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Valve, Guide and Seat Repair Options
By Larry Carley
Valve, guide and seat repairs are the cornerstones of rebuilding cylinder heads. It makes no difference if the head is a 350 Chevy or one with dual overhead cams and four-valves-per-cylinder.
The basics of refinishing the valves, guides and seats are essentially the same. The newer high tech heads will usually have smaller parts, the added complexity of one or two overhead cams, cam followers, and possibly recessed spring cavities. But once you’re past the disassembly stage, the repair options and machine work boil down to a few basic choices.
One choice you have to make is what to do with the original valves. Should they be replaced or reconditioned? Valves can stretch and fatigue with age; especially exhaust valves because they run so much hotter than intake valves. So there is always some risk in reusing valves. But new valves are expensive, and in today’s highly competitive market not everyone can afford the luxury of new valves.
If the original valves are in reasonably good condition with minimal stem wear, and the application does not involve unusually high engine speeds or loads, the old valves can often be reconditioned by cleaning, inspecting for cracks, and regrinding the valve face and stem tip.
If the stems are worn, reconditioning will require grinding the stems undersize and installing guide repair liners with smaller inside diameters, or chrome plating the valves to build up the metal, then regrinding and polishing the valve stems for use with standard or oversized guides.
Valves that are damaged, too badly worn to be reconditioned or are too risky to reuse, like exhaust valves in many diesel and high output engines, must obviously be replaced, and there are a variety of options from which to choose here, too.
There are new valves with standard and oversized stems, and rechromed valves with standard and oversized stems. Valves with oversized stems are usually the choice if the original guides are being reamed and polished to compensate for wear. But if the guides are being replaced or repaired using some type of liner, then standard sized stems may be all that’s needed to restore proper clearances.
One supplier of rechromed valves said, "One of the biggest changes in this industry today is that rebuilders are realizing their niche isn’t in salvaging small parts like valves, it’s building engines and salvaging the large high dollar parts. Rechromed valves can be very price competitive with new valves, and save rebuilders the cost and hassle of reconditioning their own valves."
Any valve that might be reused should be carefully inspected after it has been cleaned. Valves that have cupped heads, show evidence of stretching (narrowing of the stem neck just above the head), are pitted, burned, cracked, have worn keeper grooves, little or no margin left on the head, a mushroomed stem tip or a bent stem must be replaced. Stem diameter should also be measured with a micrometer and compared to specs. Many valve stems are ground with about .001" taper so measure the stem at several places for an accurate indication of wear.
If a valve has to be replaced, always replace it with one of comparable or better material. In other words, always replace stainless steel exhaust valves with the same, never a lesser grade of steel (stainless is nonmagnetic). Some intake valves in turbo, diesel and high output engines are also stainless and should be replaced with the same.
Hard-faced valves in diesel applications with stellite or other materials must likewise be replaced with the same otherwise the valves may not hold up. The same goes for valves with sodium-filled stems. Sodium helps carry heat away from the valve head to keep the head from burning. If not replaced with the same type of valve, durability will suffer.
If a valve is reusable, the face can be ground to restore the surfaces. The angle of the valve face will depend on the application, but for most it will be 45°. If the application calls for an interference fit, the valve may be ground to 44° for use with a 45° seat (or 45° and 46° respectively). A 1/32" margin should be left above the valve face after it has been ground. Less margin increases the risk of overheating, valve burning and pre-ignition/detonation.
The tip of the valve stem must also be ground to compensate for the change in installed height that occurs when the seat and face are refaced. A common mistake that’s often made here is sinking the valves too deeply into the head. This restricts breathing and changes upper valvetrain geometry.
To maintain the correct geometry between stem and rocker arm, the installed height (measured between the spring seat and stem tip) should be within specifications. Removing a little metal from the tip of the stem should restore the proper height, but on some valves with case hardened tips only about .010" of metal can be safely removed.
If the proper installed height can’t be achieved without exceeding the limit for grinding the tip of the stem, the valve is too deep in the head and a new seat needs to be installed to restore the correct geometry.
