Adapting Chevrolet & GM Generation I-IV Engines

to the

Jeep AX15, NV3550 & NSG370 Transmissions

With the advent of quality five- and six-speed transmissions in Jeeps, we now offer the possibility of marrying them to various Chevy and GM engines in a straightforward and sanitary manner using our block-to-bellhousing style adapter assembly.

* Note: This kit is in the manufacturing process and is currently backordered. Expected date of availability is late October 2019. 

Product & Features


The Adapter 

This plate style adapter is precision laser cut and machined. Precision CMM mapped and machined dowel locators and fastening points assure total concentricity for long transmission life. The adapter is plated in black zinc for corrosion resistance and appearance. 


Both kits include the adapter, fastening hardware and instructions.


Pricing and product information to the right is shown for informational purposes. We recommend calling in and ordering this kit so we can tailor it for your exact transmission. Otherwise, make your order and we will follow up with you via e-mail or phone to verify your technical details. 

Applications & Compatibility


This adapter is compatible with the:

  • AX15
  • NV3550
  • NSG370 - 4.0L I6 style, not 3.7L V6. It is possible, however quite expensive to retrofit the front half from a TJ NSG370 to the JK NSG370 and use the kit. Due to cost, our recommendation is to go with one of our HD AX15 transmissions instead as they are much stronger and have proven more effective overall. The GMAX15 would then be the best way to adapt to your engine.

This kit will work with either the earlier AX15 that uses the concentric, internal hydraulic throwout bearing, or the later style AX15 that uses the external slave cylinder. 


gm_ls6_thumbThe following Chevrolet & GM engines are compatible:

  • 4.3L V6
  • Gen. I-II Small Block V8
  • Gen. III+ GM V8
  • Chevrolet I6
  • Chevrolet Big Block V8

***This list simply demonstrates what will bolt together and does not suggest that all engines are appropriate for the strength of all transmissions. These are light duty transmissions with the notable exception of our new AX15 linked above and which would be rated as a medium duty transmission. Power capability will depend on the condition of the transmission, vehicle weight, driving style, ability of the user and the base capacity of the transmission itself.

Pilot Bushing

Depending on the transmission you have and the engine you choose, your pilot bushing will vary. The appropriate bushings are available below and are required for installation. 


For GM engines prior to the Gen. III & IV Series (LS & Vortec, 1997- present), you will use your GM / Chevrolet flywheel as per factory to your engine, which can be either the 153 tooth version or 168 tooth versions. The 153 tooth flywheel typically accepts the 10-1/2” clutch only and the 168 tooth an 11” or 12” clutch.

If you are using a GM Gen. III+ engine, we strongly recommend the use of our extended height LS / Vortec flywheel to normalize the clutch system. This model accepts both 11” and 12” clutches. 


This conversion uses readily available clutch components based on GM pressure plates and their geometries. All of these transmissions use the 1-1/8” 10 spline input shaft. Choose the 10-1/2", 11" or 12" clutch according to your preference and flywheel compatibility. Jeep Wranglers are typically lighter vehicles and the smaller clutches are generally adequate and offer easy clutch pedal pressure. The larger clutches have more holding power and still have very reasonable pedal pressure when using the diaphragm style pressure plate. A more detailed discussion on this topic is available in Novak's Knowledge Base linked below. Clutch adjustment is also addressed in detail and reading this article is highly recommended.

Starter Motor

It may be required to clearance the starter pocket of AX15, NV3550 and NSG370 bellhousings to eliminate possible interference from the starter nosecone. A deburring tool is recommended (not a grinding wheel, for safety) or the installer can choose a small nosecone or no-nosecone version starter; common on modern gear drive starters.

Related Products

Pilot Bushing

One of the following Pilot Bushings is required for all installations. On an AX15 it is a good idea to verify the pilot diameter of your particular transmission or you could order both the large ~3/4” and small ~19/32" diameter to be sure. The NV3550 & NSG370 are consistently the larger size tip.

Please choose your applicable engine and transmission combination:

Transmission Rear Mount

The AX15 and NV3550 feature a threaded base for support and isolation of the transmission and transfer case. Factory style isolator mounts are difficult to work with in any situation other than as a direct factory replacement. Novak has engineered a universal style rear mount that is easy to affix to a variety of Jeep crossmembers and is very strong and highly adjustable. We recommend it in any retrofit situation. More info on this mount can be seen here.


LS flywheel

This is a beautiful piece and necessary for proper operation on LS based engines. They must have the LS crank offset to work properly. These will accept both the 11” and 12” clutch listed below. The page is here with details.


Bolts necessary for installation


Clutch Kit

Many common GM applications used the 10 spline 1-1/8” hub and as such, it is usually a matter of selecting the correct version. Three sizes are listed below. Most common is the 11” listed in the middle but all are serviceable depending on your needs. Generally the 10-1/2” is used on the smaller 153 tooth flywheel and is exclusive to that application. The 11” and 12” are usually interchangeable with large bellhousings and standard 168 tooth flywheels. Non-standard applications will need to watch for compatibility.


The disc can be purchased separately for those applications natively using a different spline count



Adjustable release bearing

While sometimes not needed in this application, it is priceless when clearances dictate its use. You may get lucky and find a correct height release bearing but this product will guarantee correct height when set up properly. Further information is here. Be aware, it will only work with the external slave systems.

Hydraulic Clutch Actuation

Though not all applications require all of these parts, now is an excellent opportunity to upgrade from the likely worn original mechanical system. If originally hydraulic, the slave allows adjustment, and the line and adapter an easier installation. If your Jeep has the master cylinder, that part can be reused.




Installation Requirements

The adapter is easily and quickly installed between the engine and transmission. Detailed instructions are included and normal mechanics tools are required. It is again recommended to read the The Novak Guide to Clutches, Linkages & Bellhousings for Jeep® Conversions. 


As these usually are installed as part of an engine swap setting the powertrain to the proper pitch angle - usually tilted down ~5 (3-7 acceptable) degrees - is recommended. Offset will be to the right (passengers’) side because of the driver drop transfer case and front differential usually 1 to 1-1/2”. Consult our engine conversion guides for more specific recommendations on placement if the engine is being replaced.

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.

Engine 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. 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. Common sense and taking a step back to look at things overall goes a long way.


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 installers, concerned about the expense of new or modified driveshafts, attempt to let the existing driveshafts dictate engine, transmission and transfer case location, often to the detriment of the project. Our recommendation is to prioritize the correct position of drivetrain components over saving a few dollars which is usually regretted in the long run with compromised positioning.

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. 


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.

Also See