La sostituzione delle sedi delle valvole è uno dei lavori di base che è spesso necessario quando si ricostruiscono teste in alluminio o ghisa con sedi incrinate, danneggiate o molto usurate. Ma c'è molto di più nella sostituzione di una sede di valvola che nell'estrarre quella vecchia e guidarne una nuova. Se la testa è in ghisa con sedi integrate, la testa deve essere lavorata per sostituire la sede (a volte chiamata installazione di una sede "falsa").
E se la testa è in alluminio, potrebbe essere necessario lavorare il controforo della sede per accettare una sede di grandi dimensioni se il foro è allentato, deformato o danneggiato. In entrambi i casi, un macchinista deve calcolare la quantità di interferenza necessaria per il nuovo sedile prima di tagliare la testa su una macchina sedile-guida. Deve anche decidere che tipo di sedile installare.
La sostituzione di un sedile, quindi, comporta una serie di decisioni e passaggi, che influenzano l'esito del lavoro di riparazione. Come avrete intuito, abbiamo incontrato opinioni diverse sul modo giusto e sul modo sbagliato di sostituire le sedi delle valvole durante la ricerca di questo articolo, in particolare per quanto riguarda la quantità di interferenza necessaria per mantenere le sedi nelle teste in alluminio.
Una paura comune espressa da molti costruttori di motori è la preoccupazione per la possibilità che i sedili cadano, in particolare nelle teste in alluminio dove la differenza nei coefficienti di espansione termica tra la testa e i sedili può causare l'allentamento dei sedili se la testa si surriscalda.
Di conseguenza, i ricostruttori del motore hanno espresso opinioni diverse sul fatto che il composto di bloccaggio e/o la pallinatura o il picchettamento debbano essere utilizzati come "assicurazione" quando si installano sedili in teste di alluminio. Un punto su cui tutti sembrano essere d'accordo è che le sedi delle valvole svolgono un ruolo fondamentale nella longevità delle valvole. Le sedi allontanano il calore dalle valvole e lo conducono nella testata.
Ciò fornisce la maggior parte del raffreddamento che le valvole ricevono ed è assolutamente fondamentale con le valvole di scarico. Tutto ciò che interferisce con la capacità della sede di raffreddare le valvole (come un accoppiamento allentato o depositi tra la sede e il suo controforo) può portare a guasti prematuri delle valvole e costosi ritorni. Anche la lega e la durezza della sede devono essere adattate all'applicazione e compatibili con il tipo di valvole installate nel motore.
Again, we found differences of opinion regarding the selection and use of various seat materials. To better understand the issues behind the differing opinions regarding valve seat replacement, let's start with the seats themselves and why they fail. WHY VALVE SEATS FAIL Nonintegral valve seats can fail for a number of reasons.
Most of the seats that end up being replaced are replaced because they are either cracked or too worn to be reground or remachined. Seats can crack from thermal stress (engine overheating usually), thermal shock (a sudden and rapid change in operating temperature), or mechanical stress (detonation, excessive valve lash that results in severe pounding, etc.).
A small amount of valve recession results from normal high mileage wear, but it can also occur when unleaded gasoline or a "dry" fuel such as propane or natural gas is used in an engine that is not equipped with hard seats. Recession takes place when the seats get hot and microscopic welds form between the valve face and seat. Every time the valve opens, tiny chunks of metal are torn away and blown out the exhaust.
Over time, the seat is gradually eaten away and the valve slowly sinks deeper and deeper into the head. Eventually the lash in the valvetrain closes up and prevents the valve from seating. This causes the valve to overheat and burn. Compression is lost and the engine is diagnosed as having a "bad valve." The seat also has to be replaced, but it many instances it may not be recognized as the underlying cause of the valve failure.
As a rule, a seat should be replaced if the specified installed valve height cannot be achieved without excessive grinding of the valve stem tip (less than .030 in.), or if the specified installed spring height cannot be achieved using a .060 in. spring shim. This applies to integral valve seats as well as nonintegral seats. The only other alternative to replacing the seat is to install an aftermarket valve that has an oversized head (.030 in.).
This type of valve rides higher on the seat to compensate for excessive seat wear or machining, and can eliminate the need to replace the seat. A seat may also have to be replaced if it is loose or if the cylinder head is cracked and requires welding in the combustion chamber area (the seats should be removed prior to welding).
