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Timing Chains & Gears
Ever wonder why some engines have a timing belt while others have a timing chain?
By Larry Carley
As a rule, most pushrod engines use a timing chain to drive the camshaft, though some older four and six cylinder engines use a gear set. A short chain or gear set works well in this kind of application because the camshaft is located in the block just above the crankshaft. In overhead cam (OHC) engines, the camshaft is located high in the cylinder head far from the crankshaft. This requires a much longer chain or a rubber belt to drive the camshaft. In some engines, such as a Chevy 3.4L DOHC V6, a timing chain is used to drive an intermediate sprocket that drives two belts (one for each cam).
Timing belts and chains carry a heavy load. They have to turn the cam with enough force to overcome the resistance of all the valve springs. Belts are made of synthetic rubber reinforced with tough fiber cords that provide tensile strength and prevent the belt from stretching. Chains are made of steel links connected by flexible rollers. Chains are more durable than belts and typically have a longer service life. But chains are also heavier, noisier and more costly to replace. That's why belts are used on many OHC engines.
The main cause of belt wear is heat. As a belt wraps around and turns the sprockets, friction creates heat (this is in addition to the heat from the engine itself). Over time, this causes the rubber to lose flexibility, harden and crack. The cords inside the belt also weaken as the belt accumulates wear, increasing the risk of the belt breaking.
Timing chains, by comparison, are mostly immune to the effects of heat. But they do stretch as they age. The increase in length can have an adverse effect on camshaft timing and become a source of noise if the chain becomes loose enough to rub against the inside of the timing cover.
When The Cam Drive Fails
If a timing belt or chain breaks, or the cam drive gears fail, the cam stops turning, the engine loses all compression and the engine stops running. A cam drive failure can also cause expensive valve damage in "interference" engines that don't have enough clearance to prevent the valves from hitting the pistons if the cam stops turning or jumps out of time.
Interference engines include most Acura, Honda, Hyundai, Infiniti, Isuzu, Nissan and Porsche engines, also some Audi, BMW, Mazda, Mitsubishi and VW engines, as well as Chevrolet 1.5L and 3.4L, 1995 and newer Chrysler 2.0L and 2.5L, 1997 and newer Chrysler 3.2L and 3.5L, and Ford Probe 2.0L and 2.2L engines.
Timing chains or belts may also jump time if loose. The engine may continue to run but will experience a loss of performance because of the altered valve timing.
Belt and Chain Replacement
The best time for a vehicle owner to replace a timing belt or chain is before it fails. For older (typically mid-1990s and back) engines, the recommended replacement interval for timing belts is usually 60,000 miles. On newer engines, the original equipment belts are made of better materials and usually have a service life of 100,000 miles or more.
If an engine is being rebuilt, the timing belt, chain or cam gears should always be replaced. Don't take a chance reusing an old timing belt, chain or gear set. A high mileage timing chain that has stretched and is loose can't maintain accurate valve timing and should be replaced if play exceeds specifications. The camshaft and crankshaft sprockets should also be replaced along with chain. These parts are included in a typical three-piece timing set.
On OHC engines that use a timing chain, the timing set may also include a chain damper or tensioner. The chain damper, tensioner, guide or rails should also be replaced when the chain and sprockets are replaced because these components play a vital role in supporting the chain and keeping it tight.
A case in point is grinding noise coming from the timing chain cover area in 2000 to 2003 Ford Crown Victoria and Mercury Grand Marquis with the 4.6L V8. According to Ford Technical Service Bulletin 03-15-7, excessive wear in the timing chain tensioner arms can cause a noisy condition. The tension arm has an aluminum base with a nylon surface. So if you find nylon and/or aluminum particles in the engine oil it probably means the tensioner, tensioner arms, timing chains and gear set all need to be replaced.
The Ford part numbers for the 2000 model year tensioners are F6AZ-6L266-DA (RH) and F6AZ-6L266-CA (LH). For the 2001 to 2003 model years, the tensioner part numbers are 1L3Z-6L266-AA (RH) and XL1Z-6L266-AA (LH). The tension arms are 1L2Z-6L253-AA (RH) 1L2Z-6L253-AA (LH).
