For example, when reworking a set of heads, suppose you install stronger valve springs without making any other changes. Stronger springs should boost performance slightly by providing a better combustion chamber seal and reducing the risk of valve float. The fact that you initially boosted performance slightly probably won't do much to pacify your upset customer when a few thousand miles later, he returns with worn valve stem tips, accelerated valve seat wear, eroded camshaft lobes or worn/snapped timing belt because those heavier springs placed added stress on the valve control system.
Instead of helping your customers by installing better springs, you're now looking at an expensive comeback for not only your original work, but also for replacing parts that were never included in your original invoice.
Unknown Mistakes
Even worse, you may face a comeback caused not by your deliberate action, but by an unintentional mismatch of parts. More than ever, today's engines force you to rely on top quality aftermarket suppliers who warranty their parts and meet all OE standards.
Some new engines use flex valves and powdered metal seats and guides. A powdered metal part is made by literally reducing a metal alloy to powder and pressing/molding the alloy into the desired shape. Powdered metal parts are less expensive, they weigh less and can be shaped to tighter tolerances. They also give engineers more options for using alloys because metals used for casting or forging must be alloys that can be poured or shaped. Powdered metal alloys can use almost any mix of metals.
Powdered metal parts can also include lubricants blended into the alloy, thus reducing the need for oil lubrication. A powdered guide, for example, can have tighter valve stem-to-guide clearance because less engine oil lubrication is needed compared to a conventional valve and guide.
Suppose that you replace worn valves and guides, not realizing you're dealing with new alloys. To improve your profit margin, you select the lowest priced replacement parts you can find. Unfortunately, the aftermarket valve guides are casting and the replacement guides are not powdered metal. You also don't order new springs because the original units still look good. Even if the replacement parts fit into the head, changes are food that engine performance will suffer and wear will increase. The heavier valves will have a greater tendency to float because the OE valve springs were designed for a lighter weight valve.
Learning to cope
What must today's shops do to meet the challenges of tomorrow's valve control systems? Aggressively seeks out all the training and information you can get. If your parts supplier, trade association or local vo-tech center aren't supporting your shop with training, start complaining or switch. Too much is happening too fast in this industry to become complacent on training.
Prepare yourself for change. Old time practices can cause new problems. For example, many new valves feature a thin, hardened coating on the stem tip. If you grind the tip to restore the stem height after cutting the valve seats, you'll remove the hardened surface and probably change tip contour. Without that hardened tip or with a changed contour, the valve stems will quickly wear.
On any engine with two heads, avoid machining only one head. Whatever you do on one side, do on the other. Even cleaning up one head can cause a minor change in compression ratio or compression pressure between the two sets of cylinders. Today's computers are sensitive enough to detect the difference and will respond by setting a trouble code for a misfire.
You should stick with the quality parts supplier who offers guarantees. Be very careful using second tier and loss leader suppliers. Even with quality parts, when you replace a component, save the OE
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