Flame penetration into the top-land crevice of a combustion engine's cylinder is investigated by large-eddy simulation (LES) and high-speed visualization experiments. This penetration is of practical relevance as it leads to the formation of unburned hydrocarbons (UHC) wherever the flame is quenched inside the crevice. In optically accessible engines, the crevice is particularly large, so that it must be considered in simulations, which also creates an opportunity to study flame penetration in detail. The high-speed imaging shows a luminous front, subject to cyclical variation, penetrating into the top-land crevice, but cannot distinguish between a flame burning into the crevice or just being pushed into it by increasing pressure in the cylinder. The LES of the process permits to turn off the reaction source term, so that the effect of convection and free flame propagation can be separated. The simulation shows qualitative agreement for the case where the reaction continues inside the crevice, giving further evidence for the need of detailed wall and combustion modeling inside the crevice.