Well it's a long story, one I will briefly describe now and round out as time goes by...

SBC stands for Small Block Chevrolet and back in the 1950's the first SBC was born... It carried on for quite some time, until around 1985 when GM went from a two piece rear main seal to a one piece rear main seal, although some will say that the first Gen II was born in the LT engines released in the 1992 Corvettes.. I can get behind both ideas, but the fact remains that a one piece rear main seal crank from a late 1985 will fit in the 1992 and up LT engines pretty much means they are similar on the lower ends, however the blocks between an Gen II LT is in no way interchangeable between the earlier Gen 1.5 we'll call it since they are quite different. So technically, an LT1/4 Gen II is not the same as the earlier LT1/4, but the cranks will interchange between late 1985's and later LT engines all the way up until 1997.

The Gen III is a whole new small block from the ground up and introduced the LS series of engines, and like the progression between Gen I and II, the Gen III soon evolved into a Gen IV with many more offerings in sizes and combinations. While still a small block, nothing is shared between a Gen I or II with a Gen III and above. Completely different animals. Many variations of the Gen III and IV exist besides the popular LS that were fitted into GM and Cadillac's trucks and SUV's however these engines can interchange parts which is why many truck engines still use cast iron blocks rolling into the third and fourth generations, and why a lot of people like to frankenstein them with LS parts to get a stronger lower end than the typical aluminum block usually found in the LS.

All that aside there is now a Gen V, which came out in 2013 and brings back the LT1 and LT4 names. These engines are highly advanced and even I am still researching them.

Over time I will round out this post, but know that the Small Block Chevrolet is a legend and for a great deal of time people loved the SBC's because of their interchangeability and the ease of working on and finding parts for them. Yes there is a such thing as a Big Block Chevrolet, not to be confused with Pontiac, Cadillac, or Oldsmobiles engines which also looked like big blocks, but back then you dare not call a Pontiac, Oldsmobile or Cadillac engine a big block or feelings would get hurt and you might end up with a black eye.

So that's the deal in a nutshell, as I am limited with time at the moment. If you are thirsty for more I am going to attach a few links to Wiki where you can spend the next few hours getting your fill on the generations of the Small Block Chevrolet along with the newer LS generations.

More information on earlier Generations I and II

https://en.wikipedia.org/wiki/Chevrolet_small-block_engine

https://en.wikipedia.org/wiki/Chevrolet_small-block_engine#Generation_II_GM_small-block_.281992.E2.80.931997.29

More information on later model LS Generations III, IV and V

https://en.wikipedia.org/wiki/LS_based_GM_small-block_engine