http://www.wikihow.com/Determine-You...ngine-Rotation

Some Mercruiser Inboard (NOT sterndrive) engines can be RH rotation.
Engine rotation is determined by looking from the flywheel end towards the pulley end.
Left hand (LH) engines are standard rotation. Also known as Counter Clockwise Rotation (CCR).
Right Hand Engines (RH) are reverse rotation. Also known as Clockwise Rotation (CR).
Chances are you have a LH standard rotation engine especially if it's a sterndrive or a single engine installation.
Usually only older twin inboard powered vessels have one engine RH reverse rotation.
Do not confuse engine rotation with prop rotation. They might not (and usually are not) the same.
LH and RH engines have a different firing order.
Most Mercruiser LH standard rotation engines use a timing chain whereas RH reverse rotation engines use only timing gears. RH and LH engines have different camshafts.
The distributor shaft rotation is the same for RH and LH motors and it is usually almost always a distributor shaft right hand rotation as viewed from above.
Some engines have the firing order cast or stamped into the intake manifold.
Your need to know the engine rotation to set your ignition timing.
RH and LH engines have different timing specs.
Use care when using a variable timing light as it must be set to zero when timing a reverse rotation engine.

You know wnen I read this I was thinking the same thing. By the time Gen V engines came out, I thought merc had changed over to Hurth transmission (full power reverse).
But not sure about other manufacturers, Crusader, etc. Also not sure if someone could have swapped out a Mark IV 454 timing gear set and cam into a 502.
Just pull the timing cover and check for gears instead of a chain cam drive.

that merc stuff is old..they still reverse engines but they were all done at whatever marinizer was doing the engine, they do not use gear to gear anymore(I have a gen VI reverse) special cam and distributor, not just the gear...as it will eat distributors like you would not beleive...I went thru 4...as the reverse cut dist gears pulls the dist shaft down and with no thrust to absorb the load destroyes itself in no time at all....