The use of an optical proximeter to determine dynamic top center in a motored engine is demonstrated. Design criteria are formulated and a data reduction procedure is presented. The method is shown to have an accuracy of Δθ = ± 0.1°. Variations in dynamic top center with engine speed that can be attributed to structural flexing and finite bearing clearances are shown to be less than ± 0.05°. It is also shown that the compression ratio during gas exchange is slightly larger than during compression-expansion.Other methods of finding top center are discussed and contrasted with optical proximetry. In this context a rational means of examining pressure records is presented and shown to be accurate to within Δθ=±0.3°.