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The Iron Giant

Posted in Vehicle Reviews on November 23, 2004 Comment (0)
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There's got to be a jillion SM465 transmissions out there. Manufactured from 1968 to 1988, General Motors put them in everything from 1/2-ton pickups to 2 1/2-ton dump trucks. But there hasn't been much ink explaining the subtle differences in the production run, what the weaknesses are, and what options are out there for the four-wheel junkie.

We hooked up with Pat Massey of Anaheim Gear for the rundown of what's good, what's bad, and what needs to be replaced with these transmissions. Anaheim Gear has been wrenching on crashboxes for 12 years and the folks there are manual transmission experts. You can send them your transmission to be rebuilt or buy a fully rebuilt unit. Or, if you're a do-it-yourselfer, you can get rebuild kits and hard-to-find parts for your project. We followed along as Massey and technician Jack Atkins walked us through what makes for a killer SM465.

Quick Facts
Here's a few tidbits that may keep you from pulling your hair out.
*'68-- GM phases SM465 in to replace SM420, which had been in service since 1947.
*'68-'70-- Early trannies used rubber damper (bottom) to cushion gears and keep down rattles. The rubber wore quickly and just made rattles worse, plus it took away spline engagement when compared to the later gear (top).
*'68-'72-- Cases had threaded bolt holes for bottom bellhousing bolts, but smooth holes for top bellhousing bolts.
*'72-and-up-- Cases had smooth bellhousing bolt holes. The bolts were held by threaded bolt holes in the top and bottom of the bellhousing.
*Mid '70s-- The bearing on the nose of the countershaft was changed.
*'88-- Changed to thicker input bearing. Clusters and cases have different diameters so one won't work in the other.

SM465 Versions
There are basically four versions if you overlook the small design changes that occurred over the production run. All use the same gears, so don't knock yourself out looking for the big-inch input version which requires a 12- or 13-inch clutch.
'68-'79 4WD-- 1 1/8-inch, 10-spline input shaft, 10-spline output shaft.
'80-'88 4WD-- 1 1/8-inch, 10-spline input, 32-spline output that isn't compatible with aftermarket transfer-case adapters.
'68-'88 2WD-- 1 1/8-inch, 10-spline input shaft, 35-spline output shaft.
'68-'88 2WD-- 1 1/2-inch, 10-spline input shaft, 35-spline output shaft

SM465 vs. SM420
If it's a toss-up between running an SM420 or an SM465 in your truck, you may want to consider going for the easier-to-find-and-get-parts-for SM465 if it will physically fit.

Height (in.)
Weight (lbs.)
Input Shaft
1 1/8-in., 10-spline or
1 1/2-in., 10-spline
1 1/8-in., 10-spline
Output Shaft
10-spline (early 4WD),
35-spline (2WD)
32-spline (late 4WD)
BearingRetainer Diameter (in.)


What's in There

PhotosView Slideshow

Shaft Options
Anaheim Gear knows how to mix and match factory components to build a transmission that's right for your application. Because we were mating our SM465 to a Klune-V "David" underdrive with a 32-spline input, Massey suggested swapping mainshafts to the later 32-spline version (bottom). Anaheim Gear even had a fac-tory SM465-to-NP208 adapter that would work for our application.

The result is a much stronger setup than if we had used a spud shaft to connect the 4WD 10-spline output (top) to our crawler box. The only drawback is that the 32-spline shaft will be a few inches longer than the coarse output and spud shaft combo. However, we've got enough driveshaft to accommodate it.

Here's the main reason SM465s jump out of gear. The Fourth gear and Third gear are held in place on the countershaft with snap rings. Under load, Third gear wants to walk forward and Fourth gear wants to walk back. After time the snap rings flex enough to allow the tranny to jump out of gear.

Third Gear Jumpout Fix
If there's one thing the SM465 is infamous for it's its tendency to jump out of Third gear. Here's what Anaheim Gear does to prevent this problem.

PhotosView Slideshow
Problem: Worn output shaft and collar
Description: The 4WD versions had a short output that uses a collar to connect to the transfer case input. Over time, these collars wear and can damage the splines of the transmission output shaft.
Solution: Replace mainshaft with usable core.

Problems, Solutions, and Things to Look Out For
Rebuilding an SM465 isn't rocket science, but there are a few things that can go awry. The first of which is starting with a core that's hashed. Before you delve into any rebuild, make sure your transmission is in good shape. Anaheim Gear media-blasts and tumbles each part so it can be checked before assembly, then runs the transmissions on a dyno to make sure everything is perfect before it sends a rebuilt unit out the door. If you're tackling an at-home rebuild and discover part of your core is shot, chances are Anaheim Gear can supply you with a new or used replacement part to get you back into the game.

PhotosView Slideshow


Anaheim Gear


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