OPPY'S COME IN 2 DIFFERENT BORES, 3.4 AND 3.5 [I DON'T REMEMBER THE REST OFF THE TOP OF MY HEAD]. THE BIG BORES FROM BRIGGS ARE KINDA RARE. THE COMMON BORE, 3.4, ARE THE NORMAL AND CUBIC INCH IS DONE WITH STROKE. YOU HAVE 40ci THE SHORT STROKE TO THE 46 THE LONGEST STROKE WITH A 42ci IN THE MIDDLE. THIS ALL TAKE THE SAME PISTONS, WHICH IS THE SAME AS THE 28ci SINGLE. THE DIFFERENCE IS MADE UP IN ROD LENGTH. ALL THE BLOCKS ARE THE SAME DIMENSIONS. YOU CAN PUT ANY TINS ON ANY SIZE. THE BIG BORE BLOCK AND USING A 46ci CRANK IS AS BIG AS YOU GET [WITHOUT CUSTOM SIZING].