Glossary

A carrier or bearing cap bolt is a high-strength fastener used in differentials and transmissions to secure the bearing caps (also called carrier caps) that hold the differential carrier bearings in place. These bolts are critical to maintaining proper bearing preload, gear alignment, and structural stability within the axle housing.

Inside a differential, the carrier (or differential case) supports the ring gear and spider gear assembly. On each side of the carrier are tapered roller bearings that allow it to rotate smoothly inside the axle housing. These bearings are clamped in place by bearing caps, which are bolted to the housing using carrier or bearing cap bolts. The precision and torque of these bolts directly affect how well the gears mesh and how long the bearings last.

Because of the stresses involved, bearing cap bolts are typically made from hardened alloy steels such as 4140 or 8740 chromoly, with tensile strengths often exceeding 150,000 psi. They are usually fine-threaded to allow precise torque application and to resist loosening under vibration. Many designs also use shouldered bolts for perfect alignment and may include threadlocker or safety wire holes to prevent backing out under load.

If a bearing cap bolt fails or loosens, the carrier bearings can shift, causing gear misalignment, premature wear, or even catastrophic failure of the differential. That’s why manufacturers specify exact torque values and tightening sequences during assembly.