Members Login
Username 
 
Password 
    Remember Me  
Post Info TOPIC: Valve Refacing


Moderator

Status: Offline
Posts: 2150
Date: Aug 24, 2009
Valve Refacing
Permalink Closed


Valve Refacing Equipment and Procedures My Dad posted this on my site so i thought i would share it here too

There comes a time in the life of every valve when it must either be replaced or reconditioned. Valve faces become worn and eroded as the seats wear, the stems wear where they contact the guides, and the tips of the stems can become worn or mushroomed from the constant friction with the rocker arms or cam followers.

Heat also takes a toll on the valves. The hotter the valves run, the more likely they are to undergo undesirable metallurgical changes. Exhaust valves run hundreds of degrees hotter than intake valves (1,200° to 1,400° F), and may experience "burning" or failure if the valve makes poor contact with the seat or guide. Engine overheating because of a cooling problem or lean fuel condition may be a contributing factor.

As the miles add up, exhaust valves tend to stretch - sometimes to the point where the metal weakens and the head snaps off from the stem. Metal fatigue from flexing (which is often due to a concentricity problem with the valve seat, guide or valve itself) can also create stress cracks that eventually lead to valve failure. For this reason, all valves that are being salvaged for reuse should be carefully inspected for surface cracks as well as internal cracks. Ultrasonic testers can reveal hidden cracks and flaws beneath the surface while magnetic detection equipment can reveal hairline cracks on the surface.

REGRIND OR REPLACE?
Depending on the engine's mileage and the condition of the original valves, salvaging and reconditioning the original valves can usually save the cost of having to buy new valves. Valves that are not stretched, cracked, burned, bent or too badly worn can be reconditioned by refacing the heads and stem tips on a valve grinder. But some would argue that it may actually be less expensive to replace valves rather than recondition them yourself.

The price of new valves for many engines has dropped significantly in recent years, making them more competitive with reconditioned valves. If you opt to install new valves, you don't have to clean, sort, rechrome or grind the old valves, and you don't have to install guide liners. All you have to do is ream out the old guides and drop in new valves with oversized stems. For many engine builders, new valves eliminate a bottleneck in the shop and allows jobs to be turned around more quickly. This lowers overall labor costs and improves productivity and profitability.

Yet many engine builders continue to salvage and grind valves. It's a time-proven process that works very well - provided the valves are ground correctly on equipment that can hold close tolerances.

TWO BASIC TYPES
Valve refacing equipment is available from a variety of suppliers. There are basically two types of machines from which to choose: those that use a collet or chuck to hold the valve stem, and "centerless" grinders.

Generally speaking, valve grinders that use a collet holder are capable of maintaining valve face concentricity within .001" (0.025 mm) or less. Centerless grinders, on the other hand, can typically maintain valve face concentricity of .0004" (0.01 mm) or less.

Either type of machine can provide satisfactory results for today's engines as long as the equipment is maintained and used properly, and valve stems are checked for excessive wear, warpage or bending before they are ground. But the goal is to keep the face of the valve concentric with the stem, and to keep total indicated runout (TIR) to .001" (0.025 mm) or less.

"The key to valve refacing today," says Anthony Usher of Rottler Manufacturing, "is eliminating runout. It's absolutely critical on overhead cam engines with multi-valves and small diameter valve stems."

Wear in the valve stem can cause the valve to wobble as it is being ground. Valve stems can develop a fluted or lobe-like wear pattern that can't be seen with the naked eye. The wear can only be detected by placing the valve in a pair of V-blocks and measuring runout with a dial indicator as the valve is rotated 360°. If the stem is out-of-round, it will create runout in the valve head when the face is reground. This, in turn, may prevent the valve from sealing and create a compression leak. When you pull vacuum on the cylinder to check the valves, there may be enough leakage to prevent the cylinder from holding vacuum. That's why worn valve stems should be reground before a valve is refaced.

