<Slaps Forehead> Sorry lads clearly it was late  when I responded and not thinking straight.

Dynamic rotational balance is good on a boxer yet somehow I managed to think about a 180 degree V engine..... :/
Engine where the two pistons are on the same journal as per conventional V engine. Mercedes Benz built a 180degree V12 in the early nineties for sportscar racing and the Zundapp motorscycle has a 170 degree V

The poster asked about rocking couple which is a dynamic imbalance due to the one piston being staggered slightly ahead of the other. Yes that's what the balance mass is for. The two pistons and their rotational mass plus combustion forces want to precesse the crankshaft about a point somewhere around half its length. Designers need to balance the shaft to counteract that precession. Often some engine will have the balance mass incorporated into the casting of the crank either as an eccentric mass. I imagine the Brisson's crank has this. Never seen one so Final balance is usually not necessary if parts are matched to account for any imbalance cast into the crank.

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