One thing you can almost always count on when rebuilding a cylinder head is worn valve guides. The guides experience a lot of wear because of the constant friction between the guide and stem. To make matters worse, positive valve seals on late model engines prevent the guides from receiving much lubrication. Side forces on the valve stem caused by changes in valvetrain geometry or by direct acting overhead cams further contribute to guide wear.

WORN VALVE GUIDES
When the guides are worn or there is too much clearance between the guide and valve stem, the engine will use oil. This applies to both intake and exhaust guides. Though oil consumption can be more of a problem on the intake side because of constant exposure to engine vacuum, oil can also be pulled down the exhaust guides by suction in the exhaust port. The flow of exhaust past the exhaust guide creates a venturi effect that can pull oil down the 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 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 interfere with proper fuel delivery.

Inadequate valve cooling and premature valve failure is another problem that can be caused by worn guides or ones with excessive clearance. About 75% of the heat from a typical valve is conducted to the seat, and the remaining 25% goes up the stem and out through the guide. On late mode engines with three-angle narrow seats, the amount of heat transfer that takes place through the stem is even higher because less heat can be dissipated through the seat. So if the guide is worn, the valve may run hot and burn.

Worn guides can also pass air. "Unmetered" air drawn into the intake ports past the guides creates an effect similar to worn throttle shafts on a carburetor. The extra air reduces intake vacuum and upsets the air/fuel calibration of the engine at idle. The result may be a lean misfire problem and rough idle.

VALVE FAILURE
Worn guides can also contribute to valve breakage. The guides support and center the valves as they open and close. A worn guide will allow the valve to wobble slightly as it opens, which cause it to drift off-center with respect to the seat. This can cause the head of the valve to flex slightly each time it closes (much like a valve with a nonconcentric seat). After so many cycles, the metal fatigues and the head of the valve breaks off from the stem.

Generally speaking, the intake valve stem-to guide clearance for most passenger cars ranges from .001 to .003 in., and .002 to .004 in. for exhaust guides (which generally require .0005 to .001 in. 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 in. of clearance to handle the additional heat conducted up through the valve stems.

MEASURING VALVE GUIDE WEAR

To check guide wear, some machinists insert a valve stem into a guide and "feel" for looseness by wobbling the valve. Others may use a valve seat pilot tool to check the guides. Though either technique will reveal badly worn guides, neither is a very accurate method of gauging guide clearances or wear.

The best way to check guide wear is with a gauge set designed for this purpose. A gauge set will give you precise measurements and can be used to measure any portion of the guide. To check guide wear (as well as taper) using a telescoping or split ball gauge, measure the guide ID at both ends and in the middle. Subtract the middle reading from the ends to determine taper wear. Compare the smallest ID measurement (usually in the middle of the guide) to the factory specs to determine total wear.

Valve stems should also be measured to check for wear and sizing. Nominal sizes vary quite a bit depending on the application, and there is no way of knowing if the valve has been replaced previously with one of a different size without measuring.

Many late model engines have tapered valve stems. Taper stem valves are ground with the stem diameter smaller at the head end of the valve. This is done to create a larger clearance at the head where the temperatures are highest. This reduces the change of galling with unleaded fuel and narrow three-angle valve seats. When measuring a tapered stem, check the outside diameter about an inch in for each end.

VALVE GUIDE REPAIR OPTIONS

A variety of repair options are available for worn valve guides. Many professional engine rebuilders either install thin wall bronze liners, or ream the guides to oversize and install new or rechromed valves with oversized stems. Replacing guides is another option with aluminum heads as well as some cast iron heads, as is knurling.

KNURLING VALVE GUIDES

Though still used by some small shops, most professional engine rebuilders see knurling as a short term "quick fix" that does not hold up as well as guide liners, new guides or valves with oversized stems. Knurling should only be considered as a guide repair option if guide wear is minimal (less than .006 in.). And even then, it may not provide satisfactory results.

Knurling typically decreases the inside diameter of the guide where it needs it the least, namely in the center where there is the least wear rather than at the ends where the wear is usually greatest. When the knurling tool is run through the guide, it leaves behind a spiral groove. The groove acts like a furrow and raises the metal on either side. This reduces the inside diameter of the guide so a reamer can then be used to resize the guide back to (or close to) its original dimensions. The grooves also help to retain and seal oil better than a smooth bore guide. This allows somewhat tighter guide-to-stem clearances (as close as .0007 in.). But the bearing surface area created by knurling is not that great, so it will not last as long as a guide that offers greater bearing area.

VALVE GUIDE LINERS

Boring out the original guides and installing thin wall bronze liners to restore proper clearances is not only a fast and economical guide repair option, it also provides the benefits of a phosphor/bronze guide surface (better lubricity, scuff resistance and wear characteristics than cast iron).

Though liners are most often used to repair integral guides in cast iron heads, they are also a very effective way to repair replaceable guides in cast iron or aluminum heads, which saves time and eliminates the risks associated with driving out the old guides and pressing in new ones.

Liners also save the cost of having to replace the valves. If the original valves are not worn, standard sized liners can be used to restore the inside diameter dimensions of the guides. If the valves are worn, the stems can be turned down .0050 in. to accommodate liners with slightly undersized inside diameters.

