Numbers like Pi and e belong to a category called transcendental number. Even though the series are infinite, you can calculate them precisely to as many decimal places as you care to. Many have done just this first manually and later with computers to a gazillion number of decimal places. There are even competitions for people who can recite the longest Pi decimals - a genuine waste of brain cells IMHO.

As to how many decimal places you need for your calculation, you need simply ask yourself how many significant digits you require for your final answer and how precise you bore and stroke measurements are. You only need as many SD as you want in the final answer. For example, if all you want to do is to see if your engine is a 2.8 c.i. or 50cc then even 3.1 would work just fine for Pi. Plus, if your bore and stroke numbers carry no more precision than, say, 45.0mmx31.0mm, then you really only need 3.14 for Pi for a final answer of no more precision than 49.3cc.

Of course, more decimal places for Pi doesn't hurt your calculations; it just won't make the answer any more precise than your other operands.

Okay, have we peaked the geek meter already?