I have twin 350 Chevy inboards with the port standard rotation and the
starboard reverse rotation. The port has a standard timing chain while the
starboard has timing gears which means that the cranks turn opposite but the
cams turn in the same direction. Both engines use the same part number
Mallory distributor. On the starboard, the firing order is reversed by
reversing the sequence of the wires in the distributor cap. It all seems to
make sense so far.

In looking in the Mallory catalog, I note that they list a reverse rotation
distributor for this engine as well as the standard rotation model. The
helix on the reverse model is opposite hand. This would mean that it would
have to mesh with a camshaft gear which was also opposite hand helix.
Reversing the hand of the helix on the gears switches the direction of
thrust so that it would remain upward against the distributor housing thrust
washer with the distributor shaft turning backward. This would seem to
indicate that there must be some engines that reverse the cam as well as the
crank for opposite rotation. However, since the distributor shaft drives
the oil pump, the pump would also turn backwards and this isn't possible.

Can anyone clear this up for me? Thanks.