I hate to have to quote myself ...
What happens when the crankshaft is at 90? You'd think that the piston is halfway down the bore, right? Well that's not exactly true. The angularity of the rod at that point brings the piston down a little bit further - the connecting rod length is the hypotenuse of a triangle where the short side is the sideways distance from the center of the cylinder to the center of the crankpin.
Pythagoras was a Greek mathematician who lived about 2500 years ago, and who developed the most famous formula in geometry, possibly in all of mathematics! He proved that, for a right triangle, the sum of the squares of the two sides that join at a right angle equals the square of the third side. The third side--the side opposite the right angle--is called the hypotenuse of the right triangle. The two shorter sides are usually called "legs."
This formula is called the Pythagorean Theorem in honor of Pythagoras. It is usually written as the equation below, where a and b are the measures of the legs of the triangle and c is the measure of the hypotenuse.
http://www.***boat.com/image_center/...358formula.gif
So with my original post of the crank being at 90* (from TDC) ... the hypotenuse would have to be the rod angle :squiggle:
http://www.***boat.com/image_center/...hypotenuse.gif