10.2 - Transfer Case¶

Specifications¶

Gear Ratios¶

Crawl Ratio¶

With the 8HP70's deep first gear, the shallower Sport low range still yields a usable crawl ratio:

The deep 8HP70 first gear largely compensates for the 2.72:1 low range — a 4:1 Rock-Trac with the same gearing would give ≈96.7:1. ≈66:1 is a usable trail crawl ratio for this build.

Selection Notes¶

Chosen over the TJ Rubicon NP241OR Rock-Trac (4:1) deliberately:

• Crawl speed (primary reason): paired with the 8HP70 (automatic), a 4:1 case gives ≈96.7:1 geared — and the torque converter multiplies torque further at low speed, pushing the effective crawl far deeper. That is too slow to hold momentum on most terrain. An automatic wants a numerically lower geared crawl than a manual because the converter handles creep; ≈66:1 is the better target.
• Strength: a wash — both share the NP241 case, 1.25" chain, and 32-spline output, and the R2.8's torque is well within either. The Rock-Trac's reinforced 4:1 planetary is the heavier-duty variant but is not needed here.

Adapter (8HP70 → NV241)¶

An adapter is required — there is no native bolt-up. The 8HP70 is not the case's factory transmission, so it mates through a transmission-to-transfer-case adapter. The 8HP70 output and the required NV241 input gear are both 23-spline, so a 23-spline interface is the clean path.⁷

The 8HP family does pair with an NV241 from the factory — the JL Wrangler automatic bolts an 850RE/8HP75 to one — but the Ram EcoDiesel 8HP70's bolt pattern only resembles the Jeep 850RE; it is not interchangeable. So this transmission needs an adapter to any NV241 (JK or JL), and the transmission is fixed (Turbolamik TCU + Kilduff shifter are spec'd to the 8HP70).⁸

• Recommended — DomiWorks 24004001: purpose-built for the 8HP70/75 EcoDiesel Ram 1500 → NP231/241 family; adjustable case tilt. ≈8,495 SEK (~$800–900 USD + import). Includes only the adapter + integral crossmember bracket — see the parts list below.⁷
• Alternative — Advance Adapters: no off-the-shelf 8HP70 → stock NV241 kit. Their 8HP70 tooling (output assembly 50-9905, housing 51-9905) is built to mate to an Atlas; their NV241-JK input gears (52-9940 32-spline, 52-9945 29-spline) are cut for GM transmissions, not the 8HP70's 23-spline output. Call to spec a 23-spline build, or go Atlas.

Full parts list (DomiWorks path)¶

On the 8HP70 (transmission) side, the only DomiWorks part is the conditional spacer plate. The 8HP70's factory output shaft is used as-is — it is already a 4WD/transfer-case output (this is a 4x4 Ram donor) and engages the 23-spline input gear directly through the adapter; no coupler or output adapter is needed from DomiWorks.

Critical measurement: check the 8HP70 output-shaft protrusion before ordering — DomiWorks' standard fit is ~96 mm; longer EcoDiesel outputs need the spacer plate (or the axle shortened). Also confirm the donor JK case's input-gear spline before buying the 23-spline replacement.

Fluid Specifications¶

Outstanding Items¶

• Measure 8HP70 output-shaft protrusion (≤~96 mm, else order DomiWorks spacer plate or shorten)
• Order: DomiWorks 24004001 adapter; 23-spline NV241 input gear (ZTNP22390 or equiv.); NV241 input bearing + front seal; grade-10.9 hardware
• Confirm donor JK case input-gear spline before swapping to the 23-spline gear

Vendor Inquiry — DomiWorks (draft)¶

Draft email to confirm fitment and parts before ordering:

Subject: Adapter fitment — Ram 1500 EcoDiesel 8HP70 → Jeep JK NP241 (2.72:1)

Hi DomiWorks,

I'm mating a Ram 1500 EcoDiesel 8HP70 (4x4) transmission to a 2012 Jeep
Wrangler JK Sport NP241 transfer case (NV241 GenII Command-Trac, 2.72:1)
and want to confirm the right parts before ordering.

1. Is adapter SKU 24004001 the correct unit for this 8HP70 EcoDiesel →
   NP241 combination?

2. Output protrusion / spacer plate: where exactly do I measure the
   8HP70 output-shaft protrusion from, and what is the cutoff for needing
   the spacer plate? Is the spacer available for the EcoDiesel output,
   and what does it cost?

3. Input gear: do you supply the 23-spline NP241 input gear (ZTNP22390
   or equivalent), or should I source that separately?

4. Bearing / seal / hardware: can you include the NP241 input bearing,
   front seal, and mounting hardware, or are those separate?

5. Lead time and shipping cost to the US (state: ____)?

Donor details:
- Transmission: ZF 8HP70 (Chrysler 845RE), 2015–2019 Ram 1500 EcoDiesel, 4x4
- Transfer case: 2012 Jeep JK Sport NP241 (2.72:1 low range)

Thanks,
[name]

Related Documentation¶

• Transmission
• Driveshafts