Jerry Qualiana, vice president of aftermarket sales at K-Line Industries, Holland, MI, says their K-Line Bronze Bullet Guide Liner system has been authorized by Ford Motor Company and meets Ford Q-1 quality standards.

According to Qualiana, the Bronze Bullet Guide Liner design is an enhanced design over previous bronze liners, incorporating an "Interrupted Spiral" which assists in retaining oil in the guide, while eliminating oil flow through the guide. In conjunction with the previously mentioned lubricity characteristics of phosphor bronze, Bronze Bullet Guide Liners offer improved guide life in today's oil starved valve guide environment. Also, Qualiana points out that because of the lubricity in the phosphor bronze, K-Line has always advocated valve to stem clearance at the low side of the manufacturer recommended specifications.

Mike McElmurry, vice president of production at Sequal Corp.,Willow Springs, MO, says regarding K-Line's Bronze Bullet Guide Liner, "Because the final size is so easy to control, we have been able to tighten all of our valve to guide tolerances by at least .001 inch. This, along with the liner's ability to resist

galling has reduced our warranty claims by as much as 75% "I have looked at other methods of valve guide repair, .015 inch overised valves, new and rechromed, .003 in undersize valves with replacement cast iron guides, but have found nothing that was cheaper to use than K-Line with .003 in. undersize valves. We grind our own valves and enjoy over $0.80 per guide savings over any of the combinations listed above."

The key to using the Bronze Bullet Guide Liners successfully is proper installation. Qualiana says if the original guides are not worn more than .030 in. or cracked, they can be lined. Otherwise, replacement would be recommended.

Installation of the Bronze Bullet Guide Liners is a five step process:

1. First, the old guides have to be bored out to accept the liners. Qualiana recommends using a KL1725CB Black Beauty carbide reamer in an air drill with a no load speed of 2100 to 3000 rpm. K-Line's KL9900 Boring Fixture has centering pilots that center the reamer off the valve seat (which maintains seat concentricity), and an air clamping fixture that holds the head securely in place while the guides are being bored. The 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 are the proper size.

2. The liners should then be pressed in from the top side of the head using an air hammer and K-Line's Auto Installer tool. The liners go in with the tapered side facing the guide hole. The liners are then driven in flush with the top of the guide boss.

3. Next, the liners are sized. 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 K-Line's ball broach tool in an air hammer.

Sizing the liners is a critical step because it accomplishes two things: it provides the proper clearances between valve stem and liner for proper lubrication and oil control, and it locks the liner in place so it will transfer heat efficiently to the surrounding metal for proper valve cooling. Bronze actually conducts heat more efficiently than cast iron, but requires a tight fit and metal-to-metal contact with the surrounding guide for good heat transfer. If the liner isnot sized properly, it may cause the valve to run hot, or worse yet, come loose.

4. After the liners have been sized, turn the head over an trim the liner to length. The liner should be cut flush with the guide boss in the port. This step is not necessary if precut liners are being used that have the correct length for the application.

5. The final step is to Flex Hone the liner after any seat work that is necessary has been completed. The Flex honing step 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 is recommended to hone the liner. A flexible nylon brush should then be passed through the liner to clean the hole.

Though the just described procedure sounds more complicated than it really is, a typical four cylinder or V8 can be relined in six to seven minutes says Qualiana. Also, the majority of the detailed steps listed regarding cleanliness and accuracy in the guide area are requirements no matter which method of guide repair the rebuilder chooses.

Ron Bernstein, vice president of Precision Engine Parts in Las Vegas, NV, says his company sells a solid one-piece smooth bore .030 in. oversize phosphor bronze valve guide liner.

"Ours is not a split design, so all you do is ream out the guide and press it in. You do not have to broach it afterwards because the liner is installed with an interference press fit of about .001 to .0015 inch. This saves a step and prevents the liner from falling out. But the guide must be bored to exact dimensions, which means you have to use the proper boring tool and replace it when it becomes worn.

"Our liner restores the guide back to stock dimensions so a reclaimed or new valve can be installed. It is a very popular liner with Mexican rebuilders," said Bernstein.

Ertel Manufacturing Corp. in Indianapolis, IN, makes cast iron liners as well as guides. Engineer Bob Leszcynski says many people have a love/hate relationship with bronze liners. "They love the fact that anybody with a Black & Decker hand drill can install the liners, but they hate the fact that if they are not installed correctly the head will come back with loose or worn liners.

"We say rebuilders should use always some type of piloted installation equipment that centers off the valve seat so the liner will be centered properly in the guide. With a hand drill and no fixturing, you have no control. Lean this way or that way on the drill a little bit and your hole will be off.

"We also say you must always broach the liners once they have been installed to seat them, which is something we also require for our cast iron liners. Most people do not think cast iron will stretch, but it does when you broach the liner to seat it."

Leszcynski said cast iron liners cost about the same as bronze liners. "Bronze has good anti-seize properties and is popular for that reason. But cast iron wears better and performs more like an integral valve guide in a cast iron head. Cast iron is also a good replacement choice for aluminum heads. In fact, you can use cast iron guides or liners in virtually any application where bronze might be used. We also have cast iron guides for the 1993 and newer engines that have powder metal guides."

