All modern bushed engines have a steel shim thrust washer fitted between the crankcase and the prop driver. Sometimes this gets lost or overlooked if the prop driver is removed and then the crankshaft can move further back than it should when the electric starter is used. This extra movement can make the crankpin contact the back plate.

Always be certain that this shim is lubricated before using an electric starter and never try to start a bushed engine without priming it first. At least then you've got some lubrication for the crankshaft.