Cam-to-follower clearance influences several aspects of engine operation: valve-train noise, which affects acoustic comfort: valve-train dynamics, which affects engine reliability and functionality; valve lift profile and phasing, which influences combustion cyclic stability especially at idle. If mechanical followers are used on an engine, then, depending on engine operating conditions and different thermal expansion between cylinder head and valve, the valve clearance changes. For this reason the initial and final cam ramps are designed in order to minimize the effects of these variations. When a new valve-train is designed the determination of the nominal valve clearance requires an in-depth knowledge of the above mentioned variations. An experimental evaluation of valve clearance in real engine operating conditions would be desirable. In this paper an extensive experimental activity at engine test bed is presented. A simple, economical and precise indicated methodology was set-up and developed in order to directly measure the angular position of the camshaft when each valve tappet starts to move: in this way valve clearance can be directly determined. These measurements were done in different engine operating conditions with respect to speed and load, and also in thermal and speed transient conditions, in order to determine valve clearance maps. On the intake side a maximum increase of 0.13 mm was found with respect to nominal value, while on the exhaust side a maximum decrease of 0.30 mm was determined. This last value was judged excessive, so some design changes were made in order to limit it: in particular a bi-metallic exhaust valve was tested. The improvement with this type of valve varied from 25% to 70% depending on engine load conditions.

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