Stack two NP241s and gear down for the rocks — the doubler kit that turns your transfer case into a crawl box setup.

This is the FST (Froehlich Suspension Technology) NP241 doubler kit, the adapter setup that joins a second NP241 range box in front of your main NP241 transfer case. The result is a dual-case "doubler" — you keep your normal high range and low range, and gain a double-low that stacks both cases' reductions. With the NP241's 2.72:1 low range doubled, you're looking at roughly 7.4:1 out of the transfer cases alone, before your transmission and axle gears even enter the math.

That's real crawl gearing — walk your Squarebody over obstacles at idle speed with total control, then shift back to 1:1 and drive it home. The NP241 is a favorite foundation for this build for good reason: strong aluminum case and a 6-pinion planetary that holds up to the multiplied torque. If you're building a K-series crawler and want doubler gearing without the price of an aftermarket crawl box, this is the kit.

Built For

This kit is designed specifically for Squarebody Chevy/GMC trucks/blazers/suburbans, in the following configuration:

• Front donor case: driver drop
• Rear main case: passenger drop

After You Order

We reach out by email after every purchase to verify your spline/quantity count and confirm which transfer cases you're running before anything ships. This makes sure you get the correct kit for your exact build — no guessing, no wrong parts.

Why It Works

• Real crawl gearing — double-low stacks both cases for roughly 7.4:1 reduction from the transfer cases alone.
• Keeps street manners — shift to 1:1 high range and drive it like normal.
• Built on the NP241 — strong aluminum case, 6-pinion planetary, proven under multiplied torque.
• Smart money — doubler gearing for a fraction of an aftermarket crawl box.
• The Squarebody crawler move — the drivetrain upgrade serious K-series builds run.

What You Need To Know

• Adapter kit to run a dual NP241 (doubler) setup — you supply the transfer cases (your main NP241 plus a donor for the range box).
• Configured for a driver-drop front donor case and a passenger-drop rear main case.
• Built specifically for Squarebodies with the 700R4 transmission. Running a different transmission? You may need to change the transfer case input shaft, may require different transmission-to-transfer-case mounting adapters, and will need a different — or modified — transmission crossmember.
• This is a fabrication-level drivetrain build regardless: plan on crossmember work, driveshaft length changes, and a dual-case shifter setup.
• Pairs with our JB Custom Fab BBOR-Spec Dual NP241 Transfer Case Shifter for clean twin-stick control of both cases.
• Price is $550 — shipping calculated at checkout.

Planning a doubler build and want to talk through what your setup needs? Text 530-327-8811 or email blazinbiddlesoffroad@gmail.com before you order — happy to walk through it.