Here’s another tip: be sure to grind a new 1/32" chamfer around the tip of the stem. Be sure to direct coolant at the stem tip while grinding to avoid overheating the stem. Note: do not use a water-based cooling lubricant when grinding sodium filled valve stems. If the valve is cracked, the sodium inside can react explosively if it comes into contact with water.
On some applications, the amount of metal that can be ground off the valve stem tip is limited by how far the stem protrudes above the spring retainer. If too much metal is removed, the rocker arm or cam follower may contact the retainer (300 Fords, for example). One solution here is to install special keepers with repositioned grooves. The grooves may be located .030" to .060" higher than normal on the keeper, which lowers the relative position of the spring retainer.
This not only allows more of the valve stem to protrude above the retainer, but also eliminates the need for a spring shim to maintain normal spring pressure. Spring shims are normally required when refacing the valves and seats and increases the installed height of the retainer.
By the time most rebuilders see a cylinder head, the guides will show a certain amount of guide wear. The new powder metal guides that are used in some late model aluminum heads are much harder than cast iron or bronze, and resist wear fairly well. Even so, even the hardest guides will show some wear at high mileages. Powder metal guides also tend to be brittle and can be difficult to ream or reline.
Severe guide wear may be normal in very high mileage applications, but more often it indicates one of three kinds of valvetrain problems: inadequate lubrication, improper valve geometry or wrong valve stem-to-guide clearance (too much or too little).
Inadequate lubrication may be caused by oil starvation in the upper valvetrain due to low oil pressure, obstructed oil passages, push rods, etc. In adequate guide lubrication can also be caused by using the wrong type of valve seal. Insufficient lubrication results in stem scuffing, rapid stem and guide wear, possible valve sticking and ultimately valve failure due to poor seating and overheating.
Geometry problems include the wrong installed valve height, and off-square springs, rocker arm tappet screws or rocker arms that push the valve sideways every time it opens. This causes uneven guide wear, leaving an egg-shaped hole. The wear leads to increased stem-to-guide clearance, poor valve seating and premature valve failure.
The amount of clearance between the guide and valve stem is also critical. A certain amount of clearance is needed for oil to lubricate the stem and guide, and to allow for thermal expansion of the valve stem. Exhaust valves require more clearance than intakes because they run hotter and swell more. But the stem-to-guide clearance must also be tight enough to control oil consumption and to prevent a buildup of varnish and carbon deposits on the stems which could cause sticking.
Too little clearance can lead to scuffing, rapid stem and guide wear, and sticking which prevents the valve from seating fully. This, in turn, can make a valve run hot and burn. Too much clearance, on the other hand, can create oil control problems.
Contrary to what you might think, oil can be drawn past both the intake and exhaust guides. Though oil consumption is more of a problem with sloppy or worn intake guides because the guides are constantly exposed to vacuum, oil can also be pulled down the exhaust guides by suction created in the exhaust port.
The outflow of hot exhaust creates a venturi effect as it exits the exhaust port, creating enough vacuum do draw oil down a worn guide. Oil in the exhaust system of late model vehicles with catalytic converters may cause the converter to overheat and suffer damage.
On the intake side, oil drawn into the engine past worn intake guides can foul the spark plugs, cause the engine to emit higher than normal unburned hydrocarbon (HC) emissions, and contribute to a rapid buildup of carbon deposits on the backs of the intake valves and in the combustion chamber.
Carbon deposits in the combustion chamber can raise compression to the point where detonation occurs under load. Deposits on the backs of the intake valves in engines equipped with multipoint fuel injection can cause hesitation and idle problems because the deposits "soak" up fuel and interfere with proper fuel vaporization.
Inadequate valve cooling is another problem that can result when there’s too much valve stem-to-guide clearance. A valve loses about 15-30% of its heat through the stem, so if the stem fits poorly in the guide, heat transfer will be reduced and the valve will run hot. This can contribute to valve burning, especially with exhaust valves that don’t have the benefit of intake cooling.
Another problem created by excessive clearance is air leakage. Worn intake guides or ones with too much clearance allow "unmetered" air to be drawn into the intake ports. The effect is similar to that of worn throttle shafts on a carburetor. The extra air reduces intake vacuum and upsets the air/fuel calibration of the engine at idle, creating a lean misfire problem and a rough idle.
