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A damage assessment required a complete tear-down. Drivetrain specialist Richard Mellentin
Since our ring-and-pinion has already been run in several races, it's easy to see the wear pattern where the teeth have been making contact. Before he begins polishing, Mellentine uses a die grinder to knock the sharp edges off the ends of the teeth on both the ring-and-pinion. It's obvious from the wear pattern these areas are not making contact with the mating gear and are only stress points. Also, when the car is under heavy acceleration, thrust forces cause the pinion to work its way to the top of the ring gear (the outside edge). "Depending on how deep the pinion is set to contact with the ring gear, under heavy load it can actually walk itself up the ring gear and start rolling over the outside edge," Mellentine explains. "So I'll also cut the top edge down so that if the pinion teeth come near the edge of the ring gear it won't be hitting that sharp edge. When you have the pinion hitting a sharp edge, it starts putting stress points in the pinion, which causes heat and increases the chance of cracking something."
Polishing is done with a buffing wheel mounted to a bench grinder. One neat tip here if you are attempting this yourself is to use a screwdriver on the spinning buffing wheel to cut a groove into the face of the pad. This allows you to polish both the drive face and coast face of the gear teeth at the same time and to reach all the way to the bottom of each tooth. It took less than 20 minutes for Mellentine to put a mirror finish on both the ring-and-pinion. The final task before everything hit the parts washer is to smooth any rough spots inside the housing with a die grinder. Again, rough spots are the first places cracks will appear under stress and are also magnets for excessive heat.
Big Bearings Another weak area Mellentine pointed out in our center section was the inner pinion bearing. Whenever power is applied to the wheels, the inner pinion bearing is under load. The more power put to the rear wheels, the greater the load. Standard sizes are fine for street cars, but the larger bearing helps distribute the increased load caused by high-horsepower race cars. Bearing failure is not the only problem we are trying to avoid here. As the bearing wears, it slowly loses its ability to hold the pinion in the correct position. The slop that comes into play will increase heat gear wear, and the potential for breaking gears. The larger bearing requires that we also install a larger pinion retainer; both are the same pieces Mellentine used when building Winston Cup rearends.
Instead of depending on a polishing machine, Mellentine grinds and polishes all gearsets b
The rest of the assembly process is straightforward for a Ford 9-inch rearend. Mellentine recommends 20 in-lb of preload on the pinion bearing. He says that after 30 laps of practice the bearings will take a set and-if you were willing to take the rearend back apart and actually check it-that number will be down to 5 or 6 in-lb.
Along with the bigger inner pinion bearing, Mellentine also recommends using a steel spacer instead of a crush sleeve. "A stock crush sleeve is fine in a street car," he says, "but in a race car it's just going to keep crushing as you put pressure on it. What happens is you will loosen the pinion and bearing up, and you are going to start having gear failures, bearing failures, seal leakage, and things like that. By going with the solid steel spacer, you aren't going to have that problem. As the bearing takes a set, you are going to get a little bit of looseness, but you want some and it's not going to be enough to cause a problem."
Hard edges can lead to excessive heat buildup and damage to gears. Mellentine uses a die g
Polishing is performed with a buffing wheel mounted to a bench grinder.
Here, you can see the smoothed edges of our ring-and-pinion set once they've been buffed t