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.
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.
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