Particle Image Velocimetry (PIV) here has been used to measure the instantaneous velocity field within a realistic geometry motored single cylinder engine. Through-the-piston-crown illumination of a vertical plane bisecting the inlet and exhaust valves in a four valve pent roof combustion chamber and the use of a corrective optical system has for the first time allowed the velocity field in a vertical plane within a cylindrical bore to be quantified with PIV. Techniques are described which permit accurate and repeatable camera focusing, laser to engine synchronisation and seeding density control. Large scale motion observed at 180° ATDC has been interpreted as barrel swirl. Limitations of the current technique are discussed with respect to general in-cylinder applications.