Originally posted by techinspector1
Andy, according to my research, the best way to tell a 391 is by the oil drainback hole in the block low on the passenger side about halfway back, just above the pan. This was to drain air compressor oil back to the pan.

The 330MD, 359, 361, 389 and 391 were all designated as "FT" engines as opposed to "FE" passenger car engines. FT's were used in medium and HD trucks. The blocks were cast from a high-grade iron alloy with manganese, silicon and other alloys added to improve durability. Most blocks were strengthened with thicker main caps and main bearing webs. The FT uses the FE oiling system, including hydraulic lifters. A larger distributor shaft bore was used in the FT to accomodate a larger 5/16" HD oil pump drive shaft. FT blocks do not have the "352" cast into the front of the block like FE's. Most, but not all FT's have a mirror image "105" cast into the front of the block. The front snout on the FT's crank is a larger diameter than the FE's, so must be turned down to use FE components on the front of the motor. All FT's used forged cranks. Forged cranks can be identified by a wide parting line as opposed to a very thin line on cast cranks. Also, if the crank is out of the motor, hold it up in the air with a pliers or piece of wire (not held with your hand) and smack a counterweight with a hammer. If it rings like a bell, it's forged. Hope this helps.

I have a 73 motorhome with what I thought was a 390 big block does this means that it most likely is a 391 fe big block. Two barrel carb. Ed ke6bnl So. Calif