One way to check a seat for looseness is to hold your finger on one side of the seat while tapping the other side with a hammer. If you feel movement, the seat is loose and should come out (so it does not fall out later!). The seats in an aluminum head may also loosen or fall out when the head is being cleaned in a bake oven or preheated in an oven for straightening. The same thing can happen to the guides.
Whether or not this occurs depends on the amount of interference fit between the seats and head. The less the interference, the more likely the seats are to loosen and fall out when the head is baked. If you do not want the seats to fall out, turn the head upside down or stake the seats prior to baking.
VALVE SEAT REMOVAL A variety of techniques are being used to extract nonintegral valve seats from cylinder heads: Some are using their bake ovens or an open flame rotisserie thermal cleaning system to clean their heads and loosen the seats in one step. With a bake oven, the heads are loaded with the seats facing down and heated to 450 degrees F.
If the seats do not fall out of their own accord, they can be easily removed while the head is still hot. Some have success using a simple pry bar to pop the seats loose (if there is enough of an edge under the seat for the bar to grab). But using a pry bar runs the risk of damaging the counterbore. Seats can also be removed if the underside of the seats are accessible through the valve ports by using a long punch to knock them out.
But again, care must be taken not to damage the counterbore. Cast iron seats in aluminum heads are also being removed by using a die grinder to cut through the seat. This relieves pressure and allows the seat to be easily removed. The danger with this technique, however, is grinding all the way through the seat and into the head. One slip can create a gouge that can be expensive to fix.
Another technique to remove soft cast iron seats in aluminum heads is to cut them out. A cutter that is slightly smaller than the outside diameter of the seat is used to machine away most of the seat. If the thin shell that is left does not break loose and spin with the cutter (which can chew up the counterbore if you are not careful!), it can be easily pried out.
This technique does not work very well on hard seats, though, because the seats are about as hard as the cutter. To remove hard seats, you can arc weld a bead all the way around on the seat. As the bead cools, it shrinks and loosens the seat. Another trick that is sometimes used to remove a hard seat is to insert a valve that is somewhat smaller than the seat in the head and then weld the valve to the seat.
The valve stem can then be used like a driver to push out the seat. Specialty tools are also available from various suppliers for extracting seats. Diesel Cast Welding of Blane, MN (612-780-5940) makes a tool that uses a collet to remove bridge pins from diesel heads and rocker studs from 350 Chevy engines. The same tool also has the potential for easily pulling valve seats out of aluminum heads.
Though not a tool for removing seats, the "Seat Ring Factory" by the K.O. Lee Company, Aberdeen, SD (800-874-9215) is a lathe for making your own seats from semi-finished nickel alloy rings ranging in size from 5/8 in. (16 mm) ID to 2-1/4 in. (57 mm) OD, and 13/32 to 1/2 in. deep.
CUTTING VALVE SEAT COUNTERBORES IN HEADS Once a seat has been removed from a cylinder head, a determination must be made as to whether or not the counterbore needs to be machined to accept an oversized seat. If the original seat was loose, if the counterbore is flared more than .001 in.
(wider at the top than the bottom), or if the difference between the counterbore inside diameter (ID) and a standard seat outside diameter (OD) is not enough to provide the desired interference fit, then machining will be necessary. Seats are available in various oversizes. But the amount of metal that can be safely removed from most aluminum cylinder heads is minimal, so the less the amount of machining that is required the better.
Cutting a seat counterbore too large or too deep may weaken the head, cut into the water jacket or cut into the adjacent seat. Fresh cut valve seat counterbores in a diesel head.
The amount of interference required to lock a seat in place depends on the diameter of the seat (the larger the seat, the greater the interference that is required), the type of head (aluminum or cast iron), the application (hotter running applications typically require more interference to keep the seats from falling out), and in some cases the type of material used in the seat itself (hard seats cannot take as much interference as softer seats).
For cast iron heads, recommendations range from .003 to .006 in. for valve seats up to 2 inches in diameter. For aluminum heads, some say more interference is needed because of the difference in the coefficients of thermal expansion between the head and seats. Aluminum expands several two to three times as much as cast iron when it gets hot, so recommendations ranged from .004 to as much as .0085 in.
interference for valves up to 2 inches in diameter. But others said seats in aluminum heads actually require less interference than those in cast iron. Ray English of Solon Ohio says he has been rebuilding aluminum heads for 15 years and has never used more than .005 in. of interference. "You really do not need…
Fonte: AA1Car.com