Axial piston pumps are key components in modern hydraulic systems, especially where high efficiency and robustness are essential. While numerical tools for simulating lubrication gaps and internal temperature distributions have advanced significantly, accurate thermal modeling remains a challenge due to the lack of precise boundary conditions. In particular, heat convection coefficients at internal surfaces—critical inputs for thermal simulations—have traditionally been estimated with simplified geometries or analytical approximations, without experimental validation. This study addresses this gap by presenting the first direct measurements of heat convection coefficients and temperature fields on the rotating cylinder block of an axial piston pump under realistic operating conditions. A novel telemetric measurement setup was developed, enabling reliable data acquisition from within the rotating group without altering the pump design. These experimental results serve to validate a detailed CFD model of the casing oil, implemented using Simerics MP+, and a thermally coupled gap simulation in Caspar FSTI. It is shown that the choice of thermal boundary conditions significantly affects temperature predictions and, consequently, gap heights and power losses. The results demonstrate that conventional assumptions can lead to errors exceeding 30 K in component temperatures, underscoring the importance of accurate convection coefficients. By combining simulation and direct measurement, this work provides a validated methodology for thermal modeling of axial piston pumps and establishes a benchmark for future simulation efforts in this underexplored field.