One thing you can almostalways count on when rebuilding a cylinder head is worn valveguides. The guides experience a lot of wear because of the constantfriction between the guide and stem. To make matters worse, positivevalve seals on late model engines prevent the guides from receivingmuch lubrication. Side forces on the valve stem caused by changesin valvetrain geometry, or by direct acting overhead cams, furthercontribute to guide wear.
When the guides are wornor there is too much clearance between the guide and valve stem,the engine will use oil. This applies to both intake and exhaustguides. Though oil consumption can be more of a problem on theintake side because of constant exposure to engine vacuum, oilcan also be pulled down the exhaust guides by suction in the exhaustport. The flow of exhaust past the exhaust guide creates a venturieffect that can pull oil down the guide.
Oil in the exhaust systemof late model vehicles with catalytic converters may cause theconverter to overheat and suffer damage. On the intake side, oildrawn 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 depositson the backs of the intake valves and in the combustion chamber.
Carbon deposits in thecombustion chamber can raise compression to the point where detonationoccurs under load. Deposits on the backs of the intake valvesin engines equipped with multipoint fuel injection can cause hesitationand idle problems because the deposits interfere with proper fueldelivery.
Inadequate valve coolingand premature valve failure is another problem that can be causedby worn guides or guides with excessive clearance. About 75% ofthe heat from a typical valve is conducted to the seat, and theremaining 25% goes up the stem and out through the guide. On latemode engines with three-angle narrow seats, the amount of heattransfer that takes place through the stem is even higher becauseless heat can be dissipated through the seat. So if the guideis worn, the valve may run hot and burn.
Worn guides can alsopass air. "Unmetered" air drawn into the intake portspast the guides creates an effect similar to worn throttle shaftson a carburetor. The extra air reduces intake vacuum and upsetsthe air/fuel calibration of the engine at idle. The result maybe a lean, misfire problem and rough idle.
Worn guides can alsocontribute to valve breakage. The guides support and center thevalves as they open and close. A worn guide will allow the valveto wobble slightly as it opens, which cause it to drift off-centerwith respect to the seat. This can cause the head of the valveto flex slightly each time it closes (much like a valve with anonconcentric seat). After so many cycles, the metal fatiguesand the head of the valve breaks off from the stem.
Generally speaking, theintake valve stem-to- guide clearance for most passenger carsranges from .001" to .003", and .002" to .004"for exhaust guides (which generally require .0005" to .001"more clearance than the intakes for thermal expansion). Dieselengines, as a rule, have looser specs on both intake and exhaustguides than gasoline engines, and heads with sodium-filled exhaustvalves usually require an extra .001" of clearance to handlethe additional heat conducted up through the valve stems.
Checking 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 seatpilot tool to check the guides. Though either technique will revealbadly worn guides, neither is a very accurate method of gaugingguide clearances or wear.
The best way to checkguide wear is with a gauge set designed for this purpose. A gaugeset will give you precise measurements and can be used to measureany portion of the guide. To check guide wear (as well as taper)using a telescoping or split ball gauge, measure the guide IDat both ends and in the middle. Subtract the middle reading fromthe ends to determine taper wear. Compare the smallest ID measurement(usually in the middle of the guide) to the factory specs to determinetotal wear.
Valve stems should alsobe measured to check for wear and sizing. Nominal sizes vary quitea bit depending on the application, and there’s no way of knowingif the valve has been replaced previously with one of a differentsize without measuring.
Many late model engineshave tapered valve stems. Taper stem valves are ground with thestem diameter smaller at the head end of the valve. This is doneto create a larger clearance at the head where the temperaturesare highest. This reduces the chance of galling with unleadedfuel and narrow three-angle valve seats. When measuring a taperedstem, check the outside diameter about an inch in for each end.
Guide repair options
Whether you repair guideson an "as needed" basis or automatically redo all theguides anytime a head is rebuilt, you have a variety of guiderepair options from which to choose. Most rebuilders either gowith thin wall bronze liners and reclaimed valves, or installnew or rechromed valves with oversized stems. Replacing guidesis another option with aluminum heads as well as some cast ironheads, as is knurling. Most rebuilders have tried all of thesetechniques at one time or another, but usually stick with a singletechnique that fits their operation best, or gives them the leastamount of problems.
Though still used bysome small shops, most rebuilders see knurling as a short term"quick fix" that doesn’t hold up as well as guide liners,new replacement guides or valves with oversized stems. Knurlingshould only be considered as a guide repair option if guide wearis minimal (less than .006"). And even then, it may not providesatisfactory results.
