Written By, Mike McGlothlin
The 4BT Cummins is one of the best diesel swap candidates automotive enthusiasts have ever come across. Its size makes it easily packageable in tighter engine bays, its weight is considerably lighter than its bigger brother, the 6BT, and it’s arguably the most durable small displacement compression ignition engines on the planet. But for diesel swappers, deciding on the 4BT isn’t an issue—it’s which transmission to bolt behind it that often presents the true conundrum. Automatic or manual? Old-school hydraulics or modern electronic controls? Heavy-duty or light-duty? These questions are highly common in the world of 4BT swaps. Unfortunately, there are no easy answers.
The truth is that adaptability, affordability, and obtainability have to be factored into any suitable option before you begin your transmission hunt. First, the type of 4BT bell housing you have must be identified. This dictates which transmission will be easiest to integrate (i.e. most adaptable) with your specific engine. Then not only should the transmission’s obtainability be established, but the availability of replacement parts, should you need one someday. Of course, the cost of your preferred transmission will likely factor into your decision. Join us below for the ins and outs of the most commonly employed transmission choices for the 4BT.
Step 1. Identify Your 4BT
As we alluded to above, finding out which 4BT Cummins you have is paramount in determining the transmission you choose to run. Specifically, identifying the type of bell housing is what you need to know. Engines pulled from off-highway, industrial applications such as gen-sets will have an SAE type bolt pattern, while other 4BT’s will either feature the small block Chevy or small block Ford pattern. In any application, the correct engine-to-transmission adapter (i.e. “flywheel housing”) will be required. We’ll note that most 4BT’s came with the SAE #3 bellhousing pattern. That said, plenty of others came with the SAE #2 style, the small block Chevy pattern and the small block Ford arrangement.
The highly adaptable, highly durable and affordable NV4500 is always a popular choice in 4BT swaps, and its high availability (given that tens of thousands of Ram and GM trucks came with it) makes it even more tempting. Rated to handle 460 lb-ft of torque from the factory (a vastly underrated figure, in our opinion), the NV4500 is sufficient behind the 5.9L 6BT and is pure overkill in a 4BT Cummins application. It’s worth noting that there were a few different versions of the NV4500, namely the units built for ’93-’94 GM trucks and those manufactured for use in ’95-later trucks, which is a similar version to what was employed in ’94-’05 Dodge Ram 2500 and 3500’s (with either the V-10 or 5.9L Cummins). The latter NV4500 is stronger and (obviously) the most sought after.
NV4500 Gear Ratios (Dodge Ram ’94-‘05 and ’95-newer GM Applications)
So long as you’re not pursuing big horsepower and gobs of low-end grunt, the Aisin-Warner AX15 five-speed is a solid manual transmission option for a 4BT. Originally offered in 1988.5 Jeep Wrangler (YJ), Cherokee (XJ/MJ) and Comanche models, the AX15 is most noted for its steadfast durability behind the 4.0L I-6 gasoline engine. Its maximum input torque rating is 300 lb-ft, but so long as it isn’t abused the AX15 can hold up to torque output significantly higher than that. The AX15 is fully synchronized, features helical gears, is of a split-case design (cast-aluminum case and mid-plate), and four-wheel drive versions came with the New Venture NP231 transfer case.
AX15 Gear Ratios
ZF-5 ZF-5 (2)
Although the ZF-5 transmission’s predecessor (in Ford applications), the T19 BorgWarner four-speed, was employed in countless 4BT-powered bread vans and is easier to integrate, the aluminum-case ZF-5 possesses superior strength and is worth the cost(s) associated with making one work behind the 3.9L Cummins. The fully synchronized ZF-5 was available in two model numbers, ZF S5-42 and ZF S5-47, spanning 1987-1997 and each version featured a close and wide ratio option. If you plan to modify the 4BT, the ZF S5-47 gearbox (close or wide ratio) is best. This version of the ZF-5 is rated for 470 lb-ft of torque whereas the ZF S5-42 is maxed out, at least on paper, at 420 lb-ft.
