I currently don’t own anything with a manual transmission, but I am always thinking of possibility’s. I have seen some late Saginaw 3 speeds with overdrive for sale on e bay. Big $$ for sure. My question is can a regular Saginaw 3 speed (or 4 speed) be converted to a overdrive unit? If so what would need to be changed? The main Shaft? Is the Overdrive transmission longer that the regular 3 speed? Any info would be appreciated. Jay

Almost all US overdrive transmissions used Borg Warner R10 or R11 units. These were in separate housings that bolted to the regular 3 speed case. They do require a special longer main shaft. Pre '56 or so the solenoids are 6v. They are good units and fun to drive. They provide about 30% OD at the tail shaft. There is a lot of interchange of internal parts.

They can be wired so that the driver can engage OD in all 3 (4) forward gears. Normally a governor kicks in the OD at about 27 mph. The R10 has 3 planetary gears. The R11 has 4 and is a stronger unit used in some heavier cars and pickups.

The Saginaw 3 speed OD from the late 60s-early '70s (I think) is a good and reasonably strong trans though not as strong as the Borg Warner T85 OD. The 3 speed Saginaw uses the same case as the 4 speed. Many people have put the OD on their 4 speeds. The only real advantage is 4th gear OD because the closeness of other ratios are so close in most of the 4 speeds. The Saginaws use a unique solenoid that is getting hard to find. NEVER throw any BW OD solenoids away. All can be repaired most with a simple drop of solder. Hot Rod Magazine wrote on Saginaw 4 speed OD conversion In May or April 1980, maybe?

Thank you for educating me. If I ever get a car with a 3 speed I’m putting in the overdrive. Jay

There's a decent thread on this very topic over at HAMB:
Jalopy Journal circa '11

There was a great thread on The Old Truck site. It showed how to rebuild Borg OD and how rewire them so you could shift in and out of OD whenever you wanted. But they lost ALL of their old posts when they "up graded" the site.
I putting a Borg Warner T86 3 speed OD in my '26 Chevy roadster build. It will use a slightly modified T90 floor shift top plate. It'll turn my 4.27 rear gears into 3.11 or so. Low gears to get it going with the 4 cylinder Chevy II and OD to keep it going.

I worked on several of these "back in the day". They are pretty good units, but good lubrication is ESSENTIAL!! Unlike normal four speed transmissions, the Saginaw 3spd OD swapped ratios on the main shaft, direct drive being third gear and the normal 3rd gear being the overdrive. As a result, the back half of the main shaft spun faster than the input when in overdrive. The center bearing on the input shaft was subject to oil starvation and failure if the fluid became low or was contaminated.

We always slightly over-filled these units when servicing and used only premium lubricants. Today's synthetics are really the ticket when servicing them today.

Blackwater,
The saginaws actually get their O/D from a add on unit that applies to the rear of output shaft. I have converted a few 3 speeds to 4 speeds and have one in the shop for me to use.

The BW ODs were in a separate case bolted to the transmission main case. The main shaft leaving the trans in high gear (usually 3rd) was the standard 1 to 1. The special longer main shaft drove a set of planetary gears, 3 in the R10 and 4 in the R11. The overdrive ratio was attained by the increased ratio through the planetary unit. The output shaft of the OD turned about 30% faster than the input from the transmission. If someone wanted to play with these they could become a stand alone box with a modified front plate. There was a GM OD 4 speed where 4th was the OD sort of like the T-5 except the OD was in the main box not in a special housing on the back. Attention must be payed to lubrication of the old BW ODs. Some modern gear lubes will eat the bronze parts. GL-1 works.

GM Saginaw OD transmissions produced in the late '70s and early '80s were made from the 4spd standard transmissions and used the configuration I described above. There were also similar produced by Chrysler. As I said in my previous post, I worked on several of these when I worked for GM, Chrysler, and in the independent general repair and the transmission repair industries.

My mistake if all of this discussion is concerning earlier transmissions with added overdrive units. I'm aware that there are other OD units that were added to earlier standard 3spd transmissions, but I thought you were talking about more modern units.

I also was involved in the Doug Nash overdrive units and early R&D related to their installation and in early repair and modification of the early GM automatic overdrive transmissions in the late '70s and early '80s.

I have a Corvette aluminum T-10 with the T-85 R11 4 pinion overdrive behind it. Originally it was to go in my 41 but now it's slated for Bonneville. Should be a reasonably strong trans and way easy to shift. Happy as a pig in slop to find it on ebay. Back when I first built the 41 and installed a Saginaw 4speed w/OD behind it I found a 3.73 rear end a little tall for my tires and my foot a bit too heavy for long term longevity. Consequently I ended up with several trans W/OD's and being able to rebuild them. They were too weak for my habits.

The overdrive transmissions that I was originally talking about were the 3 speeds with the planetary type overdrive unit controlled by an electric solinold. The type that Blackwater mentioned is interesting also. What little I have read has been about the Chrysler units. These are 4 speeds where 3rd is direct drive and 4th is an overdrive. One question I have about the OD 4 speeds that blackwater is talking about, is the cluster gear different than a regular Saginaw 4 speed? Thanks in advance, Jay

YES!! They ARE different. The only parts that I'm sure are interchangeable are the bearings and the case parts. I only ever saw these in Camaros and one in pickup. Pretty rare. The Chrysler units were in Aspen and comparable wagons and one or two in corresponding sedans. You'll hafta' comb the junk yards for these. Like I said, most of them that I saw had bearing failures, particularly the middle main shaft bearing. Some of them were pretty spectacular. Some were catastrophic!!

I think I have one of those I picked up with some engines and other stuff from a guy who was moving. I've not looked inside it.

I ran a 48 Packard overdrive behind Chevy 261 in my 52 Chevy during high school and college (late 50's - early 60's). It would shift into OD in all 3 gears when a switch was thrown, but I seldom used that capability because the some of the non OD gears closely matched OD ratios and the shifting was tedious. I used the 3rd and 3rd OD most of the time. OD worked well for cruising. I do remember one OD safety hazard was the car would coast when you let off in OD - eliminating engine braking!

If yours used an extra switch and would engage below about 27mph (1st gear) the wiring had been modified. That probably also accounts for the free wheeling. The switch would have made the ground connection and bypassed the governor.
I have a new motor for my mill coming. When it is up and running I'm going to try making one of the Borg Warner ODs into a stand alone and hook it up on the 1919 Essex speedster project. I haven't figured out the reverse kick-out yet. The ODs don't like being turned backwards.

Those chrysler units are not Saginaw though. AFAIK, Saginaw never made a 4 spd OD. The Mopar, and Chevy variants Were New Process A833s. I have one in the shop intended for my 56 Dodge PU. And you were correct about lube is important. They were speced as needing ATF back in the 70s. My first one was a direct drive high gear version in my little Dart HP273.

The A833 OD was std. equipment on Feather Dusters and Dart Lites.

Yep, also available in 1/2 pickups and vans. Mine came from a van so it has the special forward location Hurst shifter with it.

The larger vehicles may have longer tailshafts as well.