Then a split happens the Gen I continues on till 2003 in some production and on as a crate engine. Gen II starts in 1992 through 97 these being the LT1 and LT4 they are a derivation of the Gen I with a modified block and head’s that reroute the coolant to the head’s first then returns coolant from the block and uses a redesigned front mount coolant pump and distributor. The internals of crankshaft, rods, pistons, cam and valve train will interchange to Gen I engines, with minor mods in some cases.
Starting in 86 the Gen I gets a one piece rear seal this carries through afterward. Starting in 87 everything gets fuel injection either Throttle Body or Tuned Port. Trucks and some car models keep the flat tappet cam others mostly Camaros, Corvettes and SS Impalas get roller cams. In 96 what hot rodders call the Vortec comes out for trucks and non performance passenger vehicles. These have Multipoint Fuel Injection and roller cams.
Gen III starts phasing in during 1997, it is a clean sheet of paper redesign that shares no common components with the Gen I or Gen II engines.
There’s more in the details than this but these are the big glob changes.
To get more specific for the most part the 80-85 engine is common back to about 1968. These would be long coolant pump with accessories bolting to the front machined face of the head’s. Unless you get into a performance model they all use the same hydraulic flat tappet cam. From 1973 through 86 they all use same similar low compression SMOG heads. The power differences are in the details of compression and carburetion used and the SMOG tune adjustments and external equipment used. They all use a two piece rear seal. The front oil pan to timing cover gets adjustments made to the these parts for a thicker gasket under the crank seal chin around 1975. There is a change to oil dipstick as to which side of the engine it mounts to left for older right for newer in this series. In this period engine mount positions are the same, bellhousing bolt pattern is standard Chevy for the US inline 6’s and derived 4’s and the V8’s and their derived 90 degree V6’s.
By changing cam, head’s, pistons, intake, carb, distributor and other parts any power level from stock SMOGer to screamer is possible.
Starting in 86 the blocks and crank were changed to the one piece rear seal. These can be used with pre 86 transmissions by simply by using the 86 up flywheel or flex plate and insuring that either include the small external counter weight either as an intregreal part or as a bolt on at the crankshaft. The difference is the older bolt pattern on the crank is 3.5 inches in diameter and the one piece seal crank is 3 inches and lost a small piece of external counter weight off the crank. The TBI engines use a very mild cam like 170 degrees intake at .050 this is down about 15 degrees from the earlier pedristrian cam. The TPI engines of 87 thru 95 use several types of roller cams that are more similar timing wise to the pre EFI engines. All of these are easily backdated to carb with a Professional Products or Pro Comp intake that has interchangeable intake bolt adapters that use either the older bolt angle on the bolts beside the plenum of 90 degrees to the machined head intake face or the TBI and some TPI head’s using the 72 degree angle on these bolts. All head’s from 1955 through all the Gen I and II use the same head to block bolt pattern and size. Gen II head’s while bolting on do not match the coolant transfer or oil return holes of the Gen I block. They can be modified to fit but it is more expensive than just buying aftermarket head’s. The same can be said about the Gen II block in that you can’t put Gen I heads on it. I happen to have this conversion on my daily driver but that was done before cheap and reliable aftermarket head’s were available and I was involved with a shop that had the equipment to do this in-house. The out of pocket expense was like 120 dollars, to have this done it would be about 1500 dollars or more and these head’s compared to more modern aftermarkets are not technically adequate anymore.
The above paragraph pretty much, also, describes the 96 up Vortec L30, 305 and the L 31, 350. These bring the factory (OEM) roller cam into the truck engines this actually is often found as provisions in 95 flat tappet TBI trucks. These in the 95 and 96 blocks also usually have a functioning traditional coolant bypass passage between coolant pump and block, this allows simple back dating of the coolant pumps. The succeeding Vortec blocks eliminate this feature totally so back dating these blocks to the Vee belt pumps or the 86-95 serpentine pumps takes some work plugging the unused bypass passage to prevent leaks. Vortec head’s fit the older blocks they require a unique intake that has their 4 bolts per side at 45 degrees to the head’s intake face but the aftermarket does provide a lot of choices; this is and has been for over two decades a popular choice as these head’s make power. They are well copied and improved on in the aftermarket which usually includes both the 55-86 and the 96 up L30/31 Vortec bolt pattern.
The 96-2002 L30/31 engines use a more complex fuel injection system which includes a plastic timing case cover and unique solid state distributor. Note the 87-95 TBI engines L03, 305 and L05, 350 also use a computer controlled distributor but it is a quite different design. If your converting these engines to a carb and more conventional distributor aside from the roller cam needing a Melonized drive gear older distributors are a drop in. These blocks will accept either a flat tappet or roller cam the roller can be a retro aftermarket with thrust button or the factory (OEM) thrust plate. The timing set is unique between flat tappet and retro roller cams and those for the OEM roller cam. The depth of the cam gear hub is reduced so as to accomodate the thickness of the thrust plate. The timing case cover is made from plastic and includes an integral front crank seal, it is intended to be non magnetic so as to not interfere with the crank position sensor. It can be replaced with a conventional case cover of stamped or cast metal. The factory eliminated some of the retaining bolts of the metal cover replacing them with guide pins or nothing, however, the bosses are there and can be drilled and or tapped easily. So with a little effort with at most a hand drill and otherwise hand tools these more recent blocks can be back dated. This includes using the older two piece rear seal crank with the available aftermarket adapter if needed or desired.
I rather think a lot of kudos are in order to Chevy engineering as to how they went about modernizing the SBC without losing the elements of the original design that so well stood out through its many years of product life. The way they made improvements to the SBC, especially in the 86, 87 and 96 redesigns of the Gen I allows for a lot of flexibility in using these engines in older vehicles with fairly simple and easy modifications to the induction and ignition.