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Final Check Unlike many gear specialists, Mellentine uses gear-check paint on both the drive and coast sides of the teeth all the way around the ring gear when checking lash. When doing a wear pattern test, you are looking for a consistent pattern all the way around the teeth and for the contact on the thrust face to be near the bottom of the teeth. When the gears are under torque, the contact patch will move up the face of the ring gear teeth. You want the pinion gears to make complete contact with the ring gear teeth and not walk off the top.
Also, instead of checking just a single point for lash, Mellentine marks four points on the ring gear and checks lash with a dial indicator in all four locations to make sure there are no warping or runout problems. Lash must be between 0.006- and 0.009-inch to fall between Mellentine's acceptable parameters. If lash is too tight, the rearend will suffer from excessive heat buildup and can scuff the gears. Too little and you run the risk of breaking pinion teeth. Finally, Mellentine resets the dial indicator to the back face of the ring gear and spins the pinion-again to check runout. For this check, he wants to see no more than 0.0025-inch variance. This check is not for the quality of installation but for the quality of the ring gear and spool. Too much variance means there is an alignment problem. There's no fix in this situation other than to determine which piece is at fault and replace it.