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The Novak Guide to the
GM Muncie SM465 Transmission

For a video version of this article, click here

The SM-465 or CH-465 TransmissionThe SM465 is an outstanding, heavy-duty truck transmission designed and used in ½, 3/4 and 1+ ton trucks. The SM465 was made by General Motors from 1968 to 1991. They are found primarily in GM and Chevrolet trucks, Blazers, Suburbans and many other models falling under the 1/4-ton to 1-1/2-ton platforms.

The 465 is an intellegently designed and very well-built transmission and remains easy to locate and generally inexpensive to buy and maintain. The strength, relatively short size and the gearing of the transmission make it a good choice for a variety of on-road and off-road situations. It has a very good set of road manners contrasted by its enormous off-road brawn.

The "SM" in the transmission's designation stands for Syncrho-Mesh, in the tradition of other GM gearboxes. It is also referred to as the CH-465 or CH465 by factory literature.

Identification
The SM465 is a top loaded, top shifting truck style transmission. This transmission is 12 " long and features a cast iron top cover that is retained by eight bolts. The SM465 case is of *cast iron. The case itself is generously ribbed both vertically and horizontally.

Specifications of the SM-465 Gear BoxThe SM465 has a 1-1/8" diameter, ten spline input shaft that has a stick-out of 6-1/2" from the front face of the transmission. The pilot tip is ~19/32" in diameter. The front bearing retainer flange measures an unusually large 5-1/8" in diameter and the bearing retainer tube is 1-3/8" in diameter. The front bolt pattern is the consistent GM Muncie / Saginaw four-bolt pattern as found on GM bellhousings and transmissions from the 1940's through the 1990's.

Features
The 465 enjoys a very low compound gear at 6.55:1 and as such it is a popular choice for those wanting a very low crawling gear.

The SM465 features power-take-off (PTO) ports on the passenger and driver sides of the case.

The shifting pattern for the 465 is in the standard "H" pattern with reverse gear being to the right side, and down.

SM465 Versions
There are three principal versions of the SM465, each based on the era and configuration of the transmission. There is no known mechanical superiorities to either earlier or later versions.

Earlier SM465
The earlier era of the SM465 include the 1968 - 1978 transmissions. There were both 2wd and 4wd versions. Internally, they are identical to each with the exception of the transmission mainshaft. The 4wd version of this mainshaft (or "output shaft") has a 10-spline output with a 2-5/8" stickout and an accompanying 4wd adapter housing. The 2wd version has a 35-spline threaded mainshaft compatible with a driveshaft yoke, and accompanying 2wd style tailhousing.

Spline Misunderstandings Clarified

"More splines are stronger"

Not necessarily. Larger shaft diameters are stronger, and larger shaft diameters usually require a higher quantity of splines. However, the automotive powertrain industries migrated to more splines on shafts due to their ease (speed, cost, etc.) of manufacture - smaller splines being able to be roll forged instead of the larger splines having to be machine cut.

A 32 spline output shaft vs. a 10 spline spline output shaft is probably only minutely stronger, and a decision of use here should be driven more by compatibility than perceived strength.

Later SM465
The later era of the SM465 include the 1979 - 1991 transmissions. A minor internal change of the 1st gear bushing and thrust washer occurred, in addition to a change from a shorter 10-spline, 4wd output shaft to a 32-spline 4wd output shaft - coinciding with the change of bolt pattern in the New Process 205 transfer case from a figure "8" input bolt pattern to a round, six-bolt input pattern. This pattern was also found on the 208 and 241 chain-driven transfer cases.

Later 2wd style SM465's also incorporated the same 1st gear bushing and thrust washer change, but the 35-spline, yoked output remained identical.

1988-1991 Versions
The last years of SM465 featured a direct-drive (4th gear) switch to signal a lean or cruise mode operation to the GM Engine Control Module (ECM). These final versions had an aluminum top cover and a revised input shaft and input bearing combination. They were found in the C/K Series trucks. Suburbans, however, continued to use the earlier SM465 with the cast iron top cover.

Incidentally, this laster version of the SM465 was available under GM #12345577.

 

SM-465 Ouput Spline Versions

 

A complete, adapted SM-465 ready to ship
A turn-key SM465, professionally built, adapted or OEM style and delivered to your door - ready for a variety of engines and Jeep transfer cases. Read more...

Transfer Case Adaptability
This transmission makes an excellent conversion transmission due to its adaptability into most Jeeps. Both 2wd and 4wd versions of the SM465 can be used equally well, and there are no inherent advantages to either one once you have installed our adapter assembly.

The Muncie 465 can be adapted to the popular Jeep (and many IH) transfer cases, including the:

A Chevy to SM-465 BellhousingEngine Compatibility and Adaptability
GM
As the reader will conclude, the Chevrolet and General Motors SM465 will marry directly to a GM style bellhousing. This includes Chevy car, Buick, Oldsmobile and Pontiac bellhousings. However, since this transmission has such a large bearing retainer, the car size bellhousing bore is smaller than the transmission's input bearing retainer. If you need to join this transmission to a GM bellhousing with the 4-11/16” bore, it can be done. The bore must be opened up to 5.000” and the retainer turned down to 4.995” as precision slip-fit. We can perform this service with a very quick turnaround, or you may have it done by a good machine shop local to you. We also sell new bearing retainers turned to the dimension you require.

Machining the Muncie pattern into an AMC Jeep bellhousing

AMC Jeep
The 465 may also be made to be compatible with AMC bellhousings. This modification has primarily been developed for conversions replacing the SR4, T4 and T5 Jeep® transmission as used in 1980-1986 CJ vehicles. 1972-1979 AMC 232, 258 I6 and all 304, 360 and 401 V8 engines can also be used with this transmission but must be equipped with a bellhousing, release arm, release bearing, and clutch disc from a 1980-1986 6-cylinder CJ. Our Kit #465-AMC can be purchased to simplify this process.

The 4.0L Jeep I6 (both Renix and Mopar systems) is adaptable using the same method as above, with the addition of installing a Crank Position Sensor (CPS) into the aforementioned bellhousing. This is a service that we provide affordably and in quick turnaround.

Rebuilding the SM465
The SM465 is relatively simple to rebuild. Many shadetree mechanics do very outstanding rebuilds if they have access to a press, snap ring pliers and bearing pullers. Many choose to do a full rebuild during the adaptation process, and our instruction guides feature all the details, diagrams, pics and tricks required to do professional level work.

Rebuild and repair parts for the SM465 transmission

For your information, a few Muncie 465 casting numbers include:

3871912, 3901127, 673212, 6273212, 3901131, 465453, 8672267, 867287 (aluminum top cover model)

 

When filling your 465 with gear oil, we recommend that you select a conventional mineral oil or a para-synthetic in lieu of a full synthetic oil. Properly assembled manual gearboxes do not have the thermal strains seen by combustion engines or hypoid gears. Synthetic fluid in these gearboxes, while not harmful, is probably an economic waste.

Hypoid gear oil is sulphurized higher than transmission oil and can be mildly corrosive to the non-ferrous alloys used for synchros, bushings and thrust washers in these transmissions.

An 80W-90, API-GL5 or MT-1 rated fluid is very good. Some claim faster shifts from using a 50W engine oil in their transmission and we do not consider this to be contraindicated unless you operate your vehicle in a very warm environment.

About the SM465
please enjoy our video version of this article.