Knurling typically decreasesthe inside diameter of the guide where it needs it the least,namely in the center where there is the least wear, rather thanat the ends where the wear is usually greatest. When the knurlingtool 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 canthen be used to resize the guide back to (or close to) its originaldimensions.
The grooves also helpto retain and seal oil better than a smooth bore guide. This allowssomewhat tighter guide-to-stem clearances (as close as .0007").But the bearing surface area created by knurling is not that great,so it won’t last as long as a guide that offers greater bearingarea.
Boring out the originalguides and installing thin wall bronze liners to restore properclearances is a fast and economical guide repair option. It alsoprovides the benefits of a phosphor/bronze guide surface whichoffers better lubricity, scuff resistance and wear characteristicsthan cast iron.
Though liners are mostoften used to repair integral guides in cast iron heads, theyare also a very effective way to repair replaceable guides incast iron or aluminum heads — which saves time and eliminatesthe risks associated with driving out the old guides and pressingin new ones.
Liners also save thecost of having to replace the valves. If the original valves arenot worn, standard sized liners can be used to restore the insidediameter dimensions of the guides. If the valves are worn, thestems can be turned down .0050" to accommodate liners withslightly undersized inside diameters.
Jerry Qualiana, vicepresident of aftermarket sales at K-Line Industries, Holland,MI, says K-Line’s Bronze Bullet Guide Liner system has been authorizedby Ford Motor Co. and meets Ford’s Q-1 quality standards.
According to Qualiana,the Bronze Bullet Guide Liner design is an enhanced design overprevious bronze liners, incorporating an interrupted spiral whichassists in retaining oil in the guide, while eliminating oil flowthrough the guide. In conjunction with the previously mentionedlubricity characteristics of phosphor bronze, Bronze Bullet GuideLiners offer improved guide life in today’s oil starved valveguide environment.
Because of the lubricityin the phosphor bronze, K-Line has always advocated valve-to-stemclearance at the low side of the manufacturer’s recommended specifications.Mike McElmurry, vice president of production at Sequel Corp.,a production engine remanufacturer located in Willow Springs,MO, agrees. "Because the final size is so easy to control,we have been able to tighten all of our valve-to-guide tolerancesby at least .001".
"This, along withthe liner’s ability to resist galling, has reduced our warrantyclaims by as much as 75%. McElmurry added that for Sequel’s productionrequirements, grinding valves to .003" undersize in combinationwith the liner has provided an $0.80 per guide savings over anyother valve guide repair option available.
As with any valve guideliner, the key to success is proper installation. Qualiana saysif the original guides are not worn more than .030" or cracked,they can be lined. Otherwise, replacement would be recommended.
Qualiana offered thefollowing five-step installation procedure to ensure acceptableperformance of his company’s guide liners:
First, the old guideshave to be bored out to accept the liners. Qualiana recommendsusing a KL1725CB Black Beauty carbide reamer in an air drill witha no-load speed of 2,100 to 3,000 rpm. K-Line offers a KL9900Boring Fixture with centering pilots that center the reamer offthe valve seat (which maintains seat concentricity), and an airclamping fixture that holds the head securely in place while theguides are being bored.
The guides should bebored dry with no lubricant, using steady consistent pressure.Once the guides have been bored out, they should be blown outand checked with a go- no-go gauge to make sure they’re the propersize.
The liners should thenbe pressed in from the top side of the head using an air hammerand K-Line’s Auto Installer tool. The liners go in with the taperedside facing the guide hole. The liners are then driven in flushwith the top of the guide boss.
Next, the liners aresized. Any of three different techniques may be used: roller burnishing(use with lubrication), broaching (driving a calibrated ball throughthe liner with an air hammer), or using K-Line’s ball broach toolin an air hammer.
Sizing the liners isa critical step because it accomplishes two things: it providesthe proper clearances between valve stem and liner for properlubrication and oil control, and it locks the liner in place soit will transfer heat efficiently to the surrounding metal forproper valve cooling.
Bronze actually conductsheat more efficiently than cast iron, but requires a tight fitand metal-to-metal contact with the surrounding guide for goodheat transfer. If the liner isn’t sized properly, it may causethe valve to run hot — or worse yet, come loose.
After the liners havebeen sized, turn the head over and trim the liner to length. Theliner should be cut flush with the guide boss in the port. Thisstep is not necessary if precut liners are being used that havethe correct length for the application.
The final step is toFlex Hone the liner — after any seat work that’s necessary hasbeen completed. The Flex honing step removes any burrs left fromtrimming the liner to length, and leaves a nice crosshatch finishthat improves oil retention. One pass in and out is all that’srecommended to hone the liner. A flexible nylon brush should thenbe passed through the liner to clean the hole.