ZF S5-42 Gear Ratios
ZF S5-47 Gear Ratios
It’s not the strongest manual transmission option to place behind a 4BT, but the Muncie SM465 is plenty tough. And it has something great going for it: it came as a standard transmission option in thousands of 4BT-powered bread vans back in the day. If your 4BT was originally separated from an SM465 or TH400, no adapter plate will be needed—although hunting down the proper Chevy flywheel may take some time. The SM465 gearbox itself is made from cast-iron, is rated for 465 lb-ft of torque, and is synchronized in all forward gears, as well as reverse. A solid, old-school transmission, its one notable drawback (especially for highway drivers) is its lack of overdrive. However, for a budget-built trail rig, it’s hard to beat the SM465.
SM465 Gear Ratios
Speaking of bread vans fitted with the 4BT/Chevy bolt pattern (a combination that works with the SM465), the venerable three-speed TH400 (“Turbo 400”) represents one of the easiest ways to place an automatic transmission behind the 3.9L Cummins. Despite the fact that it lacks an overdrive gear, the TH400 has long-enjoyed a reputation for being a tough-as-nails automatic. Its maximum input torque rating checks in at a respectable 450 lb-ft and its low gear count and aluminum construction translates into a lightweight, 150-pound overall package. The biggest obstacle with the TH400 exists in spec’ing out the right non-lockup torque converter, one that’s stall speed meshes well with the 4BT’s abundance of low-rpm torque (and that complements your turbocharger).
TH400 Gear Ratios
Although significant weak points surface when Chrysler four-speed automatics are joined with the 6BT Cummins, the lower torque output of the 4BT Cummins makes them suitable in most applications. The 47RH specifically benefits from simple, hydraulic operation with simple pressure switches employed to operate Overdrive and converter lockup. In fact, a fresh 47RH rebuild (ideally with its select factory shortcomings addressed) and a quality torque converter makes for a reliable, all-around-use transmission with adequate performance on the highway and on the trail. As far as integration goes, sourcing a 47RH and NP241 transfer case combination straight out of a Cummins or V-10 Dodge Ram takes a lot of the guess work out of swapping one behind a 4BT.
47RH Gear Ratios
Circling back to GM automatic transmission options, the 4L80E is a sound choice—and many argue that, stock for stock, it’s much more durable than Chrysler’s 47RH. One sticking point for this GM four-speed is that you’ll need a stand-alone transmission controller. On top of that added expense, you’ll also have to fine-tune your own shift and converter lockup points. However, the payoff is worth it. The 4L80E is a quick-shifting unit, enjoys high availability thanks to a two-decade production run, and it’s a transmission that’s even been proven reliable in 6.6L Duramax applications. Its maximum input torque capacity is 440 lb-ft, but the gearbox itself is capable of withstanding 885 lb-ft.
4L80E Gear Ratios
The Pieces You’ll Need
You already knew that the engine-to-transmission adapter plate was arguably the most vital piece in your project, but it also pays to know which starter you plan to use when searching or spec’ing out the adapter plate. Luckily, thanks to a vibrant aftermarket surrounding the 4BT (and especially the 6BT), virtually any transmission can be made to work with the right supporting components—and a variety of adapter plates are readily available in the diesel aftermarket. Beyond the adapter plate comes the myriad of additional essentials: the proper OEM, aftermarket, or custom flywheel (or flex plate), the right clutch (or torque converter), wave ring (if applicable), control module (if applicable), and slave cylinder (if applicable), to name a few.
Our Pick: NV4500 Manual
Mirroring what we said about the best 6BT Cummins transmission, we believe the NV4500 is also the best all-around transmission for the 4BT. Of course, as long as you don’t mind shifting your own gears. After all, the NV4500 is rugged like the B-series engines are, just as durable, budget-friendly, readily obtainable thanks to all of the Dodge Ram and GM truck applications it was offered in, and it’s also upgradeable. In fact, with a larger input shaft and the infamous fifth gear lock nut problem solved with a fully splined main shaft upgrade, there is no stopping how long an NV4500 will last in a 4BT application—especially when they’re known to endure considerably more abuse from heavily modified 6BT engines.