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The New Rock Box from Advance Adapters

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Posted July 1, 2002

Crawlers Delight

Step By Step

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  • Anyone who’s taken apart a regular geardriven two-speed transfer case recognizes this design. In high range the power flows straight through, while in low the flow goes from helical input to helical cluster to spur-gear cluster to spur-gear output. These all-new gears are specifically designed and manufactured for the Rock Box.

  • A test mock-up shows how the SM420 case mates to the Rock Box and the Toyota transfer case. About the same length as the regular adapter, the Rock Box offers 3.44:1 gear reduction without taking up any extra room like other competitive units do. For stock Toyota transmissions, the unit adds 5 inches to the drivetrain.

  • Assembled in the case, the cluster gear rides on a shaft with caged needle bearings. The Rock Box uses an open casting for the case, enclosed by the front plate, which is first installed on the back of the tranny. Different castings and front plates are used for various applications. Different input and output gears will be designed for other applications as they are needed.

  • We followed an install on a ’Cruiser at the Advance Adapters facility, and a completed one for testing and torturing. While there is clearance for most units, a body lift helps keep trimming of the rear floorpan to a minimum. The stock or SM420 tranny first gets an adapter plate bolted to the rear, which is about an inch thick.

  • To retain the Rock Box input gear, the transmission output shaft is whacked to length, then drilled and tapped for a retaining bolt. It’s easier to remove the shaft during a tranny rebuild for this modification, but can be done in the vehicle. Not all applications of the Rock Box will need this mod, but the stock Toyota ones will.

  • With the input gear bolted down, two all-thread aligning dowels are installed to help guide the Rock Box into place. The cluster shaft indexes into the front plate, so the front thrust washer needs to be stuck in place with some assembly lube or heavy grease. This keeps the washer in place as the unit is stabbed together.

  • The final product looks like it belongs underneath like a factory setup. The shifter goes through the floor on the driver side, and a nice boot and stick arrangement tops it off. Advance even has a twin-stick conversion for the Toyota three-speed transfer case to really make the job custom and complete.

The first adapter made by Advance Adapters was to hook a Muncie car four-speed to a Jeep Dana 18 transfer case. Back in the early ’60s the idea of a 1.9:1 First gear was cool; you could have a Jeep with 5.38 axle gears and still haul ass in the sand dunes. Nowadays, low axle and transfer case gears and lower tranny ratios are the rage, as are Atlas II transfer cases, doublers, and Klune-V units.

One of the big problems for Jeeps and Land Cruisers is the short wheelbase, which makes it hard to fit extra gearboxes anywhere, especially with a lift. The resulting driveline angle just won’t cut it, even with double CV joints. Recently, Advance Adapters had the notion to solve this riddle with an adapter replacement that doubles as a gear reduction unit. Enter the two-speed Rock Box.

The Rock Box is basically spur and helical gearsets in a nice housing, which fits between the transmission and transfer case. In low, the ratio is 3.44:1, while direct drive is of course 1:1. Currently, Advance has applications for Toyota Land Cruiser transfer cases mated to the stock four-speed tranny or, best of all, adapted to the GM SM420 and SM465 four-speeds. The SM420 has a low 7.06 First gear, and coupled with the 3.44 of the Rock Box and the 1.96 of the Toy ’case, you have a super-low crawl ratio with stock 4.11 axle gears of almost 200:1. Quite nice! In addition, the Rock Box is about the same length as the old adapter (5 inches), which negates any driveshaft mods or other problems. Of course if you have the Toyota trannies, you’ve got to make some changes.

We recently had the chance to try out a Rock Box in the rocks, as well as follow an installation and see some other new goodies Advance has on the boards. We think that this is a much needed product, and ’Cruiser owners should be rejoicing. Now all we have to do is convince Advance to offer applications for the Chevy, Ford, Jeep, and other vehicles.


Advance Adapters
Paso Robles, CA 93446