Both intake and exhaust guide clearance must be within certain tolerances to support the valves as they open and close. Too much clearance allows the valves to wobble, which leads to accelerated stem and guide wear as well as poor valve seating. Valve wobble also flexes the head of the valve which may eventually lead to breakage.
How much clearance?
Different engines have different requirements, so always refer to the factory specifications. Stem-to-guide clearances normally range from .001" to .003" and .002" to .004" for exhaust guides. The exhaust guides usually require .0005" to .001" more clearance than the intakes for thermal expansion.
Diesel engines, as a rule, have looser specs on both intake and exhaust guides than gasoline engines, and heads with sodium-filled exhaust valves usually require an extra .001" of clearance to handle the additional heat conducted up through the valve stems.
The type of guide also influences the amount of clearance needed. Bronze guides, as a rule, can handle about half the normal minimum factory clearance specified for cast iron guides or integral guides because of the anti-seize characteristics of the material and its superior oil retention qualities.
A knurled guide, one with oil retention grooves or a bronze threaded liner all provide better lubrication than a smooth guide. Consequently, clearances can for these types of guides also be tighter. Half the factory minimum specified clearance is sometimes acceptable.
The type of valve seal that’s used is also a factor to consider when determining clearances. Positive valve seals, which are used on most engines today, reduce the amount of oil that reaches the valve stem compared to deflector or umbrella type valve seals. A guide with a deflector valve seal in an older engine application may need somewhat tighter clearances than one with a positive valve seal to control oil consumption.
Guide clearance can be checked after cleaning the valve stem and guide with solvent and a brush to remove all gum and varnish. Insert the valve into its guide, then hold it at its normal opening height while checking side play with a dial indicator. If play exceeds the specified limits, measure the valve stem with a micrometer to see if it is worn excessively; more than .001" of wear calls for replacement. Also measure the inside diameter of the guide with a split-ball gauge and micrometer or go/no-go gauge. A guide will typically show the most wear at the ends and the least wear in the middle.
To determine the exact amount of guide wear using a split ball gauge, subtract the smallest valve stem diameter measured at the end of the guide rub area (most worn area) from the bellmouth reading to determine the extent of the wear. Do not use a valve seat grinding pilot to check the guide. A valve guide pilot will fit snugly in the unworn center section of the guide and not give a true indication of the amount of bellmouth wear at each end of the guide.
Liners have long been used to repair worn integral guides in cast iron heads, but can also be used to restore worn cast iron guides in aluminum heads. The main advantage with this approach is that valves and guides don’t have to be replaced.
The cost savings can be significant depending on the application. Using liners to repair cast iron or powder metal guides in aluminum heads also eliminates the labor and risks that go with replacing guides, assuming the original guides are tight and have not loosened up.
Bronze liners are also an excellent bearing material that provides good lubricity and reduces the risk of galling and seizure. On the other hand, installing liners requires a couple of extra steps which must be done properly to assure proper clearances and fit. If you opt for liners, there are several from which to choose, including thin wall phosphor bronze in various configurations (split and solid designs), and cast iron.
The key to using a split type of bronze liner is proper installation. The original guides should not be worn more than .030" or cracked, otherwise replacement would be recommended.
The first step is to bore out the guides to accept the liners. Use a carbide reamer in an air drill with a no load speed of 2100 to 3000 rpm. Special fixtures are available with centering pilots that center the reamer off the valve seat rather than the guide hole to maintain seat concentricity. Guides should be bored dry with no lubricant, using steady consistent pressure.
Once the guides have been bored out, they should be blown out and checked with a go-no go gauge to make sure they’re the proper size. Next, the liners are pressed in, usually from the top side of the head using an air hammer and installation tool.
On some aluminum heads with powder metal guides, better results can be achieved by installing the liners from the combustion chamber side. Liners go in with the tapered side facing the guide hole. The liners are then driven in flush with the top of the guide.
Sizing the inside diameter of the liner is the next step. Any of three different techniques may be used: roller burnishing (use with lubrication); broaching (driving a calibrated ball through the liner with an air hammer); or using a ball broach tool in an air hammer. This is the most important step because it provides the proper clearances between valve stem and liner for good lubrication and oil control, and it locks the liner in place so it will transfer heat efficiently to the head.
