As pointed out, Gen-2 and Gen-3 are completely different engine designs.
Gen-1: Completely standalone design. Wet Sleeve, 488ci, 4.000x3.880, 9/16 hardware, 4 bolt main, Gen-1 head and water jacket design, separate thermostat housing, later intake manifold changed to a Gen-2 like design layout. 120 degree main bearings. Few oil system problems due to deep sump pan and lower-revving nature of Gen-1 engine. Forged Crank, Forged Pistons, Forged Rods, Press fit pins.
Gen-2: Similar block "look" to Gen-1 [especially late Gen-1], but with 2 bolt + 2 side bolt mains for cap stability, dry sleeve & 1/2" hardware. Same bore/stroke as Gen-1. Completely New head and cooling jacket design. Thermostat now part of block casting. New timing cover design and oil pan with integrated oil pickup. Better Oil Filter location. 120 degree main bearings. Fewer oil system problems due to deeper sump pan design on 2000+ engines, but 1996-1999 with smaller oil pan or high revving [6500+] engines will see system instability earlier. Forged Crank, Forged Pistons up until 1999, Hypereutectic 2000-2002, Press Fit Pins, Forged Rods.
Gen-3: Complete engine revision. All new, 4.030x3.960, carry-over hardware and main cap design. New block casting, short-runner manifold design, much improved head design and cooling system. New timing cover and oil pan design, which later proved to be poor in terms of oil supply reliability. Shallow pan without integrated pickup showed issues in cornering, and timing cover design has stability issues much over 6000. Gen-3 does exceed all other in terms of total cooling system design. No other system can touch the hydraulic cooling module used in the Gen-3, even Gen-4/5. Knock Control added 2005-2006. 120 degree main bearings 2003-2004, 180 degree in 2006 to help alleviate some rod oiling issues that creep up with oil system feed instability. Forged Crank, Hypereutectic pistons, Press Fit Pins, Forged Rods 2003, Powdered Metal Rods 2004-2006.
Gen-4: Extensively modified Gen-3 type engine design. While it "looks" similar to Gen-3, it is in fact a completely new engine. The layout is similar and some parts can be retrofitted, but the technology, quality, and processes employed are light-years ahead. All new intake manifold is substantially larger in volume, flow and runner length. Substantially stronger block design, albeit with smaller hardware down from 1/2 to M12 Main/M11 heads. Complete oiling system revision. new high volume rotor with thinner profile and better back-side filling rate, shuttle type pressure relief valve with an air-bleed-off port and location to prevent cycling, swing-arm oil pickup with larger feed tube, revised pan baffling, new main bearing design. The heads flow substantially better and have better cooling, CNC chambers and throats, better casting quality throughout, lighter and faster [hollow intake] valvetrain, smaller/lighter lifters, Cam-In-Cam technology, Coil Near Plug technology, knock control, Drive-By-Wire, thermally insulated manifolds, better filter module, far superior data processing and engine position detection, etc. 160 degree main bearings w/secondary oil hole. Forged Crank, Hypereutectic pistons, Full Floating Pins, Powdered Metal Rods.
Gen-5: Nearly identical to Gen-4. Basically a Gen-4 engine with a slightly revised camshaft, new composite intake manifold, sodium filled exhaust valves, and a revised valve cover "look". Forged Crank, Hypereutectic pistons, Full Floating Pins, Powdered Metal Rods.
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