4 main 6's: That was the main change (other than displacement) from the W113 230sl to the 250sl. The 230sl had 4 mains whereas the 250 got one per piston.

smoothness: there are two components to smoothness, even firing order and balance. An even firing order is designed in by making the bank angle 360/# cyls or 720/# cyls as it has been mentioned. A slightly uneven firing order isn't the end of the world. Many racing V-6's have this due to simple crankshafts and odball vee angles. Older GM v-6's also had an uneven firing order.

Balance is trickier. For any vee engine, it has primary balance when the bank angles are at 90 degrees. This is because of the forces acting on the crankshaft. A pair of cylinders sharing a crank throw 90 degrees out of phase apply sin(angle) and cosine(angle) forces to the crankshaft. When added, these form a rotating force vector of constant magnitude. This circular rotating force vector can be exactly cancelled by a counterweight on the crankshaft. And that is how you get to perfect primary balance.

If the vee angle is not 90 degrees, the resultant force vector will be elliptical and will not be cancelled completely by the counterweights.

Once you've made a 90 degree V-6, to get even firing order, most manufacturers offset each cylinder's crank throw 15 degrees on either side of the crank throw. This makes the pistons get to TDC in an even manner restoring even firing order.

Sholin

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What else, '73 MB 280 SEL (Lt Blue)
Daily driver: '84 190D 2.2 5 spd.