Once the liners have been sized, the head can be turned over so the liners can be trimmed to the proper length unless precut liners are used. Liners are usually cut flush with the guide boss in the port.
The final step is to flex hone the liner. Honing removes any burrs left from trimming the liner to length, and leaves a nice crosshatch finish that improves oil retention. One pass in and out is all that’s recommended to hone the liner. A flexible nylon brush should then be passed through the liner to clean the hole.
With some one-piece bronze liners, broaching after installation is not necessary. The liners have an interference press fit of about .001" to .0015", which requires boring the guide to exact dimensions. Broaching is required for cast iron liners, though, to seat the liner in the guide.
If guides are being reamed to accept valves with oversized stems, an important and often overlooked step is to hone the guides after they’ve been reamed. A reamer fractures the metal leaving microscopic pullouts, tears and a relatively rough surface. Even new guides can be rough inside.
Honing adds a step to restoring the guides, but creates a straighter guide than reaming alone, especially manual reaming, and produces a superior surface finish that is both smoother and better able to retain oil for good valve stem lubrication. Honing also allows somewhat closer stem-to-guide clearances for better oil control and heat transfer.
Seats must be as concentric as possible for a tight compression seal and proper valve cooling. The rounder the seat, the better. Seat runout should not exceed .001" per inch of seat diameter. Some shops aim for .0005" or less of runout. The best way to check concentricity is with a runout gauge. Pulling vacuum on the valve port with the valve in place is another method for checking the mating of the seat and valve. But the ability to hold vacuum is no guarantee of concentricity in itself. That’s why both methods should be used to check the quality of your work.
Seat width is also important for good heat transfer, proper sealing and long valve life. If the seat is too narrow, wear resistance and heat transfer can suffer. And if the seat is too wide, there may not be enough pressure to provide a tight seal. A wide seat also tends to trap deposits that can hold the valve off its seat. This too, can reduce heat transfer as well as compression. As a rule of thumb, the ideal seat width for intake valves is usually around 1/16". For exhaust valves, it’s 3/32" - or whatever the manufacturer specifies.
The point at which the valve and seat mate is also important. If the area of contact is too high on the valve face (too close to the margin), the valve may be sunken into the head. This increases installed height, upsets valvetrain geometry and restricts free breathing.
If the area of contact is too low on the face (too far from the margin), the valve will ride too high on the seat. As the engine warms up and the valve expands, the contact point moves down the valve face away from the margin. The valve may lose partial contact with the seat causing it to lose compression and run hot. Ideally, the valve should contact the seat about one-third of the way down the valve face about 1/32" from the margin so there is about 1/64" of overhang between the margin and top of the seat.
As seats wear, valves recede into the head and close up the lash in the valvetrain. If the engine has solid lifters, it only takes about .015" to .020" of valve recession before the valves start to hang open. Once that happens, the valves burn and the affected cylinders lose compression. The same process takes longer with hydraulic lifters because the lifters can soak up about .050" to .060" of lost lash.
When rebuilding an older cylinder head without hard valve seats, therefore, the original exhaust seats should be replaced with hard inserts. Use a premium grade insert such as a nickel or cobalt alloy. The seats are usually cut to a depth of 1/4", but on thin wall castings you can’t cut much deeper than 7/32" or you’ll be into the water jackets.
If integral valve seats are damaged or too badly worn to be restored, the seats will have to be machined out to accept an insert. Cast iron inserts are rapidly becoming obsolete, although some people still use them for light duty intake valve applications. Under no circumstances should a cast iron insert ever be used on the exhaust side. The metal is just too soft to withstand the operating temperatures.
For exhaust valves, a hard insert made of high chrome stainless steel, high nickel alloy or a heat resistant alloy must be used. Stellite inserts, which are made of a non-magnetic cobalt alloy and are the hardest inserts available, are recommended for the exhaust valves in engines that burn dry fuels such as propane or natural gas, or those that are subjected to elevated operating temperatures like vehicles used for towing, truck engines, etc.
To install valve seat inserts, the old integral seats are bored out on a head and seat machine, and the new inserts are then pressed into place. The seats are usually pressed in dry not lubricated. The amount of interference that’s required will vary depending on the application.
