This is the Chevy/New Process...
This is the Chevy/New Process A-833 four-speed OD, offered as optional equipment on '81-86 (or '87) Chevy 1/2-ton trucks. Internally, it's the same as the Mopar A-833, with Fourth gear being 0.73:1 overdrive. It's essentially the same as the Mopar muscle car-era trans, but with an aluminum main case rather than the iron case. The 3-4 shifter arm on the Overdrive is installed upside-down from a normal four-speed. When the shifter goes into the Third gear gate, the transmission internals go to what is normally Fourth gear-straight through. Then when the shifter goes into the Fourth gear gate, the internals go through what is normally the Third gearset (but with different tooth counts than a regular Third gear cluster) and comes out the tailshaft as 0.73:1.
Call it old-fashioned, but there's just something satisfying about rowing your own in a vintage truck. Bench seat, vent windows, and a stick-shift transmission, it's almost the perfect package.
Most of our trucks came with manual gearboxes, most of them were three-speeds, and most were of the "bolt-action" column-shift variety, though a "Granny-Low" four-speed was also a favorite, with a long shifter coming through the floor operating the keg-like box. If you had need for a work truck (and your work was pulling down buildings with nothing more than a length of chain), the Granny four-speed with compound-low was your transmission of choice. For those times you weren't trying to crawl up a mountain or pull a freight train, you ignored First gear and just started out in Second, treating the gearbox as though it were a three-speed, the final drive being 1:1.
Times have changed. Today, everything we drive has an overdrive transmission in it, and we're used to honkin' down the road at 2,300 rpm, not 4,000 rpm, with engine life and fuel economy better than ever due in large part to overdrive gearing. We rolled with a Granny four-speed in our '62 Suburban for a couple years and more than once pushed in the clutch only to realize there wasn't a gear left.
Chevy offered this as a COPO option on 1/2-ton trucks from '81-87, known as MY-6. There are two versions of the Chevy case: the first has the regular GM mounting ears to the bellhousing, while the second retains the Chrysler mounting pattern and requires the specific factory aluminum bellhousing. We have one with a boss for a hydraulic clutch slave, and we've seen a picture of one with a Z-bar ball stud on the side. It looks to us like Chevrolet simply modified an existing mold for the bellhousing-silver plastic push-in plugs fill the unused Chevy mounting holes.
There are two versions of...
There are two versions of the main case available: one retains the Mopar trans mount. Though the mount is Mopar-style, the input shaft and tailshaft housing are still Chevy.
If you're a Chevy or Mopar owner, the New Process 833 four-speed OD is an easy and affordable way to get overdrive. Mopar guys have been hip to the trans since it came out in the early '70s, and found its way into passenger cars, vans and pickup trucks through the mid-'80s. Chevy enthusiasts, on the other hand, are barely aware of this option. Based on the Mopar 833 transmission, the overdrive version uses the same case as Mopar muscle cars, but with an aluminum main case rather than the muscle car-era cast-iron case. To get overdrive, they simply run Third gear straight thru as 1:1, employing what is traditionally the Fourth gear position for the shift arms and cluster. By inverting the lever on the shifter arm at the case, when the shift mechanism engages what is the Third gear gate, the transmission shifts into what is typically Fourth gear. For the final ratio, they run through what is typically the Third gear cluster, coming out the tailshaft as 0.73:1 overdrive, the gate on the shifter going into the traditional Fourth gear position.
The input shaft on these is GM-spec, with the tailshaft housing, driveshaft yoke, rear seal, and speedo gear being the same as a TH350. Internals are the same as Mopars. We read a single post on a website that this transmission was also available in diesel pickups, using an iron case. We haven't seen one in person, but we always check diesel trucks in the junkyards for the stronger iron case. If there's a weakness to these transmissions, it's that the countershaft registers directly in the aluminum case-behind engines with a lot of power, or after years of wear, the bore in the case can wallow out, causing the internals to bind. One fix in the Mopar camp is to have the case machined for a steel insert bushing.
 For the Mopar-style mount,...  For the Mopar-style mount, you must use the original Chevy bellhousing. This one has a boss for hydraulic clutch, and we've seen a pic of one with a ball stud. Note the silver plastic plugs in the normal Chevy bolt hole locations; It looks like they just retooled existing molds for this. |  The other version of the trans...  The other version of the trans has a retooled case with standard Chevy/GM mounting holes. Note the red goop around the countershaft. Without a wipe of sealant, trans fluid weeps from there. The Mopar's bellhousing covers that area fully, so it holds the sealant, but on the Chevy, that area is open, so there' nothing to force a seal. |  Both versions of the trans...  Both versions of the trans use a special Chevy-style tailshaft housing. It has a Mopar pattern to mount to the main case, but uses a TH350 yoke, seal, and speedo drive. The tailshaft housing is held on with a couple bolts, and a retainer snap ring that must be removed before the tailshaft can come off. A previous owner didn't know about the snap ring and broke out the pry bars, breaking the entire ring land into little pieces that went to the bottom of the housing. Luckily, we had a spare trans for parts. |