Opening and closing the intake and exhaust valves in precise synchronization with the up and down strokes of the pistons requires very accurate timing. At idle, the time interval between valve openings for each cylinder is about a fifth of a second. At 5,000 rpm it is about two hundredths of a second.
In a four-stroke engine, the intake and exhaust valves open and close every other revolution of the crankshaft, so the cam only turns at half engine speed. That is why cams have big gears on the end and crankshafts have little gears. The drive ratio is two to one, so at 3000 rpm the cam is turning at 1500 rpm.
The instant at which the valves opens and closes affects engine performance, fuel economy and emissions, so accurate timing is essential. On the intake side, valve timing not only determines how much air/fuel mixture is drawn into each cylinder, but also how efficiently the mixture is used. When the piston starts down on its intake stroke, the intake valve must open quickly so that a full cylinder worth of air/fuel mixture will be drawn in, a problem that becomes more acute as engine speed increases. If the valve doesn't open soon enough, there may not be enough time to fill the cylinder completely before the piston starts back up and the valve closes reducing compression and power. If the valve remains opens too soon, the piston may still be completing its exhaust stroke which would push exhaust back into the intake manifold interfering with efficient engine breathing.
On the exhaust side, timing is equally important. If an exhaust valve opens too soon, the still expanding gases can escape from the cylinder prematurely, wasting power. If the exhaust valve opens too late, the engine will also have to work that much harder to pump the remaining exhaust gases from the cylinder. And if the exhaust valve remains open too long, there will be excessive "overlap" with the opening of the intake valve excessively allowing unburned fuel to be drawn right through the engine.
Accurate valve timing is also essential for another very important reason, too: on most engines the cam also drives the distributor, or on engines with distributorless ignition systems the cam position sensor. This, in turn, affects ignition timing and fuel delivery on engines with sequential fuel injection.
In pushrod engines the camshaft is located in the engine block, while in overhead cam engines the cam is mounted in the head, either directly atop the valves or offset to one side. An OHC engine may have a single cam for both the intake and exhaust valves, or separate cams (dual overhead cams or DOHC).
Almost all in-block cams are driven by the crankshaft via a gear and chain set, or in some in-line four and six-cylinder engines a pair of direct drive gears. In overhead engines, the cam or cams are driven by a toothed belt or roller chain. The cam (or cams) always turn at half engine speed so the drive sprocket will always have twice as many teeth as the gear on the crankshaft.
As chain driven cams accumulate mileage, chain stretch and gear wear introduce slop into the system. Most chains will go up to 100,000 miles or more, but not always. As a rule, when there is more than about half an inch of play between the gears on a V6 or V8, it is time for a new chain and gear set (always refer to the vehicle manufacturer's specifications for maximum acceptable chain play). Don't reuse a high mileage timing chain if you are rebuilding an engine. Replace it with a new chain and gear set.
Another problem that can sometimes occur is failure of OEM aluminum cam gears with nylon gear teeth. Molded nylon teeth are used by some OEMs to reduce noise. In spite of improvements that have been made in improving the durability of such sprockets, most engine rebuilders are leery of them because of their past reputation for early failure. Overheating can cause the nylon to become brittle and crack loose from the gear. The debris usually ends up in the oil pan where it may clog the oil pump pickup screen and starve the engine for oil. Replace the old plastic timing gear with a more durable iron or steel gear.
Historically, cam sprockets have been aluminum or cast iron while crank gears have been steel or powdered metal. But now many new engines have cam sprockets and crank gears which are both made of powdered metal. The OEMs say powdered metal is as durable as steel, is lighter and is easier (cheaper) to manufacture. And so it is. Consequently, many aftermarket replacement gears and sprockets will soon be made of powdered metal, too, instead of cast iron and steel. Suppliers say the new powdered metal components will be introduced into the aftermarket within the next couple of years.
Another change in some engines today is magnetic timing sensors on the cam sprocket. A magnet mounted on an aluminum cam sprocket passes under a pickup coil that generates a signal to the engine computer. This keeps the computer informed about the firing order of the engine so it can control ignition timing and in some cases sequential fuel injection pulses accordingly. The OEMs say only aluminum replacement sprockets should be used in such applications. But according to the suppliers we interviewed, it is okay to use a cast iron replacement sprocket as long as the sensor magnet is correctly mounted on the sprocket when it is installed. The cast iron sprocket will not affect the magnetic properties of the sensor. If the engine is equipped with a camshaft thrust button assembly and it is not reinstalled (or is weak) however, the sensor magnet may make contact with the sensor on the front timing cover causing a no-start condition.