Opinions differ as to how much interference may be needed, but all agree that seats in aluminum heads require more interference than those in cast iron heads. Recommendations vary from .007" total for a 1-5/8" seat in an aluminum head (about .003" of interference for each inch of seat diameter) to .005" for a cast iron head. The amount of interference recommended will also vary according to the hardness of the seat ring itself. Harder seats generally require less interference than softer seats.
Too much interference runs the risk of cracking the head, too little interference increases the risk of the seat coming loose. One of the leading causes of seats coming loose, however, is not the amount of interference between the seat and head, but elevated operating temperatures. Anything that causes the exhaust valve to run hot may also cause the seat to loosen.
The depth of cut for a replacement seat is determined by the height of the seat. On many late model heads with thin wall castings, there isn’t a lot of metal in the head. This limits the depth of the counterbore. Go too far and you’ll cut into the water jacket. To get the right depth, therefore, set the depth stop on the cutter to each insert before you start machining the head.
When replacing seats in an iron or aluminum head, several different procedures can be used to remove the old seats. One is to counterbore most of the seat away to weaken it, then pry or pull it out. Another way to remove a seat is to drill two small holes into the seat 180° apart and then crack it apart with a chisel making sure not to drill into the head). Heating the head in a baking oven to 400°F to 450°F can also loosen the seat inserts for easier removal.
Once the insert has been extracted, check for cracks or erosion damage under the seat - a common problem on many aluminum heads. If cracked or eroded, the metal can be rebuilt by cold pinning or TIG (Tungsten Inert Gas) welding, and remachining the head to accept either a standard or oversized replacement seat.
To install a seat insert, machine the appropriate counterbore in the head. Use a lubricant on the cutter bit for a smoother finish. The head should then be heated in an oven to about 400°F. The seat inserts can also be chilled in a cooler or with dry ice or Freon to reduce the amount of interference. The inserts are then pressed into place, and locked tight by staking or peening. Some rebuilders use an anaerobic locking compound for added insurance while others say anything between the seat and head can form a heat barrier that inhibits cooling.
The guides must be reconditioned or replaced before doing the seats because all seat work is done by centering off the guides. Choices here include grinding or cutting. Grinding requires at least three steps for each seat and kicks up a fair amount of abrasive dust. You also have to be careful to keep the stones properly dressed to maintain accuracy. Cutting, on the other hand, is cleaner and faster especially if all three angles are cut at once. But cutters are more expensive.
The basic procedure for grinding a seat is to select a wheel 1/8" or larger than the seat. The grit of the stone depends upon the seat material. For best results and fast stock removal, use coarse stones on hard seats, and a finer general purpose stone for cast iron seats.
Insert a pilot tool into the valve guide to center the grinding stone. Use a spring under the grinding stone, then grind the seat with a gentle bouncing motion on and off the seat. This reduces chatter, prevents the stone from loading up with particles and gives better overall results.
Remove only as much material as is needed to clean the seat. Don’t get carried away because you can grind away the hardened surface layer on induction hardened integral seats. You can also sink the seat too deeply into the head. Grind off only enough to get rid of the discoloration, pitting and burning on the seat.
Once this has been done, switch wheels and use a 15° or 30° stone to cut the top angle on the seat to locate it with respect to the valve face. If the seat is too wide after cutting, narrow it by cutting the throat angle using a 60° or 75° stone. Be sure to dress the wheel frequently. For best results, this should be after every couple of seats.
After grinding, check concentricity and seating with a vacuum tester or Prussian blue. This will tell you if additional work is needed. If the point of contact is too low on the valve face, raise the seat by enlarging the throat cut with the 60° or 75° stone. If the seat contact is too high, lower the seat by topping with the 30° stone.
The basic procedure for refinishing a valve seat by cutting is essentially the same as grinding except that you’re using carbide cutters instead of grinding stones. The seat is cut, then positioned by making the top cut at 15° or 30°, then narrowed by making the throat cut at 60° or 75°. If the equipment makes all three cuts simultaneously, then it’s a one-step operation. Concentricity should still be checked, however, to make sure everything’s right.