I included a pic with a "cut" view of the interal engine parts. in this picture, the piston is currently at "TDC", known as top dead center. BDC, bottom dead center is when the piston is all the way to the bottom. When the piston is at the bottom of the stroke, you can mark your flywheel so you know where to turn it when setting it to BDC after running.

While you run your engine, the metals expand. You can lose your pinch at the top of the sleeve by leaving the piston at TDC or at mid stroke simply because the metal will retract as it cools. Each sleeve has a pinch, the pinch is tighter towards the top.