The OBD II Shop: It Is Necessary To Repair The Total Valvetrain System, Not Just The Parts
By Norm Brandes
Pull a cylinder head from an engine and what do you see? If you answered "the head, valves, seats, springs, guides and retainers," you’re wrong. Instead of individual parts, you’re looking at the engine’s valve control system.
Obviously, since the first four-cycle internal combustion engine, these parts have always worked together. However, the old term "valvetrain" was a good description of the relationship among the components. The individual parts were essentially coupled like cars on a train with the camshaft working as the train’s engine to keep everything moving. Unless you made a drastic change, such as putting in a full competition camshaft without doing anything else to the head or engine, changing parts was like switching cars on a train. After everything was coupled together again, the train was ready to go.
In modern engines, specifications and tolerances are tighter than ever before. The relationship among the parts is truly a system. Change anything from OE specification and you change the entire system. Sometimes the overall change is minor, but other times your modification causes major repercussions.
Change the system
Once you make a change from OE spec, you will be married to that change for as long as that engine is in use. No one is saying you can’t change the original valve system components or specifications. However, before you make a change, either by parts replacement or machining, you have to ask yourself what’s going to happen to the system. If you don’t ask this question, you can be unpleasantly surprised by unexpected wear or other performance problems.
As an engineer with one of the leading valve manufacturing companies put it, "any change modifies the whole system." All the components in the system must be matched to maintain emission control, performance, fuel economy and longevity. If you only change one component, you may introduce added stress in the system. Added stress always goes to the weakest point of the system and causes rapid wear or other problems at that point.
The danger of finding the system’s weak point is greater than ever because modern heads and valve control systems are pushing the limits of strength/ weight ratios and dimensional stability. The systems don’t have much reserve to compensate for additional load stress.
Heads are so sensitive that routine machining practices are no longer routine. On the Chrysler 2.0L, 4-valve, DOHC heads, the company requires that a torque plate and the rocker arm assembly be installed before doing any valve work. The torque plate must be at least 1/2" thick. Although using torque plates has been a standard practice for many years when doing block work, they may now be needed for heads as well.
Surprisingly, changes not directly involving the valve control system can cause problems. Recently, one of the major automakers, during pre-production engine testing, changed the ECM programming to reduce emissions. After the change, durability testing showed accelerated valve seat wear. The computer had to be reprogrammed again to keep the improved emission control, but avoid the valve seat wear problem.
Even a modification you think is beneficial can cause a comeback. Beef up one part of the system, without strengthening other components, and unexpected problems can develop.
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 a worn/ snapped timing belt because those heavier springs placed added stress on the valve control system.
Instead of helping your customer 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.
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 a 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 castings 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, chances are good 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.
Accelerated wear or other valve control system problems are also likely because the replacement valves may not be properly lubricated. The OE engine oil system was designed to work with a lubricant impregnated into the powdered metal guide. The replacement valves and guides may be starved for oil. Button up the job, send the car out the door and a few thousand miles later your customer is back with a valve control system that’s a disaster. By traditional rebuilding standards, you didn’t do anything intentionally wrong. You were just blindsided by installing a part that fit, but which didn’t meet OE specifications.
Learning to cope
What must today’s shops do to meet the challenges of tomorrow’s valve control systems? Aggressively seek 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 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 suppliers who offer guarantees. Be very careful using second tier and loss leader suppliers. Even with quality parts, when you replace a component, save the OE parts and compare them with their aftermarket replacements. Do everything you can – measure, weigh, test — to make sure the replacement matches the OE.
No one is saying that you can’t make any changes in heads and valve control systems. What’s maddening to me is that engineers tell me minor changes are probably alright. I’m still trying to find someone who will define minor in terms I can measure. Until standards are finally determined regarding how much out-of-spec a rebuilder can modify the valve system, be conservative and cautious on any changes you decide to make.
No production process is perfectly exact; all automotive systems must work within the system stack tolerances. The range from all components being as small, weak or tight as possible to as large, strong or loose as possible is system stack tolerance. As long as your valve system work falls within stack tolerances you should be fine. The catch is that stack tolerances are not available to the aftermarket. Some production rebuilders, working under contract with the OE manufacturers, get stack tolerance specifications, but I have found no source that provides this information to the aftermarket rebuilder.
Fifteen to 20 years ago, suppliers offered packaged kits that bundled valves, guides, springs, etc. The idea didn’t catch on. Rebuilders didn’t want to be considered as parts changers, and shops didn’t want to lose the income from machine work. I think it’s time to reconsider the kit concept. The engine builders designed and specified the parts to work together as a system. I think the aftermarket should have the same attitude toward replacement parts.
Adopt a system concept in all your work. Don’t just look at the obvious problem and fix or replace the damaged components. Always ask yourself what caused the damage or wear, and make sure you’re correcting the problem, not repairing the symptom.
The geometry of modern heads is getting tighter and tighter. It’s becoming increasingly difficult to take accurate measurements, and even more difficult to do accurate machining. If your shop doesn’t keep pace with modern equipment, fixtures, measuring tools and new techniques, you may very well be left behind.
Norm Brandes owns and operates Westech Automotive, Inc., a machine shop and vehicle repair service business located in Silver Lake, WI.