If there's nothing mounted on the engine, then it's normal for the crank to move fore and aft and hit the rear cover. If you have a properly-mounted flywheel on the engine, you'll get a small amount of movement, but the crank should not hit the rear cover. However, it's normal for the connecting rod to make swirl marks on the rear cover because it 'floats' on the crankpin. All glow engines tend to do this, and is not damaging to the engine.