I’ve been seeing a lot forums discussing the type 4 or BMW main bearing conversions to a type 1 case. Many of the forums explain how they machined the case to do this. They never discussed why. So what is the advantage of this conversion ?
What a great question! When a VW apposed 4 cylinder motor is run at high RPM’s, the crankshaft has a tendency to flex in the middle. This causes undue stress on the center main bearing and the center main saddle of the engine block. In the process that you are referring to the center main journal of the crankshaft is made to be a bigger diameter. On a stock style crankshaft this is done by welding, on a new racing crankshaft this is done in the machining process. In doing so the crankshaft is reinforced in this area adding strength to limit the crankshaft flex.
Unless you are running your motor and really high RPM’s of putting the motor under sever toque loads, this process is really unnecessary.