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Adapting the

GM Muncie SM420

to the

Jeep Dana Models 18 & 20 Transfer Cases

Our SM420 to Dana 18 & Dana 20 conversion creates a robust geartrain assembly for broad use in severe terrain or tame roads. It is one of the most steadily popular manual shift transmission conversions into Jeeps, providing a very compact geartrain, a supreme crawling gear, compatibility with several engines, and ease of installation.

Product & Features


Spline Count: 6 10
Bore Size: 3-5/32 in 4 in

use the options above to select the correct product for your conversion

The Adapter

The Novak SM420 to Jeep transfer cases adapter is 4.4" long for a combined transmission and adapter length of 15.2". The adapter itself is cast and machined of 356T6 aluminum heat treated alloy, featuring thick flanges, a heavy cross-section, and strength-adding gussets.

Rear Mount Provision 

This transfer case adapter's design is based on Novak's Universal Series adapters that feature a modular mounting and support system. Included with the adapter kit is a steel base that is a highly configurable mounting foot, and is ready to accept our industry standard urethane isolator mount and is available for purchase below.

Applications & Compatibility

sm420_fds_w_423_421_adapterTransmissions

Any version of the SM420, including 2wd, 4wd early, & 4wd late styles are compatible with this adapter assembly. There are no advantages between the 2wd vs. 4wd versions of this transmission.

Mainshaft replacement is largely straightforward. Good work is within easy reach of the careful home mechanic and instructions are included. We recommend at least our basic gasket kit. Better yet, this is a good time for a more thorough master rebuild and update of the transmission. Many consider one of our complete Novak remanufactured and upgraded transmission packages. The advantage of a superior, professionally built transmission, tested and ready to bolt in delivered to you is definitely worthy of consideration. 

sm420_rtpq_with_422

Transfer Case Versions

All Jeep models of the Dana Spicer 18 and Dana Spicer 20 transfer cases are compatible with this adapter assembly. IH versions of these transfer cases are also compatible. The correct input bore and spline count must be observed. This adapter is not suitable for the Ford Bronco transfer case as it is of an entirely different design in attachment and drop.

The new Novak manufactured 6 or 10 spline adapter shaft is machined from premium alloy steel and hardened for maximum strength and longevity ready to be installed in your Borg-Warner transmission and then the D18 or D20 input gear attaches directly to it.

Contents

The kit contains aluminum adapter housing, transmission output shaft, seal, transfer case to adapter gasket, transmission to adapter gasket, fastening hardware, steel mounting base, and instructions. The adapter comes assembled to the extent possible.

Transfer Case Input Gear

dana_18_20_input_gearOur adapter shaft is splined for six spline input gears.

  • Six-spline gears (shown, right) are most common and usually found behind Jeep T84, T90, T86, T89, T98, T18, T15 & T150.
  • The ten-spline version was only available on the 1966 - 1974 transfer cases against the Jeep T14 transmission
  • Factory Jeep TH400 to Dana 20 combinations had a fifteen-spline input gear, which must be replaced by a compatible six spline version.
  • International Harvester Dana 20's that had the TF-727 automatic had a 23-spline input gear, which must be replaced by a compatible six spline version.

Consult our informative Dana gears interchange chart if necessary.

All Warn and Saturn type overdrives are compatible with this conversion as long as the spline count is correct. We do not have replacement gears for the overdrive version.

dana_18_20_front_gasket

Transfer Case Bore Size


The diameter of the transfer case center input locating bore will be either 3-5/32" or 4".

  • Most Dana 18's have a 3-5/32" bore, including those found against the T84, T90, T89 or T98.
  • Some Dana 18's have a 4" bore, including those found against the T86 & T14 transmissions (Buick V6 engines), 1966-1971.
  • All Dana 20's have a 4" bore, regardless of the transmission they were joined to.

Jeeps 

This is compatible with most Jeeps as it is one of our shorter 4 speed combinations. 