Timing chains have also undergone some major engineering changes in recent years. Historically, the domestic OEMs have used an inverted tooth or "silent" type of timing chain. Most European and Japanese OEMs, on the other hand, have used a British Standard (BSI) roller chain (similar to a bicycle chain).
The domestic OEMs have preferred the silent chain design for V6 and V8 applications because it provides a very smooth, quiet drive. Tooth links engage the cam and crank sprockets almost like a flexible gear. Most older silent chains were the "rigid back" or "stiff back" design which allowed the chain to flex only one way. Most silent chains in newer engines are now "fully flexible" design that allows the chain to bend both ways. The fully flexible design is easier to install and is somewhat more durable than the stiff back design.
Some of the newer domestic engines with overhead cams, such as Ford's modular 4.6L V8, are using American Standard (ANSI) roller chain. Unlike BSI roller chain which has been used on most import applications, ANSI chain does not have a freely rotating roller, only the fixed bush. The bush is larger and stronger than that used in BSI chain, however, making it more suitable for heavy-duty applications.
Double roller chains have traditionally been considered an "upgrade" for replacing silent chains in performance applications. Roller chains perform better in such applications because they are lighter, more durable and can handle higher rpms than a silent chain.
Some aftermarket double roller chains also come with an offset cam sprocket so the cam can be advanced or retarded as needed to dial in the engine. Advancing cam timing up to several degrees is a common trick that improves the low end torque and throttle response characteristics of performance cams in street-driven engines. To dial in the cam, a degree wheel is needed to index the cam to the top dead center position of the number one piston. Most aftermarket street performance cams come with 4 degrees of initial advance already built-in (a fact which must be taken into account when degreeing in the cam). Using a degree wheel to verify correct cam timing in a performance engine s a good idea anyway because there can be errors in both cam and crank indexing from the factory. An offset bushing, keyway or crank/cam sprocket can be used to correct any errors that are found.
Contrary to what you might think, rubber timing belts do not stretch with accumulated mileage and wear. They are reinforced with strands of fiberglass which makes them virtually unstretchable. After making the crankshaft to cam drive circuit millions of times, the strands can become brittle and may begin to break. Eventually the reinforcing cords give way, the belt snaps and the engine quits. If the engine lacks sufficient valve-to-piston clearance to free wheel under such circumstances, a lot of expensive damage can result.
As a rule, most OEMs recommend replacing OHC rubber timing belts at around 60,000 miles as preventative maintenance to avert the kind of trouble just described. But there are exceptions. Some, such as Porsche, recommend belt replacement at 45,000 mile intervals on their 2.5L, 2.7L and 3.0L four cylinder engines. Volvo says the timing belt on 1992-93 240, 640 and 940 models with the B230F and FT 2.3L engines should be replaced at 50,000 miles, but allows up to 100,000 miles between changes on the B230FD version of the 2.3L engine. Acura and Audi both allow up to 90,000 miles between belt changes on most of their engines, while Chrysler says 90,000 miles is okay only for certain 1991 and up 2.5L engines. Ford, Mercury and Toyota, all allow up to 100,000 miles between belt changes, but only on their four-cylinder diesel engines.
The OEM recommendations for belt replacement vary because they are based on the type of belt used, the engine application (belt tension, belt length, number & size of pulleys, belt loading, etc.), and the "average" service life of the belt.
Changes in belt materials in recent years have improved belt durability to 100,000 miles plus, but only on those applications where the new "long life: materials are used. These belts are made of a special high temperature grade of neoprene called "highly-saturated nitrile" (HSN). HSN is rapidly becoming the OEM material of choice for new engines that use timing belts because of its significantly longer service life which reduces or essentially eliminates the need for periodic belt replacement. This change may have helped thwart a trend of going back to timing chains instead of belts in new OHC engine designs. The Olds Quad 4, Ford 4.6L V8 and Cadillac Northstar V8 all use chains instead of belts. But other new engines continue to use belts. And according to at least one OE belt supplier, most of the new OHC engines that will be introduced over the next few years will have timing belts rather than chains.
