i looked at a 57 merc.  engine today that was supposed to be a 312 but it didnt have the raised bump on the rear of the crankshaft like the other 312 engines i have.  the question is did all 312 have this raised bump or was some without it.  carl 

With the engine installed in the vehicle, the aforementioned 5/16” dot or bump on the crankshaft is most easily seen from the bottom of the vehicle when the TDC on the damper is at the six o’clock position.  The 5/16” dot will then be at about the 5:30 position on the lower edge of the crankshaft flywheel flange when viewing from the front.  Any covers or shields that block the view of the crankshaft flange will of course need to be removed to see this crankshaft flange.

Ditto on some of the 312 cranks not having the 5/16" dot as I've had one of those also.

Lorena, Texas (South of Waco)

the engine is out of the car with the bellhousing and flywheel off so it is easy to check the end of the crank.we pulled a head off yesterday and it has the standard 312 bore.next we will put it on a engine stand and  drop the pan and check the main caps.  carl