The New Rock Box from Advance Adapters

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.