Can you buy belts made of the superior HSN material for aftermarket replacement applications? Yes, but primarily for the newer engines that came originally equipped with HSN belts. To date, belt suppliers have not "retooled" their product lines to make HSN belts available for older applications primarily because of HSN's higher price. The feeling is that ordinary neoprene is perfectly adequate for most replacement applications except on the newer engines that require the higher temperature long life belts.
One thing you should be cautioned about is trying to use OHC timing belts in applications they were not specifically designed for. In other words, unlike V-belts or in some cases serpentine belts, timing belts are usually not interchangeable. Though two timing belts may look pretty much the same (same length and width), the tooth profiles may not match. There are trapezoidal, curvilinear and modified curvilinear tooth profiles, none of which are interchangeable on the same pulleys. Use a belt with the wrong tooth profile and the engine will eat the belt in nothing flat. So follow the belt supplier's applications in their catalog.
Belt length is especially critical because of the limited amount of adjustment on both manual and automatic tensioners. A belt that is too short or has too much tension may fail or cause accelerated wear in the OHC cam bearings, front main bearing and/or water pump shaft bearing. A belt that is too long or is too loose may jump timing.
One thing all the belt suppliers agree upon is that you should always replace the timing belt when rebuilding an engine. The risks are too great to reuse a belt even if it looks good because you can't judge a belt's true condition by external appearances alone. Any belt that shows obvious damage such as frayed or exposed cords, damaged teeth, hunks of rubber missing, deep cracks, excessive surface cracking or severe glazing needs to be replaced. Small surface cracks on the ribbing is considered normal. Even so, a belt that looks good as new on the outside may be dangerously weakened inside and on the verge of failure. So always replace the belt with a new one, and be sure to inspect the alignment and condition of the pulleys (no nicks or rough spots that could chew up the belt).
The cost of new belt is only 5 to 10% of the cost of replacing it if it fails after the engine has been installed in a car. And if the engine is not one that can free wheel if the timing belt snaps, your warranty costs will far outweigh any savings you might have gained by reusing the old belt.
As we said at the beginning of this article, a stretched timing chain (or worn gears) will affect both valve and ignition timing. If the wear is severe and the chain (or a belt) jumps timing, the engine may run rough and/or possibly backfire, or it may not run at all. By the same token, if someone makes a mistake when assembling the engine and gets the timing off a tooth or more, the engine may start but won't run well because of advanced or retarded cam timing. The same can happen with pressed on timing gears that are not properly aligned and OHC timing belts. If the timing is off by more than two teeth (the equivalent of eight or more degrees, depending on the application), the engine may not start or run. A broken chain (or belt) would not allow the engine to start at all, and may result in valve damage if there is not enough clearance between the valves and pistons.
One way to tell whether or not a cam is turning is to remove the distributor cap and crank the engine (a procedure which is NOT recommended on any engine that lacks sufficient valve-to-piston clearance!). The rotor should turn if the cam drive is intact. If the rotor doesn't move, the timing chain (or belt) is broken.
Another way to diagnose a broken timing chain or belt is to pull the valve cover and watch the valves while the engine is cranked. No movement means the cam is not being driven.
Yet another way to diagnose this kind of trouble is to check compression while cranking the engine. No compression means the valves are not opening and closing.
On vehicles with computerized engine controls, a failed cam drive may trigger a "no ignition pickup signal" fault code.
Timing gears and chains usually give some advance notice before they fail (but not so with belts!). Noise from inside the timing chain cover is a good indication that there is too much slack in the chain. Another way to spot excessive play in the timing chain is to remove the distributor cap and turn the crankshaft in one direction until the rotor moves, then turn it in the opposite direction until the rotor starts to turn the other way. If the crank has to be turned more than about half an inch to move the rotor, chances are the timing gears and chain need to be replaced.
Timing gears and chains should always be replaced as a set.
It makes no sense use worn gears with a new chain or vice versa. Timing gears always come as a matched set and must likewise be replaced as a set. As for timing belts, replacement of the pulleys usually isn't necessary unless one shows signs of unusual wear or is damaged.
When installing a timing chain and gear set on a pushrod engine, the new crank gear goes on first, followed by the cam gear with the chain on it. On OHC applications, there is no set procedure since chain tension is controlled by a tensioner or guide.
