On both of my cruisers the BB's begin to get play in them after about an hour of riding. Recently a new set of cranks, chain, chainring and bottom bracket was installed on an Isla Vista salvage rebuild. Same problem. Would adding a second left side locknut help the problem? Or are there other tips besides converting to 3-piece cranks? I'm careful about tightening the adjusting cone and locknut just to the point of pressure resistance in the crank movement.

My wife's Bildabike is the step thru frame version of my Bildabike and her's does not loosen. Same mechanic services all three bikes, lol.

Thanks in advance for any replies.