First, let’s tackle some terminology. There’s overall length, which is the length measured from extreme end to end. This is from the very end of the ball end to the very end of the other ball end or cup end as the case may be. A couple factors affect this measurement. One being that if the pushrod being measured has oil holes in it then the measurement could be off by the amount that the machining of the oil hole subtracted from the overall length of the end. This usually doesn’t adversely affect the length depending on the diameter of the hole.
The actual measurement if there were no oil holes in the ends is called the theoretical length. Don’t let this confuse you because the difference is usually very slight. A problem of more significance is when you measure a cup and ball style pushrod. Measuring the overall length of this style can give erroneous information as the cup depth from one manufacturer to another can and does vary by as much as .060 inch. If you do know the overall length and have a caliper to measure the cup depth then this measurement will work.
The measurement from the bottom of the cup to the end of the ball is called the “effective length.” Knowing this length is more accurate as the cup depth differences won’t be factor. One way to determine effective length is by placing a steel ball (the correct diameter for the cup) into the cup and using a caliper, measure the overall length then subtract the steel ball’s diameter. This will give you the length from the bottom of the cup to the end of the ball.
Source: Smith Brothers Pushrods