Abstract: A gauge for releasable attachment to internal combustion engines at the starter motor mounting point for determining proper positioning and selection of starter motor housings for the individual engine. The gauge includes a housing with a base surface portion having a configuration defining a gauge mounting pad, along with bores to receive bolts for attachment of the housing to the engine block. A pivot pin mounts an elongated lever within the housing for pivotal rotation about the pin, and a reference scale is attached to the housing so that the position of the lever arm within the housing is ascertainable by the scale. A pinion mounting yoke is mounted on the opposed end of the lever and is arranged to appropriately position the pinion in proper and standard engagement with the teeth of the ring gear on the engine flywheel.