The relationship of crevices to the formation of unreacted hydrocarbons in engine combustion remains to be determined. In order to help understand its processes the present paper reports the experimental results obtained from the wall effect on flame propagation in various clearances which were placed in a constant-volume charged with premixed fuel-air gases.The photographic observations revealed that the flame propagation was accelerated in some clearances with both ends open. Furthermore, it was discovered that there is an optimum clearance that creates the most rapid flame propagation through a contained combustible mixture. The smallest optimum clearance was measured in a slightly richer mixture than stoichiometric; the greater clearance was measured in leaner or richer mixtures exhibiting a characteristic quite similar to the dependence of quenching distance to fuel/air ratio. In clearances with one end open the flame propagated slower than outside the clearances. Consequently, a large portion of trapped mixture remained unreacted until the overall reaction was nearly complete.