rpm ?

On a ns90a is a cycle of one piston considered one rotation of the engine or is a cycle of all three pistons considered a complete rotation of the engine?

One rotation (or revolution) is when the crank turns through a full 360 degree cycle.

On a 3 cylinder motor, the pistons are (spaced) 120 degrees apart.

So one rotation would follow the travel of (say pistion #1) from top dead center until it gets to top dead center again (360 degrees later).

To explain it another way, let's say that the crank is at zero degrees when piston #1 is at top dead center. As the crank rotates, piston #2 will be at top dead center at 120 degrees of rotation (1/3 of the way), piston #3 will be at TDC at 240 degrees of rotation (2/3's of the way) and finally, 120 degrees later (0 or 360 degrees), #1 will be at TDC again - that's one revolution (rotation).

On a twin cylinder, the spacing is 180 degrees apart (exactly 1/2 a revolution) - a four cylinder, 90 degrees apart (1/4) etc...

thanks