Offroad Design Magnum Underdrive

GM, Ford, Dodge and Jeep all used a common six bolt circle pattern at the front of their various New Process transfer cases. They're the same pattern, but the GM pattern and the Jeep/Ford/Dodge pattern are clocked about 90 degrees different from one another.

If you're using a factory adapter, simply choose the Magnum face bolt pattern that matches the transmission. i.e. if you're using a GM transmission with a stock adapter/tailhousing, you'll want the GM bolt pattern bolt pattern on the Magnum

If you're using an aftermarket adapter, you'll need to be more careful. Many aftermarket adapters don't use the stock bolt pattern. For example, it's common for aftermarket adapters (like from Advance Adapters or Novak) to use the Jeep/Ford/Dodge six bolt round pattern even on a GM transmission. The Magnum bolt pattern needs to match the pattern on the adapter, regardless of what brand transmission it's bolting to.

GM NP205

GM NP205's came with either the figure 8 or hourglass pattern (1984 and older) or the six bolt circle pattern (1985-1991). We elected to build our GM/Dodge version Magnum underdrive only for the figure 8 pattern.

If you have one of the later GM round pattern cases, you have two options. We can exchange a big bearing figure 8 case with you and pay to ship it both ways, or you can drill/tap the figure 8 pattern into the face of the round pattern NP205. There's really isn't any advantage to either bolt pattern, they're just different.

GM NP205's with 10 or 27 spline male inputs will need to be disassembled and the housing machined as the "fat shaft" we supply with the kit uses the larger 90mm OD input bearing. We offer this service if you do not have a local source to machine your case.

GM NP205's that were attached to TH400 transmissions (any year) and SM465 transmissions ('85-91 only) will already have the larger 90mm OD input bearing and don't require any machine work

Dodge NP205

All married Dodge NP205's have the figure 8 or hourglass bolt pattern. It's easy to mistake them as a round pattern case, but if you remove the cast iron adapter on the front of them you'll see they share the same figure 8 pattern as the GM cases (we don't use that cast iron adapter with our Magnum underdrive).

23 spline cases will need to be disassembled and machined for the larger 90mm input bearing bore, 29 spline Dodge NP205's will already have the larger bearing bore.

Divorced Dodge NP205's are not adaptable to our Magnum underdrive.

Ford NP205

All married Ford NP205's are ready to accept our Magnum underdrive in factory form.

Divorced Ford NP205's can be modified to work, but it's often easier to find the married version. Converting a divorced Ford NP205 to a married one requires the case to be disassembled and bored for the larger 90mm input bearing and the married NP205 bolt pattern drilled/tapped into the face of the case.

The input splines need to match up to your transmission. We have a VERY wide range of options lined out here:

21 spline - Jeep YJ and TJ 4 cylinder 4WD applications

23 spline - Most Jeep transmissions, Dodge gas engine transmissions and Cummins automatic transmissions

23 spline 5-45RFE transmissions only

24 spline - Some Ford Superduty ZF6 applications

27 spline - TH350, 700R4 and 4L60E

29 spline - Getrag 360, Dodge diesel NV4500/NV5600/G56, GM Allison 1000, GM ZF6, GM 6L90E

31 spline - Most Ford applications

32 spline - GM TH400, 4L80E, 6L80E, GM NV4500

34 spline - Some Ford Superduty automatic and manual transmssions

Using a TH350 that was mated to an NP203 or NP205

Use the adapter here. Jeep/Ford/Dodge face pattern, 27 spline.

Using a TH350 that was mated to an NP208

We have a Magnum underdrive that will bolt directly in place of the NP208. GM face pattern, 27 spline.

Using a 2WD TH350

2WD TH350's don't work and will need to be torn down and a 4WD output shaft installed.

Using a 4WD 700R4 or 4L60E/4L65E/4L70E

We have a version of our Magnum underdrive that bolts directly in place of the stock transfer case. GM face pattern, 27 spline.

Using a 2WD 700R4 or 4L60E/4L65E/4L70E

The transmission will need to be torn down and a 4WD shaft installed

Using a TH400 that was bolted up to an NP203, output shaft sticks out ~1-3/8"

Use the adapter here. Jeep/Ford/Dodge face pattern, 32 spline.

