The Novak Guide to
Adapting AMC I6 & V8 Engines to the SM420 Transmission
We've developed a method of joining an SM420 four-speed transmission to 1972-2006 AMC I6 and AMC V8 engines. No adapter is required, but only a few straightforward machining steps to an existing or new 1980-1986 Jeep CJ bellhousing.
AMC 232, 258, 4.0L, 304, 360 and 401 engines are all valid candidates and their power will be complimented by the strength and gearing of the Muncie 420.
The SM465 transmission is also a candidate for this procedure.
Components Required
- Bellhousing, 1980-1986 CJ. SR4, T4 or T5 style
- Arm, clutch release, 1976-1986 Jeep CJ
- Flywheel; native to the engine being used
- Pressure plate; native to the engine and flywheel being used
- Disc, clutch, 1980-1986 CJ; usu. 10-3/8" x 10 spline x 1-1/8"
- Clutch release linkage; either mechanical or hydraulic, as per the Jeep
- Transmission front bearing retainer, modified
- Pilot bushing, custom
- Bearing, throwout
- Bolts, 5/16-18 x 7/8" socket head
- If you are using a Renix or Mopar 4.0L flywheel, you will need to use the appropriate 4.0L style starter for the proper starter gear to ring gear mesh
| You may send your *clean* bellhousing into Novak for this entire machining service. Turnaround is typically three days and we can ship it back with any other conversion components you may need. Cost is $79. plus shipping. Contact us. We will machine the bellhousing rear face clean and square and drill and thread the transmission holes as required. |
The Transmission Bearing Retainer
Modifying a factory SM420 front bearing retainer used to be our only option. This included sleeving, lengthening and bushing up the register with a ring. Because this is a lot of work, and the factory retainer is notoriously weak, we manufacture a retainer made from a billet of steel - that already has the right dimensions. We encourage its purchase for the best available solution.
Install this modified retainer on the GM transmission using the provided 5/16-18 socket head (Allen) bolts.
Transfer the Transmission Hole Pattern to the Bellhousing
Place the engine flange of the bellhousing face down - preferably on smooth wood or the table/bed of a milling machine. Use a transfer punch to transfer the four bolt pattern of the transmission front flange onto the bellhousing rear face. If a transfer punch is not available, you may use a bolt or pin or drill to transfer the four marks onto
this rear face, though this is not preferable.
Align the top cover gasket surface of the transmission with the dowel pin holes in the bellhousing to make sure the trans will be level. Transfer the mounting bolt hole pattern of the transmission to the bellhousing. Drill the two upper holes with a letter "U" or 23/64" size drill and tap these two holes to 7/16-14.
Procedural Variations
Your 420 will have one of two types of lower bolt holes. One offers clearance for a bolt that comes through the transmission flange ear and threads into the bellhousing, as per convention. The other type of 420 is drilled and tapped for the bolt to screw into from inside the bellhousing. Either of these types will work with this procedure.
Through-Drilled Lower Transmission Ear Version
If the SM420 has a lower driver's side hole that is not threaded, proceed as follows:
- Drill the lower driver's side hole through the bellhousing with a 17/32" drill for a 1/2" bolt clearance. This bolt will be a 1/2-13 with the head inside the housing. Note that we have used a countersunk 1/2" bolt here which offers better clearance from the rib of the clutch arm. On the backside of the transmission flange ear, use a 1/2-13 nut and lockwasher.
- Drill the lower passenger's side hole through the bellhousing with a 17/32" drill for a 1/2" bolt clearance. This bolt will be a 1/2-13 with the head inside the housing. Note that we have used a socket head cap screw (Allen) style bolt here for better bellhousing wall clearance. Blue threadlocker is recommended.
Threaded Lower Transmission Ears Version
If the 420 has lower holes that are threaded proceed as follows:
- Drill the lower driver's side hole through the bellhousing with a 17/32" drill for a 1/2" bolt clearance. This bolt will be a 1/2-13 with the head inside the housing. Note that we have used a countersunk 1/2" bolt here which offers better clearance from the rib of the clutch arm. Blue threadlocker is recommended.
- Drill the lower passenger's side hole through the bellhousing with a 17/32" drill for a 1/2" bolt clearance. This bolt will be a 1/2-13 with the head inside the housing. Note that we have used a socket head cap screw (Allen) style bolt here for better bellhousing wall clearance. Blue threadlocker is recommended.
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While these written instructions may sound complicated, the actual procedure is really quite simple and the end result is worthwhile.
Crank Position Sensor
When working with the 4.0L engine, a crank position sensor is required. We can precisely install most existing sensors (from the installer's wiring harness) or a new sensor into an AMC bellhousing. We welcome you to contact us about performing this procedure.
Novak Kits
Below are three Novak kits to assist 420 installers to the level best suited for them. Note that these kits are for the 1954+ SM420 transmissions with a conventional front oil seal. Earlier transmissions require a different seal, and to have a seal journal ground into the customer's input shaft. Contact us directly if this is your scenario.
 Retainer Kit :Novak Kit #420AMC-B
Full Bellhousing Kit :Novak Kit #420AMC-M Full Bellhousing Kit with 4.0L CPS Installation:Novak Kit #420AMC-MC
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| Some SM420 transmissions have a shift rail protrusion that will interfere with AMC bellhousings. A pocket may have to be machined to provide clearance. |
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Early AMC Cranks
Not all AMC crank flange bores are the same. Early motors 1971 and previous - as well as those with automatic transmissions around this era - featured a ~1.8" step (image, right) or a shallow recess instead of a ~1.8" centering bore as found on later AMC V8 engines. This distinction is important to make if adapting to GM SM420 transmissions. All early and late cranks (in our documentation) feature a pilot bushing bore that is just over 1" in diameter whether they have the above step-bore, or not.