bore X bore X stroke X #of cylinders is the formula, I've never heard of ".7854" being in the equasion.

I'm sure that you know the bore and stroke of the engine in question. If you take the factory bore and stroke and this simple formula- should give you displacement. Bore x Bore x Stroke x 0.7854 x # of cylinders= displacement. Just swap in your new bore measurement and there you go. May be off slightly depending on how many decimal places you go to, but will give you a very close figure. This will apply to any engine, just change your numbers and good to go!

Sources: Auto Mechanics Fundamentals