Does Your Engine’s Timing Gears Need Replacing?

Timing chains are interesting. Especially, when you find you suddenly need to replace one. I was just driving my way down the highway and had reached my exit. Letting off the throttle I let my old truck drift to bleed off my speed so I could negotiate the off ramp with a good degree of safety when the engine, with no warning simply quit. I let her roll off onto the shoulder while I still had some momentum and got out of traffic.

The next few minutes were spent in a great deal of engine cranking and snooping under the hood trying to determine just what had happened. Considering the accumulated mileage on the engine and the occasional backfire I was getting while cranking, the likely problem seemed to be that the timing gear had slipped a tooth or two..and the engine was now out of time so badly only replacement gears and chain would bring it back to life.

One tow truck ride and at the house I am under the hood beginning the process of replacing those gears and chain and wondering how long all this might take. Off comes the fan shroud. The serpentine belt and the fan and pulley assembly. The water in the radiator gets drained away and then the water pump is removed. Next it was under the truck to drain the engine oil and loosen all twenty two bolts and nuts that hold on the oil pan. Removing the pan actually wasn’t needed so I just took off all the nuts that hold it on except the ones at each corner. I just needed it to come down a little bit to gain clearance for the timing chain cover to slip on and off. And as long as I was under there, I also popped off the harmonic balancer as it needed to come off anyway. This required the use of a puller device which I rented at the parts store.

Okay…all is ready now to pop off the chain cover and get into the real guts of this latest breakdown. The chain cover is only held on by about six small bolts. I removed the bolts and gently pulled the cover away from the block. The manner in which this cover is made makes it an integral part of the oil pan seal. This is why the oil pan had to be lowered. That cover has a seal on the bottom of it that mated to the oil pan itself. Together, they formed an oil tight seal. This same cover also has a circular seal in the front so it can slip over the exposed end of the crank shaft.

The chain cover off now I can see the timing gears and the chain. Sure enough, there is a tooth missing on the large cam gear. The smaller crankshaft gear looks okay but the teeth have become pointed over time and are no longer squared off on the ends. The chain is loose as well instead of being taut as a bow string the way it should. Having stopped off at the parts store on my way back with the tow truck I had the parts I needed to fix all this right up.

First we get the cam gear off. A half inch drive socket wrench removes the single large bolt in a jiffy. Pulling the gear straight out brings it and the chain off at the same time. Selecting a sturdy flat edge screw driver allows me to wedge the crank gear off and it is soon in my hands as well. So far, so good. Now we come to the touchy part of all this. So let us talk about timing gears and chains and what they do, to gain some understanding.

The crankshaft and your cam shaft are two separate parts of your engine. They are required to work together but accomplish separate things in order for your engine to run. The crankshaft has the pistons attached to it and when the piston is forced down by an ignition..you get power and down the road you go. But that power is really the result of an intake of air, a percentage of gasoline mixed with that air, and a spark. The air we need cannot get to the piston unless it passes through a valve first. And here is where we need our cam. It has one main job essentially. To open and close the intake and exhaust valves at exactly the right time, in the exact right sequence to allow a continuous intake of air and expulsion of exhaust gases. Getting the crankshaft and cam shaft to work together and in harmony is what shade tree mechanics call timing.

I bring out the socket wrench again because I am going to need it shortly to begin this process of timing. I need to do two things. First, I need to bring the crankshaft up to what is known as Top Dead Center…or TDC. This means that the piston in number one cylinder needs to be at the very top most point in it’s movement. Top dead center !

Determining where the piston is at any given time is quite easy however. That crank gear has a slot on the inside surface of the hole that has been bored into the center of it at the factory. This hole is there so the gear can slip over the end of the crankshaft and this slot is called a key way. And the shaft has a raised ridge that will fit perfectly in that slot. This ridge is called the key. So the gear will only fit on the shaft one way, and one way only. Also, etched into the metal of the gear is an alignment dimple. You just attach the new gear to the crank with the bolt and tighten it to specifications. Then you can rotate the crank until that dimple which you have not taken your beady little eyes from..is at the twelve o’clock position. Your crankshaft..and the number one piston, are now at Top Dead Center. Now I need to remove that bolt as I will need it later for the harmonic balancer.