You should always check a database of technical service bulletins for any engine you are rebuilding to see if there are any known issues with the cam drive. Ford TSB 97-2-8, for example, describes a clattering noise at engine start-up, after the engine has sat overnight, that may be heard on 1995 Ford Contour and Mercury Mystique models with a DOHC 2.5L V6. Oil leaking out of the chain tensioners causes the timing chain to momentarily rub against the tensioner ratchet when the engine is first started. The noise only lasts a couple of seconds and goes away as soon as the engine builds up oil pressure.
The cure, says Ford, is to replace the original chain tensioner components with redesigned parts. These include two timing chain tensioners F5RZ-6L266-CA, the left hand tensioner adapter F5RZ-6C275-BA, the right hand tensioner adapter F5RZ-6C275-AA, the left hand chain tensioner arm F6RZ-6L253-AA and the right hand chain tensioner arm F6RZ-6L253-BA. The two DOHC timing chains and guide assemblies should also be replaced at the same time.
Replace, Don't Reuse
As for timing belts, they do not stretch with age, but become weak internally and may fail with no warning. Consequently, a visual inspection of the belt may not reveal much. If the mileage is not known, replace the belt. And if the mileage is known and a belt has more than about 30,000 miles on it, replace it anyway.
Many OHC timing belt failures are actually caused by faulty belt tensioners and idler pulleys. Like belts, these parts also wear out and should be carefully inspected and replaced when the belt is changed.
Reusing old tensioner components increases the risk of premature belt failure. The lifespan of many OEM tensioner components is only about 2,000 hours, or the equivalent of 40,000 to 60,000 miles or more depending on how the vehicle is driven. Tensioner bearings are "sealed for life" and are not serviceable, so there's no way to clean, inspect or re-lubricate the bearings when a timing belt is changed. What's more, it's difficult to determine the true condition of the tensioner components because they are not running under a load. Consequently, a simple visual inspection may miss tensioner bearings that are on the verge of failure.
If you find a belt with stripped cogs, that tells you something in the cam drive system has jammed or stuck, overloading the belt and causing it to shear teeth or jump time. The most likely culprit is the camshaft, which may have seized due to engine overheating or lack of lubrication (low oil level or loss of oil pressure). Be sure to check the straightness of the cam bores in the cylinder head, and straighten or align bore the head as needed before the head goes back on the engine.
When OHC cylinder heads get too hot, they usually swell up in the middle causing the cam to bend or bind. In some cases, this may break the cam, snap the timing belt or shear the cam drive sprocket off the end of the cam.
Get The Right Parts
Similar engines may use different cam drive components, so make sure you get the correct replacement parts for the engine. For example, Chevy has used three different timing gears for the 262 V6 since it first appeared in 1985. The flat tappet engines use a p/n 340235 gear, the non-balancer, roller cam engines use a p/n 12552128 gear, and the balancer engines use a p/n 10144121 gear. None of these gears are interchangeable.
On 1992-'98 262 engines with balance shafts, make sure the two balance shafts are correctly phased with camshaft. The shafts are gear driven off the cam, and there are two different gear sets, one with wide teeth and one with narrow teeth. The wide teeth gears were used on the earlier engines, and can be replaced with the later narrow gears to reduce noise.
Chevy has also used two different timing gears on its LT1 V8 engines. The earlier 1992-'95 LT1 engines with aluminum heads use a p/n 10128349 cam drive gear with a small hole with 16 splines in the center. On 1995 and later LT1s, a different gear (p/n 10206039) with a larger, unsplined center hole is used.
You'll also find three different front covers on these engines. The 1992-'94 engines have a cover with two small holes for the water pump and distributor drives, and a large hole for the crank. The 1994-'95 engines have a large center hole for the distributor, and the 1996-'97 covers have an extra hole at the bottom for the crank position sensor.
