Gas leakage through the clearance between piston and cylinder is a major source of thermodynamic inefficiency in reciprocating compressors, therefore, any simulation model should accurately predict its effect on the overall performance. Effects like misalignment between piston and cylinder and components cylindricity errors are not taken into account in the simpler numerical models usually adopted to estimate the gas leakage. This paper aims to analyze the impact of such simplifications using a three-dimensional CFD model, including transient effects resulting from the piston movement as well as actual geometric errors characteristic of the usual manufacturing process adopted for the household compressor industry. The results are compared to numerical predictions of simpler models and its benefits are emphasized