Here is a link with some more information about the
manometer method. For a 216, one degree of rotation from TDC produces about 0.0003" of piston movement and about 0.10" of oil movement in a 3/16" ID tube. Two degrees of rotation from TDC will result in about 0.40" of oil height change. Could be as accurate as the positive stop method for finding TDC. The manometer method could be good for an engine (like one of mine) where a bolt through a spark plug doesn't touch the piston.
Hoyt
