SM420 to AMC Bellhousing

Adapting

AMC I6 & V8 Engines

to the

SM420 Transmission

Let's discuss Joining 1972-2006 AMC I6 & V8 engines to the GM SM420. Their power will be complimented by the strength and gearing of the Muncie 420. The parts and procedures are detailed below.

you can order a one-and-done finished version bellhousing and all needed components, or compile and modify these parts as described.

Installation Requirements

Installation requirements will vary depending on how much of this conversion you choose to do for yourself. If all the options are purchased, typical mechanic's hand tools will generally suffice. Machining the bellhousing requires a mill while transmission rebuild necessitates pullers, a press, large snap ring pliers, etc.

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

Transfer the Transmission Hole Pattern to the Bellhousing

465_to_amc_mod

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 transmission 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 lock washer.
  • 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.
sm420_AMC_bellhousing_protrusion_pocket

Some SM420 transmissions have a shift rail protrusion that will interfere with AMC bellhousings. A pocket may have to be machined to provide clearance.

Products

Transmission Bearing Retainer Kit

Adapting SM420 or SM465 transmissions to AMC engines

Kit 420AMC-B, adapting SM420 transmissions to AMC I6 & V8 engines, retainer & pilot bushing only

This option is for those who have their own bellhousing and will do the machine work on it.

Contents

Special SM420 retainer machined from a billet steel to Late (seal style) SM420 dimensions on the transmission side and AMC bellhousing bore and sleeve dimensions on the engine side. Four 5/16-18 socket head (Allen) bolts, input seal, and pilot bushing for 1972-2006 AMC engines are included.

Full Bellhousing Kit

SM420 to AMC Bellhousing

Kit 420AMC-M, adapting SM420 transmissions to AMC I6 & V8 engines, bellhousing, retainer & pilot bushing

This kit includes everything in the above #SM420-AMC above, plus our new AMC I6 to SM420 bellhousing. It has been dialed in to center, drilled, tapped and precision bored for the 420. We've also relieved a portion in the mating face (not shown, image right) to clear the shift rail protrusion that many SM420 transmission shifter top covers have.

Contents

The kit additionally includes a machined bellhousing, clutch release arm, boot, release arm pivot, return springs, pilot bushing bellhousing full cover shield plate, as well as the Transmission Bearing Retainer Kit listed above.

Novak Bellhousing Machining Service

Send us your 1980-1986 bellhousing and we'll dial it in to center, precision bore it and drill and tap to match the SM420 transmission. This is the machining service only. Please add on the #420-AMC above for the needed retainer kit. Send in your bellhousing *clean*. If this is not possible, we will steam & detergent clean it for you for $50.

#MACH-BELLGM420, Machining, milling, CNC, AMC bellhousing machined to SM420 pattern

AMC Bellhousing CPS Machining

amc_cps_install

We'll CNC machine the accurate Crank Position Sensor (CKP) location required for your 4.0L engine into your classic AMC Jeep bellhousing. Send your sensor, or we can supply one. Send in your bellhousing *clean*. If this is not possible, we'll steam & detergent clean it for you for $50. If we are already machining your bellhousing face, the cost of the CPS machining procedure is reduced. Call in for details or a quote.

As viewed from the rear of the engine, the early 1992-2003 sensor position is about 10 o’clock (image, shown) and the later 2004-2006 is about 3 o’clock.

Does not apply to AMC 2.5L applications. These conversions are seen here.

Machining, bellhousing, machine CKP sensor in 10 o’clock position

Machining, bellhousing, machine CKP sensor in 2 o’clock position

Applications & Compatibility

Engines

1972-2006 AMC I6 & V8 engines, 232, 258, 4.0L, 304, 360, and 401 engines are all valid candidates. Earlier AMC cranks are candidates, with the following caveats.

Early AMC Cranks

early_amc_crank_flangeNot all AMC crank flange bores are the same. Early engines, 1971 and previous - as well as those with automatic transmissions around this era - featured a ~1.8" step (see image to the 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 a GM transmission. If you have this crank, call us and we will substitute a different pilot bushing for your kit.

Flywheel

The factory flywheels on all of the above engines are acceptable. Most will accept the recommended 10-1/2” clutch. There are instances of larger clutches in some trucks. If you have one of these applications the clutch diameter will need to match.

Clutch Assembly

The factory pressure plates, whether three-finger or diaphragm, are compatible with this adaptation. We have found that the diaphragm style pressure plate generally works better with hydraulic applications. 1976-1979 (T150 & T18) clutch disc hubs have a 1-1/16" spline diameter that is not compatible with the 1-1/8" input shafts of the SM420. See Related Products below to purchase a clutch kit, or a single GM-compatible disc to place in your existing pressure plate.

Clutch Release

Hydraulic clutch release

The factory clutch release arms and their linkages can be retained including mechanical bellcrank style linkages. However, some convert to a hydraulic clutch release system which has its advantages. See Related Products, below.

Related Products


Urethane rear mount for adapter to crossmember
Hydraulic release slave kit for AMC
Hydraulic release master kit for Jeeps
Hydraulic release master kit for Jeeps
Adjustable release bearing for AMC
GM style clutch disc
1976-1986 Jeep floor tunnel cover
Shifter Retention Kit, SM420, SM465, T176, NP435
Parts for the SM420
Turn-Key SM420

Summary

While dated, the SM420 offers an excellent low ratio with overall compact size and weight. All versions of the SM420 that use a standard 1-1/8” input shaft are compatible with this conversion. Because the front retainer includes a seal, early transmissions not equipped with a seal originally will require the input shaft machining option below that requires your good shaft be sent in. We do offer fully remanufactured SM420 transmissions with all our available updates and can configure it with these parts as a package. If you want a solid SM420, this is a great option.

Also See