Which bearing is least likely to be a roller or ball bearing?

The master rod bearing in a radial engine is least likely to be a roller or ball bearing because of its unique design and operational requirements. In a radial engine, the master rod plays a critical role in connecting multiple connecting rods to the crankshaft. The design of these bearings often prioritizes the need to accommodate significant axial loads and differing motion types that are not typical for roller or ball bearings.

Instead, the master rod bearing is more commonly a plain bearing or bushing, which provides a larger surface area for load distribution, reducing wear and allowing for the necessary flexibility and movement between parts. This configuration is particularly beneficial in radial engines due to their rotational characteristics and the arrangement of the connecting rods, which can complicate the application of rolling-element bearings.

Other bearings mentioned, such as rocker arm bearings, crankshaft main bearings, and camshaft bearings, are more commonly designed as roller or ball bearings due to their need for reduced friction, improved precision, and higher load handling capability. These options allow for smoother operation in applications where high-speed rotation and precise alignment are critical.