With belts, it is simply a matter of snaking the belt around all the pulleys (making sure the belt is routed on the correct side of any idler or water pump pulleys!), then adjusting tension if a manual tension adjustment is required. Tension must be within the manufacturers specifications because too much tension is not good for the belt or cam bearings, and too little tension may allow the belt to jump timing.
Never attempt to stretch a rubber timing belt over pulleys as this can damage the belt. It is important to always replace belt covers because the covers protect the belt from grease and dirt. They also help keep fingers and other objects out of harms way.
Another point worth noting about timing belts: Milling the head on an overhead cam engine lowers the head with respect to the block. Unless compensated for by a thicker head gasket, a head gasket shim or an offset key on one of the timing sprockets, cam timing will be slightly altered. In days gone by, the variation may not have been enough to worry about. But on today's emission controlled engines, even a little variation may create emission problems.
On many OHC engines, there are additional components that should also be replaced when a new timing chain and gear set is installed: chain tensioners, guides and/or rails. These components all play a vital role in supporting the chain as well as keeping it taught, so do not overlook these items.
One of the most common installation errors, according to timing chain suppliers, is misalignment. This can be caused by something as simple as installing the cam sprocket backwards, using the wrong thickness of washer under the sprocket, failing to press a cam gear all the way on, etc. If the cam and crank sprockets are not perfectly aligned, the result will be rapid chain and sprocket wear, or interference problems.
When installing sprockets on the cam and crank, do not hammer directly on the sprockets or chain. Press both sprockets on evenly, keeping them parallel. This prevents stretching or damaging the chain or sprockets.
On some applications, a separate spacer that goes behind the cam sprocket is used to control end thrust. Replacement sprockets for some of these applications have the spacer built onto the backside of the sprocket, which means you should not reuse the old spacer.
Press fit gears (except fiber gears) should be preheated to about 250 degrees to make installation easier. Attempting to press on a cold gear may ruin the interference between the gear and shaft, causing the gear to work loose later.
It is a good idea to double check the alignment of all timing marks on every engine application to make they are right, especially on engines with balance shafts. This can be tricky on some engines (especially certain imports) where multiple sprockets and/or multiple timing marks are involved. One timing chain manufacturer, for example, now includes a timing schematic for every application in their catalog.
On 1990-93 Subaru Legacy 2.2L engines, for example, each of the engine's OHC pulleys and crank pulley has an mark and an arrow. The trick here is to align the mark (not the arrow) on each cam pulley with a notch in the head, and the mark on the crank pulley with a mark on the block.
AERA bulletin #796 describes a common problem that is encountered on 1979-85 Mazda 2.0L engines. Instead of aligning the camshaft and crankshaft sprockets with marks on the head or block, the sprockets are aligned with marked links on the timing chain. When the chain is installed, the crank and cam have to be rotated until the keyways are facing straight up (12 o'clock). This puts the marks on both sprockets at approximately the 5 o'clock position. When the chain is then installed (which requires removing the cam sprocket first), the chain is aligned so one marked link lines up with the mark on the cam sprocket, and two marked links straddle a dot on the crank gear. If properly installed, there will be 20 unmarked links between the two positions.
Other excellent sources for OEM technical service bulletins include All-Data and Mitchell's electronic data bases. Both are available in CD-ROM format for quick and easy access. All-Data also offers a unique "phone in" service that allows anyone with a computer and modem to download TSBs from their extensive data base.
Having access to factory TSBs is becoming more and more essential for engine rebuilders who want to "correct" OEM problems when they rebuild an engine. All-Data bulletin #376113, for example, covers a noise problem on 1992-93 Buick Skylarks, Olds Achieva and Pontiac Grand Am models with the Quad 4 engine. A whine noise between 2000 and 3000 rpm is caused by the design of the OEM crankshaft gear. GM has released a revised gear (P/N 24574600) that eliminates the noise. If you didn't know this and either reused the old gear or replaced it with the wrong one, you'd have a noise problem and a possible warranty comeback.
Another All-Data bulletin, #90-T-156, covers a rattling noise problem upon start up in the Quad 4 on 1987-90 Olds Cutlass Calais and Cutlass Supreme models. The noise is caused by oil draining out of the filter after the engine has been shut off. This causes a delay in oil pressure when the engine is started, which delays the extension of the timing chain tensioner plunger. The noise will not hurt anything, but may not be acceptable to a customer either. The cure? Use an oil filter with an anti-drainback valve (AC Type PF1225 or equivalent).