The HD90129321004 and HD90129321007 are a matched main and driven bevel gear set (commonly known as "ring gear and pinion") used in heavy-duty vehicle drive axle assemblies. The two gears are precision-matched as a set and must be installed together.

1. Installation Location
This gear set is installed inside the drive axle housing. The driven bevel gear is bolted to the differential case, and the main bevel gear (pinion) is mounted on the input shaft.

2. Function
The gear set transfers power from the driveshaft to the axle shafts, reducing rotational speed while multiplying torque. The precision tooth contact pattern ensures smooth and quiet operation under heavy load conditions.

3. Maintenance and Service
When replacing this gear set, both the main and driven gears must be replaced as a matched set; individual replacement is not recommended. Installation requires proper bearing preload, backlash adjustment, and tooth contact verification, which should only be performed by qualified technicians using appropriate tools.

4. Important Notes
Always verify both part numbers match the original component specification for the specific vehicle model before installation. After installation, follow proper break-in procedures. Use only the recommended axle lubricant and maintain the correct oil level.