Our next move is to get the new cam gear on. It also has an alignment dimple. On my engine it is required that the dimple be at the six o’clock position. I pop the gear on and tighten down the bolt and once it gets tight enough the entire cam will move as I continue to pull on my wrench. Once the cam gear alignment dimple has reached the six o’clock position, I use a broad blade screw driver wedged into the teeth of the gear to prevent it moving and take the cam gear bolt back out. Now I can pop the gear off again and my cam hasn’t moved at all. Picking up the timing chain I drape it over the cam gear and slowly,carefully loop the bottom half of the dangling chain around the crankshaft gear. If you do this right, you can move both chain and cam gear up and onto the crank gear, get the cam gear placed , still with that dimple at six o’clock, and a bolt through it’s center with minimal trouble. With the bolt now through the gear and into the camshaft..the whole assembly is beginning to come together. Tighten the cam gear to specifications.

Now I take a look at the chain cover. There are some concerns here. First, it has a gasket or remains of a gasket, that have to be removed and a putty knife will do this in quick time for me. I also look a the engine block where this cover resides, If there are any gasket remains on there the putty knife will remove them easily. The chain cover also has a seal in the lower front of it. It is wisest to replace this seal as it can be damaged in removing the cover. And on the bottom of the cover is another seal. This is the part that mates up to the oil pan and I wanted to make dead certain these gaskets and seals were perfect to prevent oil leaks. Six bolts later I have the chain cover in place and I can go under the truck and replace all those twenty two bolts and nuts and snug the oil pan back up and against the engine block where it belongs. And again, as long as I am under there, I put the harmonic balancer back in place and torque it up to specifications.

That done I can concentrate on putting everything else back where it belongs on the front of the engine now. I replace the water pump gaskets and considering this is the original pump, I have a new replacement for it as well. The fan pulley assembly goes on next and then the serpentine belt. All that is left is the fan shroud, refill the radiator with coolant and replace the oil filter and refill with oil..and the thing should start right up.

Before starting the engine however, there is one more thing I feel I should check. I bring out my inductive timing light. This is a stroboscopic flash unit really, that has the shape of a futuristic ray gun. It’s job is to flash at the exact moment that number one piston fires. On the harmonic balancer is a shallow groove cut into the metal. And nearby, is a marked gauge showing a portion of the degrees of a circle. Mine is marked like BTC -4 -2 0 +2 +3 +4 ATC. That little code deciphered meant that -4 was 4 degrees before top dead center or BTDC. The zero meant Top dead Center. And the numbers with a plus sign are degrees of timing After Top Dead Center..or ATDC. My engine needs to be at TDC. All you have to do is use chalk or whiteout to mark that groove in the harmonic balancer so you can see it in the timing lights flash and another mark on the zero of the gauge that is mounted near enough to the balancer so you can see them at the same time.

After attaching my light I give the ignition key a twist and the engine springs to life. I let it settle down while observing the gauges looking for oil pressure to come up and find all is well. Outside again I pick up the light and aim it’s barrel down into the engine compartment so it can illuminate the marks I have made. Each time the number one piston is fired I get a bright flash of light. Being a strobe, this light has the effect of freezing the motion of the rotating crankshaft and I can clearly see the mark I have made on the balancer and the stationary gauge. They are perfectly aligned and unmoving. My engine is now said to be in time. I had little doubt this would be the case, but it is always good to confirm things. Since the distributor had remained undisturbed, there was no reason for it be out of time anyway. The timing was precisely where it had been before that gear tooth had broken off. The entire job took several hours and I can rely on this fix to last another hundred thousand miles.

On average, Timing gears and chains should be replaced at the hundred thousand mile mark. if your engine uses a Timing Belt however, You should replace the belt at about sixty thousand miles. One thing to note here about engine’s that use a belt. There are two kinds of engine’s in the timing belt world. Interference and non interference engines. The interference engine is one that can cost you a small fortune if the timing belt should break..or even slip a tooth. And the reason is that the tolerances are so close in this engine..that if it ever comes out of time due to a fault with the timing belt, the rising pistons will impact directly onto any valves that happen to be down at that moment. The valves have ” interfered” .This leads to broken valves, damaged pistons, broken heads and multi-hundred dollar repair bills. Possibly leading up to a replacement engine. At that, will set you back a lot financially.

A non interference engine does not have this damage potential if the belt should slip or break. So be on the safe and wise side of timing belts. If your clock has over sixty thousand on it, and you have never replaced the belt…you may want to get this done. And also on the plus side for timely belt replacement..it isn’t all that expensive. The belt might go 30 dollars or so, and a good garage can have the new belt installed in under 2 hours. Or you can do it yourself if you have the proper tools and a free afternoon. But be sure and get a repair manual for your exact engine. It will tell you about proper gear position, torque information and they often include handy shortcuts you can use.