on the belt. A restraint rail helps keep the part positioned (55). Some machines have air-operated holddown fixtures. Resurfacing quality depends on operator skill and factors such as belt condition, machine horsepower, and the holddown pressure applied. Milling Machines: Milling machines cut away thin layers of metal to create a level, properly finished surface (56). Cutters remove up to 0.050 inch (1.27 mm) per pass. Both rough and finish cuts are usually made to create the desired finish. Broaching Machines: Broaching machines use an underside rotary cutter or broach. A block, cylinder head, or intake manifold is held in an inverted position as the broach passes underneath. Surface Grinders: Surface grinders use a grinding wheel to remove metal stock (57). They set up and operate similarly to milling machines. Fgr__55 (A) A typical belt surfacer and (B) how it’s used. Fgr__56 Milling an aluminum cylinder head. Fgr__57 A surface grinder. Fgr__58 CC-ing a cylinder head to find the combustion volume. Stock Removal Guidelines The amount of stock removed from the head gasket surface must be limited. Excessive surfacing can lead to problems in the following areas. Compression Ratio: After resurfacing a cylinder head, the combustion chamber will be smaller. This will raise the compression ratio. How much depends on how much metal was removed from the surface and the type of chamber. A good rule of thumb is that when 0.010 inch (0.2540 mm) has been removed from the head, the compression ratio will increase by about 0.141:1 to 0.20:1. To determine if the compression ratio increased too much, it may be necessary to measure the volume of the combustion chamber. Measuring the volume is called cc-ing the head. This is done with the valves and spark plugs installed. The cylinder head is mounted upside down, and a glass or plastic plate is installed over the combustion chamber. A graduated container called a burette is used to fill the combustion chamber with thin oil. The oil is poured through a hole in the plate, as shown in Fgr__58. The amount of oil required to fill the chamber is equal to the volume (in cubic centimeters) of the chamber. The measured volume is compared to specifications. If the chamber volumes are unequal, individual chambers can be matched by grinding the valve seats to sink the valves and by grinding and polishing metal from the combustion chamber surface. Either method can be used to equalize the volumes and adjust them to specifications. On some engines, a thicker head gasket can be used to decrease the compression after the head has been resurfaced. There can be as much as 0.040-inch difference between various types and brands of gaskets. For instance, changing from a soft-faced to a steel or copper shim gasket can increase the compression ratio by as much as 0.50:1. Valve Timing: On many OHC engines, it’s necessary to restore the distance between the camshaft and crankshaft gears after the head has been resurfaced. Special shims are used to raise the cylinder head, which raises the camshaft. If 0.030 inch (0.7620 mm) was removed from the head surface, the camshaft must be moved up 0.030 inch (0.7620 mm). If this is not done, valve timing will be altered. Piston/Valve Interference: When the block or head is surfaced, the piston-to-valve clearance becomes less. To prevent the valves from making contact with the piston, a minimum of 0.070 inch piston-to-valve clearance is recommended. Misalignment Removing metal from the head or block also causes valve tips, rocker arms, and push rods to be positioned closer to the camshaft. This causes a change in rocker arm geometry and can cause hydraulic lifters to bottom out. Also, on V-type engines, the mountings for the manifolds will be lower. This can present a sealing problem with the intake manifold. Also, the ports might be mismatched and manifold bolts might not line up. In order to return the intake manifold to its original alignment, metal on the sealing surfaces of the manifold must be removed. SHOP TALK -- If one cylinder head on a V-type engine is re surfaced, the other one must be cut so equal amounts of metal are removed from each head. USING SERVICE INFORMATION Specifications for valve angles are listed in the engine specifications section of a service manual. Fgr__59 Cutter-type valve re-surfacer. Neway Manufacturing, Inc. RECONDITIONING VALVESWhenever the valves have been removed from the cylinder head, the valve heads and valve seats should be resurfaced. The most critical sealing surface in the valve train is between the face of the valve and its seat when the valve is closed. Leakage between these surfaces reduces the engine's compression and power and can lead to valve burning. To ensure proper seating of the valve, the seat area on the valve face and seat must be the correct width, at the correct location, and concentric with the guide. These are accomplished by renewing the surface of the valve face and seat. Valve grinding or re-facing is done by machining a fresh, smooth surface on the valve faces and stem tips. Valve faces suffer from burning, pitting, and wear caused by opening and closing millions of times during the life of an engine. Valve stem tips wear because of friction from the rocker arms or actuators. Valve tips are machined after the valve face is refinished. Valve Face: Valves can be refaced on either grinding or cut ting (59) machines. Although grinding machines are common, they should not be used on some late-model valves. The manufacturer may recommend cutting the valve face. The process for grinding or cutting a valve face is much the same. However, grinding uses a stone and cutting uses a hardened blade. Before using a stone to grind a valve face, it should be dressed (60). A special diamond cutting tip is passed by the stone to clean it and provide an even surface. Place the valve as deeply as possible into the machine's chuck. This eliminates stem flexing while pressure is exerted on the valve. Set the angle of the stone or cutting bit to the desired angle. SHOP TALK -- Many manufacturers recommend that the angle of the valve seat and face have an interference angle of 1 degree. This provides a good seal through a wedging effect. The 1-degree difference can be made at the valve face or seat, or split between the two. Take light cuts (61) using the full width of the grinding wheel. Make sure coolant is flowing over the contact point between the valve face and the grinding wheel. Remove only enough metal to clean up the valve face. A knifelike edge will heat up and burn easily or might cause preignition (62). The width of the edge of a valve head between the top of the valve and the edge of the face is the valve mar gin. As a general rule, it’s not advisable to grind a valve face to a point where the margin is reduced by more than 25% or to where it’s less than 0.045 inch (1.143 mm) on the exhaust and 0.030 inch (0.7620 mm) on the intake valves. After grinding, check valve head runout. Use a dial indicator on the valve margin and rotate the valve while it’s still in the chuck. Valve runout should not exceed 0.002 inch (0.0508 mm) TIR. The face should not show any chatter marks or unground areas. After grinding, examine the valve face for cracks. Sometimes fine cracks are visible only after grinding. Sometimes they occur during grinding due to inadequate coolant flow or excessive wheel pressure. CAUTION! Always wear eye protection when operating any type of grinding equipment. Fgr__60 Before grinding a valve, the stone should be dressed. Fgr__61 Take light cuts on the valve using the full width of the grinding wheel. Fgr__62 A sharp edge on the valve face is not recommended. CORRECT Fgr__63 Measuring valve stem installed height. Valve stem installed height; Spring seat Valve Stem: Removing metal from the valve face and/or seat will set the valve deeper into the port. As a result, more of the valve stem extends from the other side of the head. If the stem height is greater than that specified by the manufacturer, the valve stem tip must be ground down to bring the overall height of the stem back into specs. In some cases, if the stem height is excessive, the valve and/or valve seat must be replaced. Valve stem installed height is measured from the valve spring pad on the cylinder head to the top of the valve (63). When the installed height is excessive, valve train geometry can be thrown off or there can be valve lash problems. The valve tip is ground so that it’s exactly square with the stem. Because valve tips have hardened surfaces up to 0.030 inch (0.7620 mm) in depth, only 0.010 inch (0.2540 mm) can be removed during grinding. If more than 0.010 inch (0.2540 mm) is removed from the tip, the valve must be replaced. Follow the manufacturer's specifications for the allowable limits. After the tip has been ground, the edges should be chamfered. This is done by setting the valve at a specified angle to the grinding or cutting tool. The valve is then rotated until the entire edge is smooth. === Next: part 6 Prev.: part 4 Home Article Index top of page |
