I was talking with my son and trying to come up with a definition for a small block vs. a big block.  I remember when Chevy had a 396 big block and a 400 small block.  I also read where the new corvette engine is a 427 small block.

I looked on the web and there are some postings from people who don't have a clue, i.e. a big block is a big motor and a small block is a small motor, or it depends on the transmission, a big block transmission won't fit a small block transmission.  DUH!!

Does anyone have a real explanation on what determines a big block vs a small block?  Bore sizes, stroke sizes?  Is there minimum measurement that determies this or is it just a subjective term that the manufactures use.

Any facts on this would be appreciated.  

With Chevy it was just the block used.  The big block was physically bigger.  With Oldsmobile a big block has a 1" taller deck height than a small block.  Pontiac didn't have a "big block"  just larger displacements.

For the other brands you'll have to wait for the rest of the forum to show up.  

As CSIROC said, pretty much it is a definate change in dimensions of the block to one larger.  Buick's, Ford's and Mopar's Big Blocks are just that, bigger casted blocks than the small blocks. Displacement as you noticed has nothing to do with it. (Hence why Pontiac as it was pointed out has no distinction between big and small blocks)  

Aside from the external dimmensions, the big difference between a big block and small block Chevy is the bore spacing on the engine.

While the 400 small block had just a few more cubes than a 396 big block, the bore spacing was smaller on the small block. On the LS-series engines, even though the LS7 is 427 cid, it still has the same bore spacing as a 4.8L, 5.3L, 5.7L, 6.0L, 6.2L LS-based engines.