It's also a good idea to inspect the seal surface on the water pump drive shaft when changing the timing gears and chain on an LT1. The water pump is driven off the cam on this engine, and if the shaft is worn it will leak.
If you are replacing a timing chain on a late model overhead cam engine with variable valve timing (VVT), be sure to look up the assembly procedure so the VVT unit on the end of the cam is installed correctly. On GM's Vortec 4200 DOHC inline six (introduced in 2002 on the Chevy TrailBlazer, GMC Envoy and Oldsmobile Bravada), the VVT "phaser" unit is mounted on the end of the exhaust camshaft and uses oil pressure to change exhaust valve timing up to 25 degrees. The VVT unit is controlled electronically by the powertrain control module via a solenoid valve that opens to route oil pressure to the unit. When replacing the timing chain on one of these engines, you have to make sure the VVT unit is in the correct position (fully advanced).
Timing Belt Installation Tips
Make sure the replacement timing belt is identical to the original. Belt length, width, tooth profile and pitch must all be the same, and the material must be the same or better. Do not substitute a less expensive neoprene belt for one made of HSN (Highly Saturated Nitrile). Use a quality brand of belt because some no-name brands use inferior materials that won't last as long as the OEM belt or a quality aftermarket belt.
When changing a belt, never attempt to "stretch" a belt over a pulley. Belts, remember, do not stretch and forcing one to do so will likely damage the cords and cause it to fail.
Also, never pound on a sprocket to force it into place. If it isn't sliding into place, check its alignment and fit.
Misalignment problems in the cam drive can also occur if the cam sprocket is installed backwards, the wrong thickness of washer is used (incorrect end play), a thrust button is forgotten, or the crank sprocket is not positioned properly on its keyway.
Belt tension is critical to belt longevity. Adjust to the recommended specifications (always refer to the manual because the amount of recommended play can vary depending on the application). If the belt is too tight, it won't last. If the belt too loose, it may jump time.
On a Chrysler 2.0L engine, for example, here's the recommended procedure for adjusting the belt:
- Loosen the tensioner pulley bolt.
- Using tool MD998752 and a torque wrench, turn pulley clockwise with 22-24 in.lbs., hold the pulley and torque the tensioner bolt to 31-40 ft.lbs.
- Remove rubber plug from engine mount bracket and screw in tool MD998738 until it moves the automatic tensioner pushrod in.
- Remove the 0.055" rod from the tensioner body.
- Remove tool MD998738, turn crankshaft clockwise two revolutions and wait 15 minutes.
- Measure pushrod protrusion. It should be .15" to .18".
- If not, repeat tensioning procedure.
Some engines such as GM's 3.4L DOHC V6, a hydraulic tensioner is used to keep the belt tight. On these engines, the tensioner must be prepared before it is reinstalled by draining out the oil, fully retracting the plunger and refilling it with 5W-30 motor oil.
Also, check for revised or updated belt adjustment procedures that may be more recent. An example here is the timing belt and tensioner setup procedure that Ford originally published for the 1998-2000 Contour, Escort ZX2, 2000 Focus, 1998-2000 Mercury Mystique and 1999-2000 Mercury Cougar with 2.0L engine. Ford TSB 99-25-4 provides the following corrected information:
To achieve proper timing belt tension on this engine, Ford says the camshaft sprocket bolts should be loosened enough to permit the sprockets to turn freely on the camshafts. Once this has been done, rotate the crankshaft clockwise so cylinder #1 is at top dead center. Then install the camshaft alignment tool in the camshaft slots to hold the cams in place.
- Back out the tensioner bolt four full turns and position the tensioner so the locating tab is at approximately the 4 o'clock position. Line up the hex key slot in the tensioner adjusting washer with the pointer that is located behind the pulley.
- Working counterclockwise from the crank sprocket, install the timing belt.
- Rotate the tensioner locating tab counterclockwise and insert the locating tab into the slot in the rear timing cover.
- Position the hex key slot in the tensioner adjusting washer to the 4 o'clock position.
