Also known as: sm465, sm-465, sm 465, muncie sm465, muncie 465
General Motors' heavy-duty four-speed manual for light and medium trucks, built at Muncie in Indiana from 1968 until 1991 and fitted to the full-size Chevrolet and GMC pickups, the K5 Blazer and the Suburban.
The first thing to understand about it is that it has three usable gears, not four. First is a granny — 6.55:1, and on most units unsynchronised — meant for pulling away under load or crawling, not for moving off in traffic. Second at 3.58, third at 1.70 and fourth at 1.00 are what the truck is actually driven on. A driver new to one will report that it "won't pull away in first without stalling" or that first "grinds going in"; neither is a fault. Start in second on the flat and treat first as a tool.
It is a top-loader with a cast iron case, and the cover on the top is where the shifter mechanism lives. Later units — 1988 to 1991 — went to an aluminium top for a cleaner change, and from 1985 the clutch release became hydraulic rather than mechanical. That last change matters more than it sounds: on a hydraulic truck a heavy or non-releasing clutch is very often the master or slave cylinder rather than anything inside the gearbox.
Its reputation is durability, and it is deserved — this is one of the most-swapped gearboxes in the four-wheel-drive world precisely because it is hard to break and its low first is worth having off road. Dual power take-off provision was standard, which is the other reason it survived in service vehicles long after the trucks around it were scrapped.
It replaced the SM420 and was itself replaced, for the 1992 three-quarter and one-ton trucks, by the New Venture 4500.
Cars fitted with this gearbox
1 generation. Only where a source names the model and the generation together — a gearbox is documented as a product far more often than as a fitment, so this list is deliberately shorter than the truth.
GMC
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