OVERSIZED VALVES

"We tried knurling and found that it only lasted about 5,000 miles. We also tried liners but saw failures as early as 10,000 to 12,000 miles. So we went to chrome plated oversized valves and have not had any warranty problems since!"

Lynaugh says the chrome plated valves conduct heat better than unplated valves and resist scuffing like an new OEM valve. The standard oversized stems also make installation easy because all that is needed is one pass with a reamer through the guide to size the hole.

Fred Calouette of Cal Valves (Grinding Inc.) in Escanaba, MI supplies Cloverland with their chrome plated valves. Calouette says ease of installation, better durability and service are just a few of the many reasons why our customers, large and small, prefer the "+8" valve program. Cal Valves are plated with .008 in. of hard chrome (not just a flash coating as is used on many OEM valves), and finished to OEM specs plus .008 in.

"Our +8 oversized valve program really simplifies installation because all you need is one reamer for each basic valve size. The reason for this is we finish grind all valves to one common size, be it a 7mm +8, 5/16 +8, and so on. All valves are ready to install and do not require any additional machining."

Calouette says chrome plating makes a valve more durable, and at the same time improves its wear characteristics five to ten times over an unplated valve. Most OEM valves are chrome plated to prevent galling and scuffing during dry starts. Ford, for example, has specified chrome stems on both intake and exhaust valves since 1985. Chrome prevents the stem from galling when cast iron guides are used, and it helps prevent positive valve seal wear on intake valves.

When used valves are salvaged and the stems are reground, grinding removes the chrome flashing. This means a reground valve must be used with either a bronze liner or guide, or replated to restore the original scuff protection if used with a cast iron guide.

Gene Hailey, vice president of tech services at Enginetech Inc. in Carrolton, TX says his company offers a .015 inch oversize "Rebuilders" valve for restoring worn valve guides. "The larger oversize can compensate for greater wear in the guides. The stem diameters have also been consolidated to a common size so only one reamer is needed regardless of the application. You can use the same reamer on a Ford, GM or Chrysler head."

Hailey says his exhaust valves are made of 21-4N stainless, and the intakes are HNV3 alloy. All valves have a hardened tip, a .030 in. oversized head (to compensate for seat wear and refinishing), and a triple chrome flashing on the stem .002 in, thick. The chrome plating makes the valves more scuff and wear resistant when used in cast iron guides. All valves also follow the OEM design with respect to taper. If an OEM exhaust valve is tapered (which many GM and stelite diesel valves are), so is the Rebuilders valve. Intake valves are .0005 in. larger than exhaust valves.

John Brehm, shop manager at Densmore Engines in Fresno, CA says Densmore has been using the Rebuilders valve for eight years with excellent results. "We use all new valves in every head we remanufacture, including our Ultra heads which we warranty for 30 months or 50,000 miles. So we usually go with an oversized valve unless a valve is not available, in which case we reline or replace the guide and use a new standard sized valve."

Brehm says he uses the oversized valves mostly in cast iron heads, and prefers to replace the guides in aluminum heads.

HONING VALVE GUIDES

When guides are reconditioned by reaming to oversize (or knurling), the passage of a reamer through the guide fractures the metal leaving microscopic pullouts, tears and a relatively rough surface. This less than ideal bearing surface will not wear as well as one that has been honed. So honing is usually recommended to smooth the guide bore by knocking the peaks off the ridges left by the reamer. This produces a superior bearing surface that will retain oil better and last longer than an unhoned guide.

Even new guides can benefit from honing. New guides are often rather rough. Honing provides a more uniform surface finish which will reduce stem and guide wear, and generally extend the life of the guides. This includes cast iron guides as well as bronze guides.

VALVE GUIDE REPLACEMENT

On aluminum or cast iron heads with nonintegral guides, worn guides are often replaced. Pressing out the old guides and installing new ones can be difficult with some aluminum heads where the interference fit is considerable. Cracking the head or galling the guide hole is always a risk. One recommendation here is to preheat the heads in an oven prior to guide removal and to lubricate dry liners before driving them out. The head should also be preheated before the new guides are installed. Chilling the replacement guides can reduce the amount of interference during installation. Lubricant also helps prevent galling. With tapered guides, care must be taken to install them from the right side. Most wet guides are tapered, and also require sealer to prevent leaks.

Replacement guides come in various alloys and varieties including bronze, cast iron and powdered metal. Phosphor/bronze, silicon/aluminum/bronze and manganese/bronze are generally more expensive than cast iron but are harder and usually provide superior wear resistance. With leaded gasoline, bronze guides typically lasted 3 to 5 times longer than cast iron. But with unleaded gasoline the difference in longevity between cast iron and bronze is not as great according to one supplier of bronze guides. However, bronze still provides superior heat transfer, resists seizing and can handle closer tolerances (which improves valve life and reduces oil consumption). That is why thick wall bronze guides are preferred by many performance shops.

Adapted from an article written by Larry Carley for Engine Builder magazine