- Tighten the attaching bolt enough to seat the tensioner firmly against the rear timing cover, but still allow the tensioner adjusting washer to be rotated using a 6 mm hex key.
- Using the hex key, rotate the adjusting washer counterclockwise until the notch in the pointer is centered over the index line on the locating tab. (The pointer will move in a clockwise direction during adjustment).
- While holding the adjusting washer in position, torque the attaching bolt to 18 ft.lbs. (25 N.m).
NOTE: If the pointer does not remain centered over the index line during final tightening, loosen the bolt and repeat the procedure beginning with step 4.
Special Timing Chain and Belt Tools
Before you replace a timing belt, chain or gear set on some engines, you will have to look up the timing reference marks. Some engines have multiple timing marks that can cause confusion if you don't know which ones to use or how to line them up.
Many engines also require special tools when changing a timing belt or chain. At the very least, you should have a belt tension gauge to make sure the tension on a timing belt is correct. Special camshaft positioning tools (or a bolt or pins) may also be required on some dual overhead cam engines to hold the cams in place while the belt or chain is replaced.
Here are just a few of the special OEM timing tools that may be required:
- Audi 2.8L V6 (1992-'94) - Requires camshaft holding tool 3243, and crankshaft holding tool 3242.
- Chevrolet 3.4L V6 DOHC (1991-'94) - Requires two camshaft timing clamps J38613-A.
- Chrysler 2.0L OHC - Requires tensioner tool MD998752 and tensioner adjuster tool MD998738. The tensioner adjuster tool threads into a hole under a rubber plug in the engine mount bracket.
- Chrysler/Dodge/Plymouth 2.2L & 2.5L (1981-'95) - Requires tensioner wrench C-4703.
- Ford Escort 1.9L (1985-'91) - Requires crankshaft pulley wrench D85L-6000-A and camshaft holding tool D81P-6256-A.
- Ford Taurus SHO 3.2L V6 DOHC (1992-'94) - Requires tensioner tool T93P-6254-B and torque wrench adapter T93P-6254-A.
- Honda Accord 2.2L (1990-'94), Prelude 2.2L V-TEC (1993-'96) and 2.3L (1992-'94) - Requires the following for balancer shaft belt installation: locating pin 07LAG-PT20100 (or a M6x100 mm bolt).
- Honda Prelude 2.0L & 2.1L (1988-'91) - Requires two 5mm pin punches to hold dual overhead cams at TDC.
- Hyundai Sonata 2.0L & Elantra 1.8L & 1.6L - Requires tensioner pulley socket wrench 09244-28100 and tensioner adjusting screw 09244-28000.
- Isuzu 1.5L (1985-'93) - Requires belt tension gauge J326468-B and crankshaft wrench J37376.
- Lexus ES250 2.5L - Requires crank pulley holding tool 09213-70010, pulley tool handle 09330-00021, and puller 09213-60017.
- Lexus SC400 & LS400 4.0L V8 - Requires crank pulley holding tool 09213-70010, pulley tool handle 09330-00021, and puller 09213-31021.
- Mazda pickup 2.3L - Requires tensioner adjusting tool 49-UN10-067.
- Mitsubishi 3.0L V6 DOHC Diamante & 3000GT (1993-'95) - Requires tensioner tool MD998752-01.
- Mitsubishi 2.4L Eclipse & Montero 3.5L V6 DOHC - Requires tensioner tool MD998767.
- Mitsubishi 2.0L Eclipse & Galant (1988-'94) - Requires tensioner pulley tool MD998752 and tensioner tool MD998738.
- Nissan 3.0L V6 (300ZX) - Requires tensioner tool EG 14860000.
- Toyota 3.4L (4Runner) and RAV4 2.0L - Requires crank pulley puller 09950-50010.
- Toyota 3.0L V6 4Runner (1993-95) and Camry (1993-96) - Requires crank pulley puller 09213-31021.
- Volkswagen 2.0L Golf, Jetta & Passat (1990-94) - Requires tension gauge VAG 210 and 2-pin wrench - Matra 159.
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