Building a BW1350-1354 Transfer Case Doubler

Introduction

Are you building a 1983-2011 Ford Ranger and need a transfer case doubler for rock crawling? You can always build your own. This article will teach you how. By proceeding you agree to assume all risks related to building and operating a doubler, including but lot limited to loss of vehicle, accidents, injuries, loss of sleep, broken relationships, and chronic rock crawling.

Table of Contents

• Preface
• Tool And Material Requirements
• Splitting The BW1350
• Making The Adapter Shaft
• Cutting The BW1350
• Plate Fabrication
• Preparing The BW1354
• Assembly/Installation
• Driveshaft Changes

Preface

Why build a doubler? As tires get larger and larger, more gear reduction is required to provide enough torque at low speed to turn them. Changing differential ratios provides more reduction, but Ford Ranger based vehicles (RBV) are limited to around 5.13 gears before the pinion gets too small and weak. 5.13s though helpful, provide only limited reduction. If you’re running 3.73s and an M5OD, you’ll go from 35 to 47 in final crawl ratio with 5.13s. A doubler will take you from 35 to 85.

Benefits – Doublers are for trail trucks that need a massive amount of low-end torque and a very low crawl ratio. In addition to excellent reduction, they provide the driver with true 2-wheel low range, as the doubler case can be put in LO and the transfer case in 2 HI.

Basics – This article will describe how to put a Borg Warner (BW) 1350 transfer case in front of an existing BW1354. This is done by splitting the BW1350 and keeping only the front half, then cutting the front output housing off of that front half. Then the BW1350 half is connected to the BW1354 by means of a steel plate between the two. A custom shaft connects the two cases. To compensate for the 6” addition, the front and rear drive shafts must be lengthened and shortened respectively.

Costs – The system can be built for around $400. This includes parts, materials, and tools. Costs will vary for each person and by location. You may already have all the materials laying around in your garage or you may have to buy everything. You might have a buddy with an old Ford Ranger that lets you grab the 1350, or you might have a stuck-up yard that charges a few hundred for a case. Plan on spending at least a couple hundred on the entire project. For me, it cost $300 but I already had all the tools.

Who should build a doubler? This is a very advanced project. It is harder than rebuilding an engine. One mismeasurement and the entire setup could be shot. This project requires patience and mechanical experience. Plan on at least 7 working days to get it done. This is only a guide. Each setup may be different. There are so many steps in the project that this would turn into 100+ pages, so each step is not necessarily listed.

Tools And Material Requirements

Components

You will need a Borg Warner 1350 manual shift transfer case. You cannot use an electric shift case. Your best bet in finding a manual 1350 is a 1980s Ford Ranger.

You also need a Ford Ranger / Bronco II, or early Explorer 4WD transmission output shaft. You only need the end. I went to a junkyard and asked if they had any junk Ranger transmissions. Then I had them cut off the shaft with a torch.

Tools

• Normal tools, i.e. socket set, wrenches, c-clamps
• 5” hole saw
• Drill with high-quality metal bits
• Angle grinder with cutting and grinding blades
• 3/8” 16 pitch tap
• If you are going to shorten/lengthen you shafts yourself, you’ll need a chop saw and a welder.

Materials

• 12”x12”x1/2” steel plate
• RTV
• JB Weld
• Brake parts cleaner
• Rags
• 6 – 3/8” bolts, 1.5” long
• 2 – 3/8” bolts, 1/2” long

Other

You’ll want to find another transfer case skid plate to protect the new case.

In order for the doubler assembly to fit without major body modification, you need a 3” body lift on Ford Bronco II’s, and a 2” body lift on Ford Ranger and Explorer vehicles.

Splitting The BW1350

This is one of the easiest parts of the job.

Remove the center bolts holding the two halves of the case and pull it apart.

You can then remove the bolts holding the front plate and pull that off; the planetary set will come out with it.

The rear housing with all the components is junk.

Making The Adapter shaft

This is a part where you’ll be heading to the local machine shop, unless you have an arbor press and a metal lathe. The cost for this should be between $50 and $100.

Take the input shaft (the main shaft inside the case) and the 4×4 transmission output shaft.

Tell them what the application is. Tell them the shaft needs to be built to handle the most torque possible. They need to first cut down the transfer case shaft, then bore it out. Then they need to lathe down the 4×4 output and press the two together. Finally, the two need to be welded together. You’ll start with this:

4×4 transmission output shaft (top) and transfer case shaft (bottom). Remove that little ATF pump and discard it.

And you’ll end up with this.