Engines

Any engine that can be made to work with the chosen transmission will work with this adapter. We have available parts to adapt several versions to various engines. See the SM420 information page linked below for that discussion. A procedure adapting to the Jeep engines is shown here.

420_with_422_drivers_side_viewsm420_with_422_rear_drivers_quarter_view

Related Products

Transmission Rear Mount

This transfer case adapter's design is based on Novak's Universal Series adapters that feature a modular mounting and support system. Included with the adapter kit is a steel base that is a highly configurable mounting foot, and is ready to accept our industry standard urethane isolator mount, available for purchase. This is our preferred rear mount unit. Our kits are designed for it, and its usage is essential to protect powertrain components from the flex and movement typical of any automobile, but especially Jeeps.

Whether you are assembling onto a stock crossmember/skidplate or if you are setting up a high-clearance skid plate, you will find it to be a clean and simple process.

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GM Transmission Pilot Bushing

A pilot bushing is always required for installation choose the correct version for your application from these choices

AMC Cranks



GM Cranks



SM420 Rebuild Kits 

Though not necessarily required, it is our recommendation to do a full rebuild on these units while you are in the transmission. These are the best parts available. Additional hard parts are no longer manufactured and get increasingly more difficult to obtain. If you have pieces that need to be replaced, it is often best to look at the full transmission option above or keep additional cores on hand. If you need other parts, you are welcome to call in. Please don’t be offended if our sales tech chuckles when you ask for first or second gear or simply quotes you the price of a complete unit. Our instructions accompany all rebuild kits. At the very least you will want the gaskets, but we do recommend the full rebuild kit.

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Gasket Kit

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Sticks

Straight sticks are available if yours is work or you simply want to start with a clean slate to get the bends right.

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Retainer Kit

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Billet PTO Cover 

Why not add a little “Bling” to that transmission? These beautiful billet covers also seal up substantially better than the stock stamped steel piece. Depending on the transmission you may need 1 or 2 of them.

Block_K-PTO

 

Transmission Tunnel Cover

Because the transmission and transfer case shifters are often in a different location than factory in these Jeeps, this new transmission tunnel cover gives the installer a clean slate. There are no screw holes nor transmission & transfer case shifter holes, allowing the installer to drill and cut them as necessary. An easy way to get a perfect fit is to use the old cover as a template, making cuts as necessary, then overlay on the new aluminum piece for perfect holes.

The finish is natural brushed aluminum, which looks pretty terrific, but it can easily be primed and painted to suit, if desired.

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Dana 18 & 20 6 Spline Input Gears

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D20 Transfer Case Shifters

The stock transfer case shifter may be used, possibly requiring minor modifications. On the Dana 20 it is often advantageous to use a new twin stick style shifter, especially if you are able to remove the interlock pins during the transfer case rebuild process. Multiple models are available depending on your original transmission mated to the D20. It is also possible to retrofit a longer length shifter by using our replacement tubing and TX20-COM4 sold below. You will likely need to re-bend the shifter levers for correct positioning but this gives a sound starting point. Going this route allows the shifter to be positioned further forward in relation to the transfer case which allows a location closer to stock as the transfer case moves back with the longer length TH350.

Block_TX20-CJ1S
Block_TX20-CJL
Block_TX20-COM3
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This is the tubing necessary to fabricate a custom length shifter, sold by the inch.


All would require the

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D18 & D20 Rebuild and Gasket Options

Block_18MR-1




Gasket and seals kit

Block_GS318

Dana 20 Billet Pan and PTO Cover

These rigid and precisely machined billet bottom pans are the answer to a few Dana transfer case dilemmas. The thick, smooth-finished flange replaces the thin, warped factory pans and they virtually assure a drip-free seal. The PTO cover does the same for the back, plus they add a little “bling” to your case.

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Dana 18 & 20 3.15:1 Upgrade Gearset

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Installation Requirements

422_mod20Tools 

Normal mechanics tools are required for the installation.

