There are always changes going on in engine design that can drive machinists to distraction, even as they wonder at the technology behind the developments.
One of the key areas of focus for engine technology engineers at the original equipment level has been metallurgy. In the desire to find stronger, lighter and less expensive ways to build ever more powerful engines, engineers have developed tubular camshafts, practically skirtless pistons with ring lands practically at the crown, valves with multiple alloys and stems half the diameter of an HB pencil (or at least is seems that way). Engineers have even developed metal alloys that aren’t really alloys at all, called Metal Matrix Composites, because they wanted to achieve special properties of wear and expansion that normal metallurgy couldn’t provide.
Over the past few years, more and more engines contain components made from powder metal. Powder metal, as the name would suggest, is a single metal or mixture of metals in atomized form. It’s a concept that has been around for at least 40 years, but has become more popular among car makers over the past few years. Efficiency is a key reason.
Powder metallurgy is a highly developed method of manufacturing reliable ferrous and nonferrous parts. Made by mixing elemental or alloy powders and compacting the mixture in a die, the resultant shapes are then sintered or heated in a controlled-atmosphere furnace to bond the particles metallurgically. Basically a “chipless” metalworking process, it typically uses more than 97% of the starting raw material in the finished part. Because of this, it is an energy and materials-conserving process.
The process is cost effective in producing simple or complex parts at, or very close to, final dimensions in production rates which can range from a few hundred to several thousand parts per hour. As a result, only minor, if any, machining is required. Powder metal parts also may be sized for closer dimensional control and /or coined for both higher density and strength.
This lack of a need for initial machining has however led to the use of some very hard alloys that are extremely difficult to machine during the head rebuilding process. In fact, some of these latest alloys work-harden after one or two turns of the cutter blade, blunting the cutter almost immediately. In most passenger car type heads running on gasoline, these seats are overkill and can be replaced with a material with appropriate hardness, but that are much easier to machine.
Powder metal as a single term can be misleading. Using this process allows manufacturers to produce a wide variety of mixtures with different characteristics in a way akin to alloys. Alloys, and the way we refer to them, have become second nature to machinists. Powdered metal has not.
Different compositions can provide different characteristics, but in general powder metal valve seats are a combination of tungsten, molybdenum, chromium, vanadium, carbon, cobalt, nickel, manganese, silicon, copper, iron, and other proprietary components. The precise mixture of each component will vary by the characteristics of hardness and thermal expansion, typically extremely low, that is required by the application.
The end result has, however, yielded very positive results for machinists.
“We handle a line of guide and seat machines,” says Mike Howden of S&M Distributing in Port Coquitlam, B.C. “and what we’re were finding is that when you cut the powder metal seats using carbide tools, they come out as just a gorgeous looking seat.”
That, he says, helps him sell the benefits of the machines, and it’s not hard to see why it could also help machine shops sell the quality of a job. He says too that the failure rate is virtually zero–only one in a dozen years that he still insists was a goodwill replacement–and wear rates are very good, particularly on alternative fuel applications.
He remembers one customer who worked on forklift engines. “Many customers just wanted to fix the bad seat and get the head out.” Howden says a powder metal seat was in-stalled, but that when other seats failed the customer sent the head to another shop that put in more conventional materials. Then, six months later, the head was back suffering from the failure of that second set of seats.
“The powder metal seats that were six months older weren’t punched out. They were still perfect seats,” says Howden.
Cutting, or grinding, powder metal seats does require a different touch, though.
Gary Swan, machine shop manager of the Lordco cylinder head division in Mission, B.C., says that it is hard to describe the feel required to get the best result, but he did his best to do so for us.
It should also be noted that his technique has been developed from experience and may differ from that found in manufacturers’ literature, which may be based on using other equipment. It is recommended that any reader consider this when developing a procedure that works for them and the equipment they have.
Swan says that he’s come around to powder metal seats over the years, and now prefers to use them. An early dislike was the result of trying to run tooling too fast and burning up cutters as a result.
“I find the that the powder metal makes a really nice finish. On the regular seats, I can almost get a ragged finish, it can almost look torn.
“On powder metal, I like to run the speed a little bit slower than for standard seats. If the chips are coming off blue I try to turn it down a little bit; in the range of 400 rpm on a Serdi seat and guide machine, whereas it’s at 500 to 600 for a standard seat.
“That can change too because a lot of it is a feel thing.” The same applies to how fast you feed the tooling in. Feed speeds need to be a bit greater on the powder metal seats, he says.
“If you’re getting chatter, you want to make sure that the head is in there tight and the tool holder is in good shape, but if you’re getting a bit of chatter anyway, you might be feeding it in a bit lightly. You need to put in a little bit more on the powder metal, though I like to use a lighter touch than some on the regular ones because it makes the finish better.”
For those who prefer grinding, Swan says that the best route can be a matter of opinion. “You have to use stones that are harder than conventional ones. Some guys like ruby, some guys like nickel-chrome, some like stellite.” On some OEM seats he will use a stone to touch up the finish after cutting because they are extremely hard even compared to aftermarket powder metal seats, but prefers cutting seats as a rule.
“What I find is that powder metal seats always produce a really nice quality finish. You get good definition between the angles with no lines running through. On some seats it’s hard to see where your 30 degree and 45 degree start, but there’s always good definition on the powder metal.”
For best results, he recommends paying attention to the basics: a good sharp cutter and proper tool maintenance. “It will affect what the customer sees, and if there are any deficiencies in the cutter, you’re going to see it more readily.” He says, though, that tool wear has not been a problem since he learned to adjust his cutting technique.
Swan says that today he is completely over his initial uncertainty about working with powder metal seat materials. In fact, he prefers them now.
“If I can get a powdered metal seat, I will use it.”
Special thanks also to the people at Dura-Bond Bearing for their assistance with this article.
Advise Technicians to Use Caution
The powder metal process yields extremely strong components–due to the very fact that the production process permits very accurate and consistent densities within the part.
However, powder metal components, like all highly stressed parts, have been known to fail though rarely, and usually after other components go first–belt breaks, piston breaks valve, valve breaks seat–but they can do so in a particular way. Rather than bending and balling up into tiny bits, like a few connecting rods I once knew, they can seemingly revert back to t
heir powder form.
The Automotive Engine Rebuilders Association even produced a tech bulletin (TB 1939) regarding valve seat failure on 1997-2001 Ford 2.0 L VIN P Engines. The powder metal seats, says the bulletin, can shatter and leave pieces in the intake and exhaust manifolds. The pieces are small and can go unnoticed if the technician is used to pulling back the intake and exhaust manifolds just enough to allow the head to be removed.
Without proper inspection and cleaning, these small fragments can easily find their way back into the engine and cause a repeat engine failure.
Powder Metal on the Market
There are a few suppliers of powder metal valve seats, but they don’t necessarily identify these parts using that term; sometimes the term sintered valve seats is used. Here is how three suppliers indicate which of their offerings are powder metal.
Dura-Bond: This company’s powder metal offerings are its 30000 (Gold) Series for light to medium duty applications, 70000 (Diamond), for high temperature, alternate fuel applications like propane, LPG and natural gas, and 90000 (Platinum) for extreme duty applications.
S.B. International: This company uses the suffix of “PM” on its valve seats using powder metal technology.
Qualcast: This company refers to its powder metal valve seats under the series name JP345.
Other companies may also supply powder metal offerings.