As for the length, you cut the transfer case shaft off at the gear. The 4×4 transmission output shaft goes in with the splines ending right where the transfer case shaft begins. The total length of the shaft should be 5-7/8” and the output spline length should be 2-1/16”.

Cutting The Case

You should have the front BW1350 housing with the planetary set removed. The next step is to cut the front output housing off. As looking at the above picture, you want to keep the bottom hole and cut through the top hole. Below is what you should end up with:

Throw away the housing on the right. You now officially have a doubler housing. There is a 1/2″ gap at the end of the case, this can be filled in with some of the 1/2″ plate bolted in and sealed with JB Weld.

Drill and tap drain/fill holes in the top and bottom of the case. You can thread a short bolt into each of the two holes as a plug. Be sure to seal the drain plug/bolt with RTV or it will leak.

Plate Fabrication

Line up the doubler housing on the plate. Make a center mark on the plate through the input hole and get to work with the 5” hole saw. It will take at least 20 minutes to cut through the plate. Use cutting oil and allow for the drill to cool from time to time.

Once the hole is cut you can line the case back up on the plate. Drill out and tap the two most outer boss holes into the plate. These will thread into the plate. The remaining case boss holes around the 5” hole will thread into the case from the opposite side.

You can clean out the case and reinstall the front plate. Above: the first of the outer two holes has been drilled and tapped into the plate. The bolt is threaded in on the bottom right.

Next, drill 7/16” holes for bolts to thread into the case from the opposite side as shown below. You can use the stock bolts that used to hold the two halves of the BW1350 together.

Finally, place the doubler case on the plate and cut it out.

You’ll need to cut down the shaft that the shift fork rides on and grind a recessed hole in the plate for it to sit.

Preparing The BW1354

The BW1354 front plate has to be ground down in three places in order to fit over the bolt heads going into the BW1350. You’ll need to remove the BW1354 front plate and see what needs to be ground down.

Once you get the plate seated you can drill the holes into the steel plate and tap them. You may wish to clock up the transfer case a little if you think you’ll have driveshaft clearance issues with the radius arm cross member. One bolt goes through the plate and threads into the BW1354, the remaining go from the BW1354 front plate into the steel plate. These are the 1.5” long 3/8” diameter bolts you purchased. You’ll have to drill out the threads in the bosses, as the stock setup has bolts thread into the BW1354 plate from the transmission.

Since the BW1354 breather tube output is now right in front of the plate, you’ll need to cut down the existing fitting in order to be able to fit a hose over it. You can also find and thread in a 90-degree fitting.

Additionally, some minor grinding may need to be done on the actual case for bolt head clearance. Clearance issues are huge in this project. Basically, grind where the need arises.

Assembly/Installation

It is recommended that you do a trial assembly to make sure everything fits before RTV sealing it.

Order:

1. Begin with both front plates removed from the cases
2. Bolt the doubler case onto the plate
3. Bolt the BW1354 front plate onto the steel plate, with the BW1354 plate-to-case bolts in place
4. Assemble the internals in the doubler case
5. Install the doubler case front plate, check to make sure the case shifts ok
6. Connect the BW1354 to its front plate
7. Fill the doubler case to the top with gear oil; the transfer case can be filled with ATF after installation

Driveshaft Changes

Your front shaft will need to be lengthened, and the rear shortened, both by about 6”.

You can have a shop do this or do it yourself. You’ll need a chop saw and a welder if you want to do it yourself.

Cut the front shaft right in the middle. Find a piece of pipe that has 1/4″ thick walls and is the right diameter to fit in or over the shaft and weld it on.

For the rear: Grind off the weld where the male splines for the slip yoke come out of the driveshaft. Cut the necessary amount off the shaft, and then hammer out the splined cup. Carefully and evenly re-weld it to the shorter shaft. Either of the two shafts can be dynamically balanced by filling them with some oil.

WARNING – DO NOT put the doubler case in LO and the transfer case in HI. This could break your adapter shaft. Also, you should have the newly created splined shaft heated prior to welding and then cooled slowly to prevent cracking.

Related Articles:

Transfer Case Doubler Overview & FAQs

1983-2011 Ford Ranger Transfer Cases

BW1354 Transfer Case Exploded Parts Diagram

D&D Transfer Case Doubler Install

Dual Transfer Case Conversion

Behemoth Drivetrain Strongbox

Co-Authors

Written and photographed by Evan Vaughan / Process developed by Todd McIntosh and presented to the author for publication at TheRangerStation.com to share with other Ford Ranger enthusiasts.

Last Updated: November 18, 2025