Runout is a big deal with valves because any runout in the valve face causes the valve to deflect when it bangs shut on the seat. If the seat also has too much runout (more than .001") or is not concentric with the centerline of the valve guide, tolerances can stack up and may create too much total runout resulting in a valve that leaks or fails prematurely from flexing and metal fatigue.

Suppliers who make centerless valve refacing equipment say one of the advantages of centerless grinding is that it eliminates a potential source of runout that may be present in machines that use a chuck or collet to hold the valve. If the collet is not rigidly supported, or there is play in the shaft bearings, it won't hold the valve straight as it rotates against the grinding wheel. With centerless grinding, the valve rotates around its own center and is supported on a pair of rollers. This maintains better alignment between the valve and grinding wheel.

Equipment suppliers have a variety of options for centerless grinding.

Tim Meara of Sunnen says the new version of his company's VR7000 centerless Valve Refacing machine introduced at this year's PRI Show offers operators better control. "The new VR8000 adds variable speed control so valves with smaller stems can be ground at a consistent surface speed."

Severine Guevara of SERDI Corp. says "Centerless grinding allows valves to be ground with unparalleled accuracy. We offer two machines - the VVR120 and HVR90 - to service the entire engine rebuilding industry, from motorcycle, automotive, racing to heavy-duty. The valve stem capacity of these machines is from 4 mm/.157" up to 20 mm/.787". "

Makers of collet-style equipment insist machinery advancements allow a high degree of precision as well.

Dennis Mullarkey of Kwik-Way, says his company's Model SVS II Deluxe Valve Refacer comes with a standard chuck or a high performance chuck. "Our high performance chuck is extremely accurate and can maintain runout at .0002 inches or less. Our tests show that no other valve refacer, including centerless grinders, can do any better than this, whether its a new valve or a used valve that's being ground."

Jim Schultz of Neway Manufacturing, says the VFR1000 valve refacing machine uses a special precision-machined collet that can hold tolerances to .0005" or less, and a portable valve cutter called the "Gizmatic" that uses a carbide blade to cut 45° and 30° angles on the valve head. "The Gizmatic is primarily a clean up tool and will not correct runout. But it can do titanium as well as stainless steel valves," Shultz says.

Chip Brown of Peterson Machine Tool, explains that the Berco RV20 is a collet-type machine that also offers a high degree of accuracy. "This valve refacer can hold concentricity to .0005" or less and is a good choice for performance engine builders or production engine rebuilders. It has a quick release collet, and can do rocker arms, too."

If you already own a collet style valve refacer and need a smaller chuck to handle smaller valves, Goodson Tools & Supplies has a UVRC46 adapter that fits 1/2" chucks for grinding 4 to 6.5 mm (.157 to .255") valves.

Sioux Tools currently sells two valve refacers, the Sioux 2075 which comes with three chuck sizes from .170" all the way up to .750", and the Sioux 2075HPA which has a high precision collet system with an accuracy of .0003" or less. The Sioux 2075HPA also features a variable speed chuck (150 to 300 rpm). Both machines can cut valve angles from 0° to 46°.

GRINDING WHEELS
Valve refacing machines have two grinding wheels, one for refacing the valves and a second for grinding the tips of the stems. A liquid coolant is usually used during the grinding process to cool the valve and wheel, and to wash away debris.

Maintaining concentricity and a high quality finish requires dressing the grinding wheels periodically. This is usually done with a diamond lapping tool. David Monyhan of Goodson says the diamond should be carefully inspected and rotated every time the wheel is dressed. "If the diamond is not maintained and has any flat spots, it won't dress the wheel properly. The wheel will act as if it is loaded up."

As a rule, different types of grinding wheels are recommended for grinding different valve materials such as titanium. Stainless steel and stellite faced valves are usually ground with a gray aluminum-oxide grinding wheel. Titanium valves, on the other hand, tend to smear and are best ground with a green silicon-carbide grinding wheel. Monyhan also recommends using a special grinding fluid when refacing titanium valves to prevent the stone from loading up and to wash away the debris.

