Gear Swap

Ditching 4.88s For Hopes Of Road Speeds

Andy Bolig Jun 1, 2001 0 Comment(s)

Step By Step

The first step is to remove the side yokes from the differential. Once you pull the snap rings, the yokes will slide right out. Check the yokes for wear after you’ve removed them.

You can see the difference in wear between the good yoke on the left and the other two worn yokes. The minimum distance from the spring clip groove to the end of the shaft should be no less than 3/16 inch.

Mark the bearing caps before you remove them because they’re machined to fit. Marking them eliminates confusion and problems. You can’t tell from this photo, but Dewey removes the bolts from the ring gear while it’s still in the housing and easier to hold.

Dewey will use a prybar...

...to gently lift the differential case out of the housing. Be prepared for shims and bearing races to come out also.

Next Dewey focuses on removing the pinion. An impact gun is the best way to remove the nut from the pinion. You can also use a big breaker bar and a socket, but you’ll have to devise a way to hold the universal hub to keep it from moving.

With the nut removed, tap on the pinion and it should slide out.

With the pinion removed you can tap the seal out of the housing and remove the bearing.

Corvette Central supplied our new 3.90-ratio Richmond gears. They differ slightly from the original GM gearset, so if you have the specific tools to set up GM-only gears, they will not work.

There are several numbers on the top of the pinion gear. Notice the numbers 2.482. This is the distance from the face of the pinion to the centerline of the differential.

Dewey cut a piece of angle iron to fit in the housing, and then polished the one face on a belt sander. He lays it over the bolt holes for the bolts that hold the caps in place, and measures the distance to the pinion. This method works on the shark differentials, but it won’t work on the later Dana rears because the mating surface is off center.

To get the proper measurement to the pinion, you need to install shims under the pinion.

Dewey starts with a .028-inch shim and changes accordingly.

The shims are marked, or you can check them with dial calipers.

The pinion bearing has to be pressed on and off, so the fewer tries it takes to get the proper setting, the better.

Once the pinion distance is correct, you can install the new crush sleeve...

...and outer bearing.

Put some gasket sealant around the pinion seal before you...

...gently tap it in place with a soft hammer. Make sure the seal goes in evenly and seats entirely.

Dewey likes to put some sealant in the splines of the universal hub to prevent any differential lubricant from leaking through them.

Because the hub fits snugly onto the pinion splines, he taps it onto the pinion using a socket and hammer. This keeps the pinion in place until the next step.

Install the nut onto the pinion. You’ll need to tighten the universal hub onto the pinion, and there are two ways to do that.

Rather than use an impact gun to tighten the hub, Dewey put the complete housing into a press with the hub facing down, and pressed on the pinion from the inside. Be careful not to over-press the pinion. You can feel when there is no more slack in the bearings by spinning the housing.

Once you feel them just start to tighten, remove the differential and finish tightening the nut by using an impact gun (or a long breaker bar) and a socket.

We set our new bearings so they had a preload of 20 lb/in. Used bearings should be set at 5 lb/in because they will not “seat” like a new bearing.

Because of the difference in gearing, we had to exchange the differential case. The original case works for 4.11 gears and higher, but would not work with the 3.90 set. The ring gear had to be pulled before we could do anything else with the differential case because the teeth of the ring gear will not let the pin slide out.

We already removed the ring-gear bolts so, with a few slight taps from a punch, the ring gear dropped off the differential case.

There is a small bolt that holds the pin in place. With that bolt removed, the pin can simply be tapped out.

With the pin removed, you can tap out the pressure plate and springs.

Now spin the small gears out of the differential case. Use one of the side yokes to spin one side of the differential to bring the small gear around. There will be spacers between the small gear and the differential case.

With the small gears out of the case, we can remove the clutch disks.

We checked each disk for wear and applied some GM limited-slip additive to each disk upon reassembly.

When you install the clutch disks into the new differential case, make sure the disk guides are properly seated.

When installing the small gears, make sure the pin will go through both gears. If the small gears are not exactly opposite each other, the pin won’t go through both. Don’t forget to install the shims behind the gears.

Install the clutch springs next. Dewey likes to wire the retainers to keep the springs compressed. You can also use large pliers. Install the pin and the small retainer bolt.

The ring gear must be aligned perfectly, so we put a threaded stud with a groove into one of the bolt holes.

It aligns the ring until it’s seated in place; then we just unscrewed it from the ring gear and installed all the bolts.

We installed the new bearings from Corvette Central by using a large socket to apply even pressure. Be careful and make sure the bearing goes on evenly.

We used the large spacer as a starting point, but we had to install the bearing caps and torque the ring-gear bolts before we could check the backlash. The ring-gear bolts should be torqued to 45 lb-ft and the cap bolts should be 65 lb-ft.

The backlash should be between .004 and .008 inch. Also, check the mating pattern between the two gears. Richmond gears come with contact pattern instructions to show you how to obtain the proper pattern.

The final step is to install the side yokes into the differential. The spring clips have a rounded outside edge and a sharp square edge. The rounded edge goes toward the inside. You may have to tap in the yoke to get the spring clip to seat properly. When seated, the ends of the spring clip should not be any farther than the distance of one tooth on the yoke.

One of the best ways to get a great launch from your Corvette is to swap out the ring-and-pinion for a lower (higher numerical) gear. This is where sound reason and restraint can be very valuable. The thinking that “some is good, so more is better” can turn an enjoyable driver into a nightmare.

Our big-block-equipped shark is one such example. The 4.88 gears were insanely high for this driver. It had the launch of a lightning bolt, but the top speed of a golf cart—not good for upholding the Corvette heritage!

We joined forces with Corvette Central and Dewey’s Just Vettes in Longwood, Florida, to give this Corvette hope of seeing the speedometer accelerate faster than the tachometer. Follow along as we bring the speedometer to new heights by swapping in a set of 3.90 gears.

When the swap is complete, you can install the differential and fill the appropriate fluids to see what a difference a gear makes. It should become immediately obvious the first time you hit the interstate onramp.

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