We are the sole supplier of Inchworm Adapters and are currently building products with them! This adapter kit allows you to mate a 1995 and newer 23-spline Chain Drive transfer case behind a Gear Drive Dual Case Reduction Unit!

Made in USA, this billet adapter allows you to bolt a 1995+ Toyota VF1A Top Shift Chain Drive Transfer case (23-Spline) behind your RF1A gear-driven transfer case reduction housing or crawl box (1979-95).

Top-shift style 1979-83 or 1985-88 RF1A transfer case core required to assemble into a Crawl Box. A forward-shift style RF1A transfer case can be used with the appropriate top shift rods/fork.

Includes Ultra Grey silicone, Dished MC08 Coupler, Heat Treated Thrust Washer, and high quality OEM JIS Japanese hardware.

This listing is for the rear adapter. Front adapter also required & sold separately.

What does a Dual Case do?

A Dual Case has three shift positions that may be engaged from a stop:

• High: High Range is 1.00:1 so shaft speed & torque enters and departs at the same rate and is used for normal City and Highway driving. The ECU and all associated systems such as cruise control, traction control, stability control, Toyota Safety Sense, ABS, ATRC, and Crawl Control, function normally. City and Highway Fuel Economy is unaffected. The truck will feel and operate 100% like factory both on-road and off-road, 2WD or 4WD. The Dual Case is simply bypassed.
• Neutral: A Neutral position which many newer vehicles do not have.
• Low: Low Range may be either a 2.28:1 or our famous 4.70:1 ratio. When engaged, your vehicle instantly has more traction, more torque, more control, with longer reaction for increased safety, as well as less bouncing, less breakage, less trail abuse, and less risk to vehicle occupants.

The end result will look like this:

Engine > Transmission > Dual Case > Transfer Case

… thus transforming your 2-speed (High/Low) factory transfer case into a 4-speed Dual Case transfer case as follows:

Required modifications?

Required modifications include contracting with your local driveline shop to lengthen, shorten, and rebalance your front and rear shafts, lengthen the wires of your transfer case shift solenoid (if applicable), lengthen 6mm vacuum hose to shift base and solenoid (if applicable), and install a new shift handle into your interior by cutting a 2.5″ hole in the center of your cab and modifying your center console. For automatic owners this usually involves modifying a cup holder, coin slot, or cubbyhole. For manual owners, a simple modification to your transmission boot is all that is needed. Please see our Interior Shift Handle Installation Picture Gallery for ideas!

No factory crossmember modifications are required, however, modifications to under belly skid plates may be required depending on brand. The Marlin Crawler Dual Case includes provisions for an additional transfer case mount which is optional but when incorporated is a great addition to help lengthen the lifespan of your engine mounts! If desired, we offer a Heavy Duty 6cyl wide-style OEM Toyota mount here.

Below is an example of a fully assembled Marlin Crawler Dual Case Unit
using this rear Inchworm Dual Case Adapter Kit