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Jeep FSJ Fullsize Pickup Guide

Posted in How To: Tech Qa on October 16, 2015
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Few pickups garner the same attention and respect as a ’63-’87 fullsize Jeep J-series truck. These trucks aren’t exactly rare, but they are certainly uncommon, especially in the Rust Belt of the United States. They came in many different GVW ratings, wheelbases, and derivatives over the years. Thriftside, Townside, stake beds, and even a chassis-cab version were made available. There were many different drivetrain and suspension changes over the years, so not all FSJ trucks are desirable for off-road use.

The ’63-’70 Gladiator trucks are considered the most collectable, but they have many disadvantages over the newer J-trucks. If you are interested in turning your FSJ pickup into a simple, multi-purpose, reliable trail rig and daily driver using mostly bolt-on parts, then you’ll likely want to steer clear of the ’73 and earlier trucks. They feature a less-desirable post-mount suspension, drum brakes, and a weaker closed-knuckle Dana 44 front axle that doesn’t turn very sharp. Some of the early engines are less desirable as well, namely the 230ci Tornado inline-six and 327ci Vigilante V-8. They can be unreliable and difficult to find parts for. The later 232ci inline-six, 258ci inline-six, Buick 350 V-8, and the three AMC V-8s (304 ci, 360 ci, and 401 ci) are far better powerplants with lots of aftermarket support. The best bang for the buck can be found in the ’74-’79 pickups, although the ’74 model has some oddball suspension bits. The ’80-’87 trucks have more modern amenities, but they are choked down with low-horsepower engines, smog equipment, and they often have wiring gremlins. All of these issues can be remedied, but it is something to consider. If you plan on hacking out the engine, transmission, suspension, and axles, then any J-truck will do. However, keep an eye out for rust, which is commonly found around the windshield frame and on the floor panels among other areas. Severely rusted trucks should be avoided. There are some new replacement body panels available from companies like BJ’s Off-Road ( It also offers new J-truck seals and other trim and interior bits. For larger and more-difficult-to-find used items, you can try Montana Overland (

Lots of different transmissions were available in the J-trucks over the years, many are not notable. The most durable and desirable optional transmissions include the GM TH400 (optional from ’65 to ’79) and Chrysler TorqueFlite 727 (from ’80 to ’87) three-speed automatics and T-18 (from ’68 to ’87) four-speed manual with a 6.32:1 First gear. All three of these transmissions have a great reputation and are capable of handling far more power than the engines they came behind.

The available transfer cases were sort of ho-hum. Look for the part-time cast-iron gear-driven Dana 20 in trucks up to ’79 and the aluminum chain-driven NP208 in ’80 and later trucks. The optional full-time transfer cases can be problematic if worn out or filled with the incorrect fluid. Both are extremely common. Here are some more suggestions for upgrading the J-trucks.

The stock closed-knuckle Dana 44 front axle found in ’63-’73 J-trucks doesn’t offer much strength, braking, or steering angle. It’s fine for collectors, but for those that want a trail truck with a front locker, you’re better off swapping it out or starting with a ’74 and newer J-truck with an open-knuckle Dana 44 front axle. The open-knuckle Dana 44 front axle enjoys tons of aftermarket support in the form of gears, lockers, heavy-duty axleshafts, and more.
For bigger tires and off-road use, it’s a good idea to replace the spindle bolts on the closed-knuckle Dana 44 with open-knuckle Dana 44 studs or quality 1 1/4-inch-long 3/8x24 button-head bolts. Minor machining is required to fit the hardware from the backside of the knuckles. This modification helps keep the spindles from blowing off of the knuckles in abusive off-road conditions.
The closed-knuckle Dana 44 front axle came with drum brakes. It can be upgraded to discs using common CJ hubs and vented rotors along with Dana 44 caliper brackets. Some grinding for caliper clearance is required and steering angle becomes even more limited. The calipers will make contact with the shock mounts if you don’t properly adjust the steering stops.
The Dana 20 transfer case found in many J-trucks can be upgraded with an Advance Adapters ( 32-spline rear output. The stock 10-spline output is susceptible to failure under heavy loads or if the driveshaft binds.
None of the transfer cases that came in the ’66-’87 J-trucks are excessively beefy. For heavy off-road use and tires over 35-inches tall, you might consider swapping in an Advance Adapters Atlas. This aftermarket transfer case is easily adapted and fits well in the J-trucks. It’s available in many different low-range ratios to match your off-road needs.
Most J-trucks came with the durable 30-spline semi-floating Dana 44 rear axle with flanged axleshafts. However, some early-to-mid ’70s J-trucks came with 35-spline semi-floating Dana 60 rear axles. Full-floating 8-lug 30-spline Dana 60 rear axles (pictured) were available on the heavy-duty trucks. There is lots of aftermarket support available for the J-truck rear axles.
The ’63-’73 trucks feature a less-desirable post-mount suspension design. Aftermarket suspension lifts are very limited. However, Hell Creek Suspensions ( offers front and rear 4-inch lift springs for post-mount trucks.
The ’75 and later J-truck suspension has an abundance of aftermarket support. Front and rear leaf springs and lift kits up to 6 inches are available. Companies like Skyjacker ( offer complete lift kits with U-bolts, shocks, extended brake lines, and a drop pitman arm.
The steel fuel tanks in the early FSJ pickups are usually full of filter-clogging rust. MTS ( offers polyethylene replacement fuel tanks for nearly every J-truck model. The company also has new fuel sending units and hard-to-find filler hoses.
The AMC 360ci and 401ci V-8s are by far the best engines that came in the J-trucks. However, they still have some issues. The factory ignition is completely anemic and problematic. Performance Distributors ( offers a Davis Unified Ignition (DUI) that replaces the entire wonky AMC ignition system. It’s basically a GM HEI distributor that has been machined and custom tuned to work in an AMC V-8. The performance difference is like night and day.
Unfortunately, J-trucks were discontinued before factory fuel injection became common. The good news that there are many aftermarket fuel-injection systems available from companies like Edelbrock (, FAST (, Holley (, Howell EFI (, and Professional Products ( to help improve fuel economy and off-road performance at any angle.
Some purists hate the idea of it but cutting off the fender flares on the J-trucks provides a lot more room for bigger tires. With a 4-inch lift and removed flares, you can easily fit 37-inch tires on a ’63-’87 J-truck.

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