Though this proceduremay sound complicated, a typical four cylinder or V8 can be relinedin six to seven minutes, says Qualiana. Also, the majority ofthe detailed steps listed, regarding cleanliness and accuracyin the guide area, are requirements no matter which method ofguide repair the rebuilder chooses.
Ron Bernstein, vice presidentof Precision Engine Parts in Las Vegas, NV, says his company sellsa solid, one-piece smooth bore .030" oversize phosphor bronzevalve guide liner. "Ours is not a split design, so all youdo is ream out the guide and press it in," explained Bernstein.
"You don’t haveto broach it afterwards because the liner is installed with aninterference press fit of about .001"_ to .0015". Thissaves a step and prevents the liner from falling out. But theguide must be bored to exact dimensions, which means you haveto use the proper boring tool and replace it when it becomes worn.Our liner restores the guide back to stock dimensions so a reclaimedor new valve can be installed. It’s a very popular liner withMexican 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 relationshipwith bronze liners. "They love the fact that anybody witha Black & Decker hand drill can install the liners, but theyhate the fact that if they’re not installed correctly the headwill come back with loose or worn liners.
"We say rebuildersshould always use some type of piloted installation equipmentthat centers off the valve seat so the liner will be centeredproperly in the guide. With a hand drill and no fixturing, youhave no control. Lean this way or that way on the drill a littlebit and your hole will be off.
"We also say youmust always broach the liners once they’ve been installed to seatthem, which is something we also require for our cast iron liners.Most people don’t think cast iron will stretch, but it does whenyou broach the liner to seat it."
Leszcynski said castiron liners cost about the same as bronze liners. "Bronzehas good anti-seize properties and is popular for that reason.But cast iron wears better and performs more like an integralvalve guide in a cast iron head. Cast iron is also a good replacementchoice for aluminum heads. In fact, you can use cast iron guidesor liners in virtually any application where bronze might be used.We also have cast iron guides for the ’93 and newer engines thathave powder metal guides."
Another popular guiderepair option is reaming the guides to oversize and installingnew valves with oversized stems or used valves with oversize,chromed-plated stems. Those who prefer this technique say it’sa fast and easy way to restore guides because all you have todo is ream the guide to oversize and drop in a new valve.
In many instances, theexhaust valves have to be replaced anyway because of wear or burning,so the added cost is not that great a factor. New valves alsoeliminate the need to regrind the old valve stems, as well asthe sizing hassles that go with reusing reground valves or stockvalves that come in so many different nominal sizes.
According to Alan Carver,director or marketing at SB International, Nashville, TN, a supplierof replacement valves and seats, about 30% of production enginerebuilders are installing new oversized valves. This comparesto about 30% that are using guide liners, and 30% that are grindingor rechroming valves. Carver says that although they offer oversizedvalves in .003", .005" and .015" oversizes, the+.015" valve is the most popular because it can be used inabout 85% of all engine applications.
"It used to be thatrebuilders could reclaim about 80% of their valves. But becauseof the accelerated valve stem wear in so many older engines thatdidn’t have chrome plated valve stems, they now have to throwout about 80% of those valves. The cost of replacing the valveson popular engines is not that bad because the valves are relativelyinexpensive," said Carver. "But on less popular engines,they can be rather expensive."
Bill McClusky at ManleyValves, Dynagear Co. in Dallas, TX, said his company offers oversizedvalves in +.003", +.005" and +.015" sizes, withmost of the demand being for the +.015". He said there hasn’tbeen a lot of growth in the demand for oversized valves primarilybecause many rebuilders are still reclaiming the old valves.
"It’s still abouta dollar per valve cheaper to reclaim valves than to replace themeven when you figure in the added labor cost to recondition theold valves," he said. "Even so, some rebuilders aregoing with all new valves because they eliminate problems. Beingable to offer a customer a head with all new valves can also bea good selling point," said McClusky.
John Lynaugh, warrantysupervisor for Cloverland Manufacturing in Escanaba, MI, saysCloverland Mfg. rebuilds about 150 cylinder heads a day. Of these,99% go out the door with oversized, chrome-plated valves. Lynaughsays they prefer to use +.008" oversized valves for severalreasons:
"We tried knurlingand found that it only lasted about 5,000 miles,’ said Lynaugh."We also tried liners but saw failures as early as 10,000to 12,000 miles. So we went to chrome-plated, oversized valvesand haven’t had any warranty problems since!"
