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Atlas 31, 28, 34 spline input (Ford)

Atlas 31, 28 and 34 Spline Input (Ford)

Advance Adapter produces a variety of Atlas transfer case spline input shafts and adapters for use on Ford vehicles. Depending on the stock transmission, a 31, 28 or 34 spline of different lengths may be required. In some cases, spacers and other parts may be needed. You’ll also need to account for the stick-out length of the output shaft.

In this guide, we look at the three primary Atlas transfer case Ford spline options along with the variations of each to accommodate different stock transmissions — and in the case of the two-speed transfer case, whether you’re using a left-drop or right-drop setup. This documentation was produced to help off-roaders find the right conversions products and accessories for their specific application.

If you still have questions after reading our information on Ford Atlas transfer case inputs, browse our other Atlas documentation or call Advance Adapters toll-free at 800-350-2223. We have been manufacturing these cases in the U.S. since 1996 and can tell you anything you need to know about setup or compatibility.

Ford 31 Spline:

The 31-spline Atlas was designed to fit 4WD full-size Ford transmissions with a matching 31-spline output such as the Ford 4R100, C6 with automatic trans, Top Loader and T45. The stock transmissions used in these vehicles were normally coupled to an NP205, NP208 or Borg Warner 1356 transfer case. The bolt pattern on these transmission tail housing adapters is identical to a stock Jeep. Ford used various output shaft stick-out lengths.*

Caution should be used to avoid the spline portion of the shaft from “bottoming out” in the Atlas input shaft. Some applications that have an output shaft stick-out past the tail housing will require a one-inch spacer adapter (Part No. AS-0404). Late model full-size Fords with the Borg Warner 4406 transfer case and the vacuum-activated electric lock front axle will require a low-range switch on the Atlas shift housing and some electrical work for the front axle.

Ford ZF six-speeds work fine and are equipped with a 31-spline output. However, the six-speed ZF diesel will not since Ford used a 24 spline on this transmission.

If you have a Ford 6R80 or 10R80, we make a new adapter to fit these transmissions to the Atlas. This combination does require a shorter 31-spline input on the Atlas, which is Part No. A31S.

An Atlas 31-spline input is available for the following transmissions. 

Atlas Two-Speed Left and Right Drop: A31 with 0.325-inch transmission shaft stick-out length

Atlas Four-Speed: A4-31L (left drop), A4-31R (right drop) — Inputs accept 0.5-inch transmission shaft stick-out length

Ford 28 Spline:

The 28-spline Atlas was designed to fit only the 1966-77 Broncos with a three-speed manual transmission. The stock transmissions used in these vehicles were normally coupled to a Dana 20 transfer case. The bolt pattern on the face of the Dana 20 is a six-bolt hex pattern, which we mirror on the Atlas transfer case. The rotation options of the Atlas into these vehicles are 42 degrees, which is the stock location; 35 degrees for additional ground clearance; and 28 degrees, which requires some internal frame modifications.

Note: When bolting the Atlas to an automatic C4 transmission, we offer a shorter Atlas input shaft to avoid spline run-out. When using an NP435 or T18 transmission with one of our adapter housings, the threaded portion of the transmission output shaft must be removed.

The Atlas two-speed is the only unit available for these transmissions. A body lift is recommended to ensure a good fit.

Left Side Front Driveshaft Atlas Two-Speed: Part No. A28

The Atlas four-speed can be used with a C4 four-speed transmission. The C4 must be equipped with a new kit (Part No. 50-8100). This is the adapter that is used for Jeeps and comes with a new 23-spline output shaft. The adapter is 6.25 inches long and would require a new crossmember to be used. The Atlas four-speed would be an A4-23L (left drop).

Ford 34 Spline:

The 34-spline Atlas was designed to fit the 2004 and newer 5R110 (Torqueshift) transmission. This 4WD transmission has the speed sensor in the transmission tail housing, so when ordering the Atlas tail housing, a speedometer hole plug is required. The 2003-09 5R110 Torqueshift is a five-speed automatic that was used in the E250-350 and F250-450 6.0 and 6.4 diesels. The gear ratios are 3.09, 2.20, 1.538, 1.096, 1.00, 0.712 and R2.88. To install an Atlas using the stock 4WD tail housing, you will be required to use a one-inch spacer,  Part No. AS-0402.

An Atlas 34-spline input can be used for the following transmissions. 

Atlas Two-Speed Left and Right Drop: A34 0.325-inch transmission shaft stick-out length

Atlas Four-Speed: A4-34L (left drop only) — Input accepts 0.25-inch transmission shaft stick-out length

*Stick-out length is the length that the shaft protrudes from the rear of the transmission with the adapter installed. The measurement is the length that the Atlas will accept.