BorgWarner T-18
Donor Vehicles: Various Ford trucks, Jeeps, and International Trucks from '66-'90 with the 70's and early '80s seeing the bulk of the production.
Swap Into: Anything that needs a bulletproof four-speed transmission
Identify: This is the tough part. Jeep alone used 18 variants of this transmission. The Ford T-18s are the more standardized ones. Not all T-18s had the desirable 6.32:1 First gear. Turn the input and count output revolutions. If less than 6:1 it's not the T-18 everyone wants. Case length is 11 7/8 inches with a height of 17 1/2 inches and features both a cast-iron case and cast-iron top cover.
Gears: The ratios vary, but the most desirable unit has: 6.32, 3.09, 1.69, 1.0 (First through Fourth); 7.44:1 Reverse
Pros: Very heavy-duty transmission, the best of all OE Jeep manual transmissions in terms of sheer strength and abuse it can take.
Cons: No Overdrive, long-throw shifts, and finding the right one can be tough.
Comments: If you ever stumble across a Jeep with a T-18 you owe it to yourself to at least check the First gear ratio. If the transmission doesn't have the low First gear, grab the 1-inch spacer/adapter between the transmission and bellhousing and the bellhousing itself. These parts are often not included with second-hand T-18s. Check out novak-adapt.com for more info.
GM Muncie SM420
Donor Vehicles: Chevy Trucks from '48-'67, busses, and various military applications behind diesel engines.
Swap Into: Any Jeep that needs the lowest low gear in a manual transmission, or has rear driveshaft length issues.
Identify: It has a cast-iron case, cast-iron top cover retained by eight bolts, the case is 10 3/4 inches long, a power take-off port only on the driver's side of the case, and a bulge near the front on the passenger side are dead giveaways.
Gears: 7.05, 3.58, 1.71, 1.00 (First through Fourth); 7.05:1 Reverse
Pros: More heavy-duty than any aluminum Jeep transmission, its ultra-short length makes it a great swap for shorter Jeeps, and a very low First gear means greater control for the rocks.
Cons: The bulge in the passenger side of the case can mean driveshaft clearance issues in passenger-drop front axle Jeeps, the low First gear is useless on the street, and the newest SM420 you are likely to find is at least 40 years old. To bolt it to a Jeep engine will require some kind of adapter, incurring more cost.
Comments: While they are rare to find in junkyards anymore, they can still be had for dirt cheap from the guys who are now hot-rodding the trucks these originally came in. The long throw of the shifter can get tiresome living with day to day, but if you can get past that, and only essentially having a three-speed Jeep on the street, then this is a good little transmission. Certainly it is the best manual transmission to swap into a flatfender due to its small size and legendary durability
GM Muncie SM465
Donor Vehicles: GM trucks, Blazers, and Suburbans from '69-'01
Swap Into: Any Jeep that needs brute strength. Can live behind high-horsepower engines with ease
Identify: At 12 inches long, and 18 inches tall with a cast-iron case and top cover it isn't all that hard to pick this transmission out of the Chevy/GM lineup. The clinchers are the two PTO ports (passenger and driver's side), the large ribs of the case, and the 6.55:1 First gear.
Gears: 6.55, 3.58, 1.57, 1.0 (First through Fourth); 6.55:1 Reverse
Pros: Most heavy-duty four-speed light truck transmission and likely the most reliable to boot. It is still easy to find in junkyards, and easy to get parts for with tons of aftermarket support.
Cons: At 18 inches tall, it will hang down lower than your transfer case and could take some planning to make work. Also, like the SM420 it replaced, the low-low First gear is almost unusable on the street rendering your Jeep essentially a three-speed for most uses. It won't just "bolt-in" to many Jeeps, requiring adapters, machine work, or fabrication to install.
Comments: Big and beefy, put any engine you want in front of it and you won't have to worry about it. Like the other truck-style transmissions, the throw is pretty long, especially that Reverse gear. Some stick massaging might be required to keep the stick out of the passenger in Reverse or out of the dash in First.