Learning how to machine, install and grind valve seats, can help solve many, valve leakage and valve burning issues.
Firstly, the growth of the valve seat market, can be traced back to the early 1970s.
Because, this is when the switch to, unleaded fuel took place. Consequently, most of the engines in use, had to have replacement valve seats inserted.
However, valve seats were only installed in the exhaust side, as that was where the most heat was created.
Firstly, this was to prevent, valve seat recession. But, engines running on unleaded gas, were more affected. Many people thought, that lead was a lubricant and somehow prevented wear. In fact, the lead caused a chemical reaction, with the cast iron cylinder head and the stainless steel valve. Consequently, forming oxides and halides, that locally hardened the wear surfaces.
Local hardening, is what actually helped. As a result, preventing seat recession. Then, vehicles running on leaded fuel switched to unleaded fuel. The initial use of leaded fuel, had created the local hardening required, making the switch to unleaded fuel easy.
However, if these same heads were then reconditioned, the machine shop would machine away the protective layers. As a result, valve seats recession would occur very rapidly, sometimes in as little as 3,000 miles.
The (OEM)s used an induction hardening technique, to locally harden the valve seats areas. The depth of hardness was about .070”. Unfortunately, it was not deep enough, for re-machining, during cylinder head rebuilding. As a result, these early unleaded fuel heads, needed to have exhaust seats installed.
The Switch To Aluminum Cylinder Heads
The cylinder heads today are mostly aluminum. With the exception of diesels and truck engines. These heads have inserts already fitted at the factory. Meanwhile, this has contributed to the growth in the seat insert market, at the (OEM) level.
When the time arrives for these heads to be rebuilt, they are often cracked around the valve pocket areas. But, before welding the cracks, the valve seats must be removed.
New Valve Seats Materials
The growth in the (OEM) seat market, has led to the widespread use of, powder metallurgy. Allowing (OEM)s to produce inserts in large volumes. These valve seats are engine specific and can almost exactly replicate, the heat transfer characteristics of the parent metal.
The use of powder seats, demands very large production runs, to justify the tooling costs. But, using powder does produce a part that is very close, to finished size. Requiring very little machining.
This lack of machining, has led to the use of some very hard alloys, that are extremely difficult to re-machine.
In fact, some of these latest alloys work-harden, after one or two turns of the cutter blade. The result is, blunting the cutter almost immediately. In most passenger car type heads running on gasoline, these seats are overkill.
Upgraded Materials For Diesels With Higher Operating Temperatures
These upgraded materials are often nickel or cobalt based and come with, a corresponding increase in cost. The composition of these nickel based alloys, is about SAE610b, numbers 11, 12 or 13 compositions. These seats are capable of withstanding, higher operating temperatures and higher levels of corrosion.
And, would end up being a common replacement, in (LPG) type engines. Gasoline leaves behind an ash content that acts as a lubricant, between the valve face and seat insert. (LPG) type fuels burn very cleanly, but this ash content is missing.
Above all, (LPG) engines have to have the correct insert, to avoid failure. Very often the valve material must also be changed, to provide good service life in these applications. The last series of materials are, the cobalt or stellite-based alloys. And, most of these are application specific.
A good example of this is, the Cummins K Series engines. The intake valve in the premium engine, is made from tribaloy. And, must be run, with a tribaloy seat insert.
These alloys have hardness values, around 50 to 55 (HRC) and maintain higher hardness, at elevated operating temperatures. And, tribaloy is abrasion resistant. But, tribaloy also costs more money to produce. Because, it contains about 30% chromium, also known as Stellite. These seats are normally the hardest to machine, of all the seat alloys used in the replacement market.
Press Fit And Surface Finishes
The powder (OEM) seats, are often made of a material that matches, the expansion rate of the parent material. For this reason, they often have press fits of about .003″; but can be as low as .002″. The replacement cast seats, however, need varying press fits, to prevent them from falling out during heat soaks.
Most aftermarket seats need about .005″ press, when installed in iron heads. And, about .007″ press when installed in aluminum heads. Seat suppliers usually build the required press fit into the O.D. of the seat. So, a 1.500″ O.D. seat will measure 1.505″ for cast iron applications and 1.507″ for aluminum heads.
NOTE: Always use the press fit recommended by your seat supplier, not the value listed in (OEM) manuals.
Selecting a seat by size only, could create a problem in obtaining, the correct interference fit. If the fit is too small, problems can happen. Also, oven cleaning can cause huge problems. It is not uncommon for seats, to fall out during the heating process.
Cleaning the cylinder heads upside down, is a preferred process. Doing this will help to prevent these type of problems from occurring. Most valve seats have a 15 Ra surface finish. The finish in the counterbore should be, equally smooth and round to within .001″ T.I.R. This will ensure a good contact area and excellent heat transfer properties, for the valve to operate against.
Valve Seats Cutting Techniques
More and more shops are changing to, seat cutting equipment to replace, their older grinding systems. To ensure good tool life with these systems, it is necessary to keep close control over, feed and speed rates. Always adjust spindle speeds, when changing from, intake to exhaust valves. Especially, where there are large diameter differences. Because, the cutting speed increases with the increase in the diameter, from the exhaust to the intake side.
Generally speaking, uncoated carbide inserts, work best for seat inserts. A sharp cutting edge (no hone) on the uncoated carbide, will provide lower cutting forces overall. So, C2 grade carbide can provide satisfactory results. But, C4 carbide, will provide the best overall tool life. Finally, check with your tool supplier, for availability of both these grades.
Carbides used for steel (grades C5 to C8) do not work well, with valve seat insert materials. As a result, you will have increased productivity, by using ceramics. Ceramic cutters, also will provide excellent results, on iron-based materials.
There are three main requirements to be aware of, when doing a valve job:
- Seat width.
- The seat angle.
- Seat runout.
Valve Seat Width
The seat width is important because, about 70% of heat transferred from a valve, goes out the seat contact area. The old rule of thumb used to be, to try to maintain a seat width, of about .070″. Today’s engines, have valves that are so thin, it is impossible to locate a seat that wide, on the valve. It is important to remember that valve seat width problems, show up on the valve and rarely, burn the seat.
Valve Seat Angle Mistakes
The seat angle, is also very important. Seat angles are responsible, for most mistakes. This happens on the 6.9/7.3L Navistar, more than any other engine. The mistake made, is to cut the exhaust seat at 30 degrees, instead of 37.5 degrees. Because, reducing the contact point, will burn out the valve. Also, keep in mind that tool holders, do wear out. Consequently, allowing the cutter to tip, during operation. The runout requirement is generally, between .001″ and .002″. The larger the valve head, the more runout allowed.
Valve Seat Runout
Excessive runout will eventually break the valve head off, at the under-head radius. This is due to the flexing that occurs every time the valve, opens and closes against the seat. The most common causes of excessive runout are, a loose fitting pilot and the condition, of the machine spindle bearings.
So, valve seats must to be cut, concentric to the center of the valve guide. Lack of concentricity in the valve seat itself, can also prevent the valve from, sealing tightly against the seat. As a result, causing a compression leak and a possible misfire. Finally, by applying vacuum to the intake and exhaust ports, valve-to-seat sealing can be confirmed.
Thank You !