Put an Old Trans Behind Your New Gen III V-8

Engine and transmission swapping is the heart of car crafting. The GM Gen III/IV (4.8L through 6.2L engines from 1998 to present) engine swap is quickly becoming the new classic with GM musclecar movers and shakers. Up until now, a Gen III/IV engine also demanded using a late-model transmission, either a Gen III/IV-dedicated 4L60E automatic or a six-speed manual. These trans choices not only limit creativity but also tend to be more expensive. The latest move is using older transmissions, such as Gen I four- and five-speed manual gearboxes along with the classic TH350/TH400 three-speed autos and the 700-R4 and 200-4R four-speed automatic overdrives. All these transmissions can be adapted to Gen III/IV engines if you know the tricks, and that's what this story will detail. Car Craft has done all the research so all you have to do is follow the outline and save yourself money, time, and effort.

We'll break this story into sections for manual transmissions and automatics. The automatic side is pretty easy, so we'll save that for last. The manual trans info isn't difficult, but there are a few essential details that make the swap a bolt-in, so we'll start there.

Manual Trans Swaps
When GM decided to build the new Gen III small-block that would become the LS1, GM's engineers carried over a couple of important design characteristics that make it easy to retrofit newer engines into older cars. It's doubtful that GM considered car crafters' requirements when it designed these new Gen III/IV engines, but regardless of the company's agenda, it did us a couple of big favors. The first gift is that the Gen III/IV bellhousing pattern is almost identical to the original small-block Chevy. Five of the six bellhousing bolts are in exactly the same location, as are the size and location of the two dowel pins. This one fact allows us to make everything else work. The Gen III/IV bolt pattern is not symmetrical, which is odd, and the top center bolt is employed while the upper passenger side bolt is not. But if using five of the six bolts works for you, then everything else is easy. As you will see, there are plenty of options that allow you to use all six bolts.

Unfortunately, GM engineers also added a few hurdles just to make this swap game more challenging. First and foremost, the crankshaft flange on a Gen III/IV engine is essentially even with the bellhousing flange on the block. On the original small-block and big-block Chevy engines, the crankshaft flange extends 0.400 inch farther rearward (aft, if you're nautically inclined) of the bellhousing flange. This is the critical differential measurement that defines all the other modifications we have to make to adapt earlier transmissions.

Another important piece to this swap puzzle is that all Gen III/IV engines use a distinct crankshaft mounting flange. This means now we have three separate crank flanges: the Gen I small-block with a two-piece rear main seal, the Gen I with a one-piece rear main seal, and now the Gen III/IV engines with their one-piece rear main seal. While this should come as no surprise, the Gen III/IV engines are now completely metric, demanding all new fasteners. Obviously, this means Gen I flywheels and flexplates will not interchange with a Gen IIII/IV engine. One other important tech tidbit is that all Gen III/IV engines use a 168-tooth flywheel or flexplate.