Lynaugh says the chrome-platedvalves conduct heat better than unplated valves and resist scuffinglike a new OEM valve. The standard oversized stems also make installationeasy because all that’s needed is one pass with a reamer throughthe guide to size the hole.
Fred Calouette of CalGrinding Inc., Escanaba, MI, supplies Cloverland with its chrome-platedvalves. Calouette says ease of installation, better durabilityand service are just a few of the many reasons why his customers,large and small, prefer the +.008" valve program. Cal Valvesare plated with .008_ of hard chrome (not just a flash coatingas is used on many OEM valves), and finished to OEM specs plus.008".
"Our +.008"oversized valve program really simplifies installation becauseall you need is one reamer for each basic valve size," saidCalouette. "The reason for this is we finish grind all valvesto one common size, be it a 7mm +.008", 5/16 +.008",and so on. All valves are ready to install and do not requireany additional machining."
Calouette says chromeplating makes a valve more durable, and at the same time improvesits wear characteristics five to 10 times over an unplated valve.Most OEM valves are chrome plated to prevent galling and scuffingduring dry starts. Ford, for example, has specified chrome stemson both intake and exhaust valves since 1985. Chrome preventsthe stem from galling when cast iron guides are used, and it helpsprevent positive valve seal wear on intake valves.
When used valves aresalvaged and the stems are reground, grinding removes the chromeflashing. This means a reground valve must be used with eithera bronze liner or guide, or replated to restore the original scuffprotection if used with a cast iron guide.
Gene Hailey, vice presidentof tech services at Enginetech Inc., Carrollton, TX, says hiscompany offers a .015" oversize "Rebuilders" valvefor restoring worn valve guides. "The larger oversize cancompensate for greater wear in the guides," said Hailey."The stem diameters have also been consolidated to a commonsize 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 exhaustvalves are made of 21-4N stainless, and the intakes are HNV3 alloy.All valves have a hardened tip, a .030" oversized head (tocompensate for seat wear and refinishing), and a triple chromeflashing on the stem .002", thick. The chrome plating makesthe valves more scuff and wear resistant when used in cast ironguides. All valves also follow the OEM design with respect totaper. If an OEM exhaust valve is tapered (which many GM and stelitediesel valves are), so is the Rebuilders valve. Intake valvesare .0005_ larger than exhaust valves.
John Brehm, shop managerat Densmore Engines in Fresno, CA, says Densmore has been usingthe Rebuilders valve for eight years with excellent results. "Weuse all new valves in every head we remanufacture, including ourUltra heads which we warranty for 30 months or 50,000 miles. Sowe usually go with an oversized valve unless a valve isn’t available,in which case we reline or replace the guide and use a new standardsized valve."
Brehm says he uses theoversized valves mostly in cast iron heads, preferring to replacethe guides in aluminum heads.
When guides are reconditionedby reaming to oversize (or knurling), the passage of a reamerthrough the guide fractures the metal leaving microscopic pullouts,tears and a relatively rough surface. This less than ideal bearingsurface will not wear as well as one that’s been honed. So honingis usually recommended to smooth the guide bore by knocking thepeaks off the ridges left by the reamer. This produces a superiorbearing surface that will retain oil better and last longer thanan unhoned guide.
Even new guides can benefitfrom honing. New guides are often rather rough. Honing providesa more uniform surface finish which will reduce stem and guidewear, and generally extend the life of the guides. This includescast iron guides as well as bronze guides.
On aluminum or cast ironheads with nonintegral guides, worn guides are often replaced.Pressing out the old guides and installing new ones can be difficultwith some aluminum heads where the interference fit is considerable.Cracking the head or galling the guide hole is always a risk.
One recommendation hereis to preheat the heads in an oven prior to guide removal andto lubricate dry liners before driving them out. The head shouldalso be preheated before the new guides are installed. Chillingthe replacement guides can reduce the amount of interference duringinstallation. Lubricant also helps prevent galling. With taperedguides, care must be taken to install them from the right side.Most wet guides are tapered, and also require sealer to preventleaks.
Replacement guides comein various alloys and varieties including bronze, cast iron andpowdered metal. Phosphor/bronze, silicon/aluminum/bronze and manganese/bronzeare generally more expensive than cast iron but are harder andusually provide superior wear resistance.
With leaded gasoline,bronze guides typically lasted three to five times longer thancast iron. But with today’s unleaded gasoline the difference inlongevity between cast iron and bronze isn’t as great accordingto one supplier of bronze guides. However, bronze still providessuperior heat transfer, resists seizing and can handle closertolerances (which improves valve life and reduces oil consumption).That’s why thick wall bronze guides are preferred by many performanceshops.
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