Using a TH400 that was bolted up to an NP208 or NP241, output shaft sticks out ~2-1/2"

We have a version of our Magnum underdrive that bolts to the stock adapter directly in place of the stock transfer case. GM face pattern, 32 spline.

Using a TH400 that was bolted up to an NP205, 1985 and newer, output shaft sticks out ~2-1/2"

You'll need a TH400/NP208 adapter, we carry those here. GM face pattern, 32 spline.

Using a TH400 that was bolted up to an NP205, 1984 and older or a 2WD TH400, output shaft sticks out ~4-1/2"

If you can find one, an '85 and newer TH400/round NP205 adapter works. They aren't available new and are rare. This would use the GM face pattern, 32 spline input.

Alternatively, we carry a brand new adapter that is the right length for this output shaft here. This would use the Jeep/Ford/Dodge face pattern, 32 spline.

Using a 4WD 4L80E

We have a version of our Magnum underdrive that bolts to the stock adapter directly in place of the stock transfer case. GM face pattern, 32 spline.

Using a 2WD 4L80E with a slip yoke output shaft

The 2WD slip yoke style 4L80E shaft is splined all the way back to the transmission case, it can be cut down to mimic the 4WD output shaft. Using a stock 4WD 4L80E adapter, we have a version of our Magnum underdrive that bolts up directly. GM face pattern, 32 spline.

Using a 2WD 4L80E with a fixed (bolt on) yoke output shaft

Output shaft needs to be changed to stock 4WD output shaft. Using a stock 4WD 4L80E adapter, we have a version of our Magnum underdrive that bolts up directly. GM face pattern, 32 spline.

Using an SM465 that was bolted up to an NP208 or NP241

This version uses a 32 spline output shaft, we make a version of our Magnum underdrive that bolts directly to the stock adapter in place of the original transfer case. GM face pattern, 32 spline

Using an SM465 that was bolted up to an NP205, 1985 and newer (NP205 has six bolt circle pattern)

This version uses the same 32 spline output shaft that the SM465/208 and SM465/241 uses, but with a longer adapter. The transmission works as-is but will need to source an SM465/208 or SM465/241 adapter. GM face pattern, 32 spline.

Using an SM465 that was bolted up to an NP205, 1984 and older (NP205 has figure 8 or hourglass bolt pattern)

This version uses a 10 spline output shaft that will need to be changed. There are two methods:

Install our short 32 spline SM465 output shaft and use our short SM465/D300 adapter. This is the shortest way to put these together, the adapter is 3-5/8" face to face, Jeep/Ford/Dodge face pattern, 32 spline.

OR

Install a factory 32 spline output shaft and use a factory SM465/208 or SM465/241 adapter. This is the longer way to do it with an 8-1/4" adapter, GM face pattern, 32 spline.

Using a 2WD SM465

One of the 32 spline outputs noted above will need to be installed and the corresponding adapter used

Using a GM 4WD NV4500, any engine

This version uses a 32 spline output shaft, we make a version of our Magnum underdrive that bolts directly to the stock tailhousing in place of the original transfer case. GM face pattern, 32 spline

Using a Dodge 4WD gas engine NV4500

This version uses a 23 spline output shaft, we make a version of our Magnum underdrive that bolts directly to the stock tailhousing in place of the original transfer case. Jeep/Ford/Dodge face pattern, 23 spline

Using a Dodge 4WD Cummins diesel engine NV4500, NV5600 or G56

This version uses a 29 spline output shaft, we make a version of our Magnum underdrive that bolts directly to the stock tailhousing in place of the original transfer case. Jeep/Ford/Dodge face pattern, 29 spline

Using a GM or Dodge 2WD NV4500

One of the 4WD output shafts and tailhousings need to be installed, the output shaft and tailhousing need to match (i.e. GM/GM or Dodge/Dodge)

Most Dodge 4WD automatic transmissions share the same Jeep/Ford/Dodge face bolt pattern and a 23 spline output shaft

As long as it uses the common six bolt circle pattern and is from a full size truck, we should have a version of our Magnum that bolts up. These will use the Jeep/Ford/Dodge face pattern, most will be 31 spline, some newer Superduty diesel applications are 24 or 34 spline.