dead center

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The point at the end of each stroke of a moving crank and connecting rod at which the two lie in the same straight line and the turning force applied by the connecting rod is zero.

from The Century Dictionary.

• noun In mech., that position of the arms of a link-motion in which they coincide with the line of centers—that is, when the links are in the same straight line.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The position of the crank of a piston when it is in line with the connecting rod

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the position of a crank when it is in line with the connecting rod and not exerting torque