If you're grinding a valve and see chatter on the finish, the wheel you're using may be too hard for the type of valve. Other possible causes of chatter on the finish include:

Too much motor end play. This can create vibration in the grinding wheel and cause the stone to dull quickly. Some motors have a preload spring to maintain zero end play - if the spring is weak or loose, it can't maintain zero end play. Other machines may have a plastic thrust spacer on the motor that may be worn. Check for end play with a dial indicator placed against the end of the motor shaft.

Loose motor mounting. If the base bolts are loose, the motor may vibrate. Tighten and loosen each mounting bolt and/or shim as needed to eliminate any vibration.

Wrong coolant. Follow the wheel manufacturer's recommendations for the type of oil or coolant to use when grinding.

Improperly dressed wheel. This is often caused by a flat diamond or a clogged stone. The pores of the stone must be open to carry coolant and debris past the valve as it is being ground. When the pores are plugged the grinding debris stays on top of the wheel surface causing the wheel to skip over the valve and chatter.

A scalloped finish on the valve face. If there's too much end play in the chuck or chuck bearings you may see a scalloped finish. Play allows the valve to bounce in and out as it is being ground. End play should be .0005" or less. Inspect and lubricate the check bearings as needed.

Inconsistent chuck rotation This may also cause the valve to skip as it is being ground. Ch
eck, realign and/or tighten the chuck drive belt as needed.

Tom DeBlasis of K-Line says his company's business has been strong because a lot of engine builders continue to reclaim valves to keep costs down. "We have a full selection of refacing wheels to help them out. While we don't have anything new in valve refacing equipment, we do have a great valve stem polishing machine that does a fantastic job," says DeBlasis.

STEM LENGTHS
Valve refacing machines are also used to grind the tip of the valve stem and to apply a new chamfer. The amount of metal removed from the tip of the stem should be limited because the depth of the hardened heat treated layer may be .010" or less. Usually a couple of thousandths is all that has to be removed to restore the tip.

Keeping valve stem lengths consistent is also important on today's overhead cam engines. The installed height of a valve depends on the depth of the seat in the cylinder head as well as the overall length of the valve. Valve and seat machines can be set to cut seats to a consistent depth, so it's important to grind the tips of the valve stems so the valves are also a consistent length. This will help maintain correct valvetrain geometry and save assembly time.

"Maintaining a consistent valve length is a three-step process on our centerless valve refacer," said Anthony Usher of Rottler. "First, you select the worst valve in the set. Then you manually feed the valve against the wheel to grind the tip. Then you push a lever to move the valve away from the wheel. This establishes the length so the rest of the valves can all be ground exactly the same."

HAND LAPPING
Though hand lapping valves was once a common practice, the need to do so is much less today thanks to precision valve seat cutters. Even so, some performance engine builders still hand lap new and reground valves to make sure the valve has a perfect seal with the seat. A tighter seal eliminates any compression leaks and also provides maximum cooling between the valve and seat to prolong the life of the valve. This is extremely important in high revving high performance engines as well as hard-working engines of any type.

Hand lapping involves applying lapping compound (an abrasive paste) to the valve face or seat, and then using a suction cup on the end of a short stick or a power lapping tool to rotate the valve back and forth while slight pressure is applied. This smooths off any imperfections on both surfaces and results in a near perfect seal. The area of contact between the seat and valve can then be checked with Prussian Blue to see if any additional lapping is required to clean up any gaps that may be left.

Goodson sells a pneumatic lapping tool that can be a real time saver, while a company called Samplex in Boca Raton, FL, makes a product called an "Eezilap" tool that fits in an electric drill. Both tools oscillate the valve left to right to eliminate the need for hand massaging the valves.


__________________
Pres. WICKED RIDES car club
www.wickedridesnj.com
www.wickedmontes.com
Page 1 of 1  sorted by
 
Quick Reply

Please log in to post quick replies.



Create your own FREE Forum
Report Abuse
Powered by ActiveBoard