Transfer Case Shifter  

This transmission conversion will allow for the retention of the factory Dana 18 twin-stick shifters, Dana 18 single-stick shifter, and many of the Dana 20 shifters. Some tube-mounted Dana 20 shifters may need to be modified, or replaced with a twin-stick shifter kit.

Placement 

Engine placement in these applications is an effort in compromise to find the best position overall as a package. Generally the engine will be about 1” or even more towards the left, (driver’s side in the USA) away from the front differential in a CJ application. This gives the best balance of weight, and more importantly clearance, for the driveshaft going to the front axle. Your tight spots will be steering to exhaust on the left (hold that as tight as you can) and clearance for the front driveshaft on the right. On a driver’s side drop transfer case, usually the later Jeeps, things often get a little easier as steering and front driveshaft are pushing you the same direction. Fore and aft position will vary with the Jeep model and engine.

Have a CJ5 and Gen I with a rear distributor? You’ll be better off a little forward for more rear driveshaft length and clearance for that HEI. If you are in a longer Jeep and using an LS engine with no distributor, you’ll have more fan clearance and better balance if you hold it to the rear. Usually for ground clearance tuck things up nicely for height to avoid damage to oil pans and other life giving parts in the Jeep. On later Jeeps with a Driver’s side drop transfer case and front differential, things are a little easier as the steering and offset are both pushing the drivetrain to the passenger’s side. Again, that 1 to 1-1/2” dimension is usually where you want to be. Common sense and taking a step back to look at things overall goes a long way.

Clocking Information & Driveshaft / Yoke to Pan Clearance 

Use of the factory size driveshaft and yoke is recommended. Our adapter design has maximized clearance between the transmission pan and yoke, but it is necessary that you install your GM engine offset 1-1/4" to the driver's side. If you are installing against a factory AMC engine with our 437 AMC adapter kit, your factory engine should already have this proper offset. Our adapter is drilled for multiple clocking locations from near-factory (20 degrees down) to near-flat (3 degrees down). However, pan clearances and front axle width may inhibit the flatter options.

Engines & Engine Mounting

mmtj-g3If converting to a V8 in conjunction with this upgrade, aftermarket engine mounts are available to ease its installation. Because of the broad spectrum of engines and vehicle combinations used with this adapter, it would be difficult to list them all here. Whatever your application, Novak likely has the perfect solution. From a 225 Buick V6 in a CJ2A to an LS3 in your JK, we’ve got you covered. 

Novak's engine mounts, featuring excellent vibration dampening, superior strength, and impressive adjustability - adjustable even after the engine is installed. The design, strength, and affordability of our mounts are second to none.

Driveshafts

Driveshaft length changes are often required. Most conversions to these transmissions will require that the rear driveshaft be modified to be shorter and front driveshaft longer. Some of our customers, fearful at the perceived expense of new or modified driveshafts, attempt to let the existing driveshafts dictate engine, transmission and transfer case location, usually to the detriment of the project. Our recommendation is to always value the correct position of drivetrain components over saving a few dollars which is usually regretted in the long run with compromised positioning. Sometimes the application allows use of the existing shafts without modification. While that is perfectly acceptable, it should not be the first priority. Driveshaft modifications and rebalancing can be affordable when performed by driveline, RV or tractor implement specialists. New driveshafts are an option, but not necessarily a requirement in regards to the actual successful conversion if your existing driveshafts are in good condition. Jeeps that require extensive travel or specialty-built driveshafts have this option available through several fabricators across the nation. These are normally specified after placement of the new transmission and measured at vehicle ride height. As the rear driveline gets shorter, it is often advantageous to us a Double Cardan or “CV style” rear shaft with the correct geometry at the axle to minimize vibrations and possible binding. Novak can also accommodate your needs on this if provided with dimensions for the shafts at a reasonable rate.

Axles

Use of factory axles is completely acceptable with this conversion. Axle upgrades are not necessary, but they may be chosen for reasons external to this transmission upgrade.

J. Heikka's writeup of this installation is an excellent guide. Read it here on the Early CJ forum.

Also See