kircher is spot on. Top and bottom dead centre are both points of stoppage and also the points of maximum acceleration towards the opposite end.
Regarding the imbalance of 4 cylinder engines, that's caused by the difference in acceleration away from each dead centre due to the rod length. ie. 10 degrees of rotation of the crank away from TDC will not produce the same piston movement as 10 degrees of rotation away from BDC. The only engines that get close to being balanced out are 6 or 12 cylinder engines due to the three "wave forms" generated but even they can't be truly perfectly balanced as the induction, compression, power and exhaust strokes all add to or subtract from the inertial forces present.
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