This is normal with ABx engines. Because of the pinch near the top the piston can't quite make it as far as it normally will when hot and running. This loads up the bushes at the top and bottom of the conrod and in fact compresses the conrod slightly. When you turn the crankshaft a little further all this strain gets released and you hear that click.

Putting the glow plug in and flicking it gives compression on top of the piston which pushes the piston back down so you don't notice that pinch.

As rsieminski said, it's bad practise to slowly turn over a new ABx engine before it's got some decent oil in it. The oil that's in it when new isn't there to lubricate, it's just to stop any rusting. Because there's an interference fit at the top there's more than enough time for any oil to get squeezed out when turned over slowly. Then you have metal to metal contact and the piston could be damaged.