In This Article:
The harmonic balancer is removed with a special tool. The timing chain and sprocket are removed. The rear oil seal and main bearing caps are removed. The crankshaft is lifted out and stored carefully.
1 - 2 Hours
Bruce W. Maki, Editor
To reach the timing gears, the harmonic balancer must be removed. The harmonic balancer fits very tightly on the front end of the crankshaft, and is secured by the snout bolt, which also holds the serpentine belt pulley which drives all the accessories in the engine compartment.
BUT... you cannot use an ordinary gear puller to remove a harmonic balancer, because there is section of rubber between the outer ring and the inner metal section.
For $75 I bought a deluxe kit for removing and installing harmonic balancers.
I installed the puller plate (red arrow) on the harmonic balancer and turned the hex on the end of the jackshaft to push the shaft into the end of the crank.
I held the jackshaft with a 1-1/16 inch wrench while I turned the back end of the shaft with a 5/8 inch socket and breaker bar.
While turning the puller, I could see this gap opening up between the harmonic balancer and the timing chain cover
The shiny area between the arrows is not the crankshaft... it's a smooth part of the harmonic balancer. The front oil seal rides against this surface.
Note that the front oil seal is built into the plastic cover for the timing chain, so if the front seal leaks, the cover must be replaced.
At this point I was able to remove the timing chain cover.
The arrows indicate two of the six bolts that hold the cover to the front of the engine block. These bolts require a 10mm socket or wrench.
The big sprocket is for the camshaft.
Removing the timing chain simply requires that the cam sprocket be removed. But first there is a "reluctor" for the crankshaft position indicator that needs to be removed.
I used a 3-jaw gear puller to remove the crankshaft position sensor reluctor.
This part has a light press fit. I barely started turning the gear puller (by hand) when the reluctor came off. It might be possible to remove this part by lightly and evenly tapping on the back side with a small hammer.
The timing gears.
Note the triangle-shaped marks on the crank sprocket and the cam sprocket, which are aligned for this photo.
These marks need to be aligned during reassembly or else the camshaft timing will be off, and that can create a whole slew of problems.
I unbolted the camshaft sprocket using an impact wrench and a 1/2 inch socket, and lifted off the chain.
The red arrow points to the gear for the balance shaft, which lies directly above the camshaft (when the engine is upright). I believe GM's V8 engines don't use a balance shaft.
I used a 3-jaw gear puller to remove the crankshaft timing sprocket. But there was a small problem... the center of the puller has a pointed tip which almost fit inside the 7/16-20 threaded hole in the end of the crankshaft.
To prevent damage to these threads, I placed a 5/16 inch nut in the end of the crank, and placed the tip of the puller in the center of the nut. This allowed me to push against the end of the crank without ruining the internal threads.
I removed three small bolts and a nut to remove the rear oil seal.
The seal assembly slipped off the rear end of the crankshaft easily, but it was trapped by the engine stand, so I couldn't actually remove it until the crankshaft was lifted off the engine.
I'll need to remember to slip the rear seal assembly over the end of the new crank when I install it.
Now I'm ready to remove the main bearing caps and remove the crankshaft.
The main bearing caps are numbered 1, 2 and 3.
The 4th (rear) cap isn't numbered because it is different from the others.
Closer view of the main bearing caps.
(I used a paint marker so the numbers would show up better in pictures.)
I removed the main bearing caps using the impact wrench and a 5/8 inch socket.
The main bearing caps are quite difficult to lift off the engine block because they fit so tightly in the machined recesses in the bottom of the engine block. I had to tap the caps sideways with a small hammer to loosen them.
When I was ordering a remanufactured crankshaft, the parts store needed to know the forging number on the crank. The guy rattled off a long list of numbers, and I went home to look at the engine.
The number 255 was molded into the crankshaft in 5 places, so this must be the forging number that the parts store needed.
At the rear of the crank, there was this balancing hole (red arrow) drilled into the side of number 6 rod journal, which is consistent with the description of a number 255 crankshaft given by the auto parts store.
At the front, there was another balancing hole drilled into the side of rod journal number 1.
I carefully lifted out the old crankshaft and stood it on the front end, and attached the flex plate.
I tightened the bolts snug. Then I turned the crank over and let it stand on the flex plate. I've read that a crankshaft should never be laid on the ground, because it can actually warp if it's not supported by the main bearing journals. I think that's more of a problem if the crank gets dropped or bumped.
Note the small tab (or tang) on the main bearings. This tab of metal prevents the bearing from turning.
I inserted a small flat screwdriver behind the tab and pried gently to separate the main bearings from the engine block.
The fourth (rear) main bearing has a different design. The shoulders on the side of the bearing act as thrust bearings, which prevents the crankshaft from moving front-to-back.
Note the spots on the bearing. These areas felt slightly rough when I dragged my fingernail across them. This may be normal wear, or it may have been caused by "galling", which is metal-to-metal contact. Galling is a sign of a serious problem, basically failure of the lubrication system to deliver the proper amount of oil. This makes me suspect that there may be a plugged oil gallery near the back of the engine block or in the crankshaft.
The other main bearings had a slight amount of these spots too, with the 3rd bearing being worse than numbers 1 and 2. I wonder if my decision to add SeaFoam to the engine (because I thought the knock might have been a stuck lifter) may have diluted the oil too much.
I have a vague memory of a discussion of journal bearings when I was in engineering school more than two decades ago. I recall that the rotating shaft riding on a film of oil actually rides off-center slightly. Basically, the shaft tries to "climb" uphill on the film of oil, squeezing the oil film thinner in one area while allowing for a slightly thicker film on the opposite side of the shaft. Keep in mind that on a new engine the space between the crank journal and the bearing, on average, is between .00055 and .00115 inches. (That is, the total clearance between the journal and bearing is supposed to range between .0011 and .0023 inches. That dimension measures the space twice.)
Given that there is essentially no oil pressure for a few seconds every time the engine is started, and the way the crankshaft tries to pull itself uphill, I guess a little galling near the bearing seam would be normal.
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Note the diagonal scratch pattern on the engine block behind the 4th main bearing. That's a pattern from the original machining process when the engine was made, so it's a good sign that the scratch pattern is still visible.
Note the small black line underneath the 3rd bearing (red arrow).
I'm told that these black marks are caused by the metal getting too hot, possibly from running a little low on oil.
Years ago I heard an interesting point on that "Car Talk" radio program. Engine oil is not just for lubricating the internal parts, it also helps carry heat away from moving parts and dissipate that heat.
When an engine is low on oil, even if the oil level is still inside the acceptable range, there is less liquid available to absorb all that heat, so the oil will be hotter. While this GMC Jimmy has an oil cooler (which runs inside the radiator), that may only reduce the oil temperature a few degrees and may not prevent the oil from getting too hot.
The bottom line is: It's important to keep the oil level topped up, especially in hot weather or under "severe service" conditions like towing a trailer, or driving in the mountains.