The Stovebolt Site Forums The Shop Area Engine Shop Swapping to 3 speed with OD

Anybody have any experience swapping out the original 3 speed manual transmission to an original 3 speed with overdrive?

I am considering this on my 1959 GMC 9314, which is a Canadian halfton, shortbox stepside. Canadian GMC's have Chevrolet drivetrain. Therefore I have a 235 I6 with a 3 speed column shift manual transmission.

I would like the overdrive to improve highway speeds. Ultimately I would like to be able to cruise at 60 mph without the engine screaming at me.

I would like to know if there are any tricks to the swap.

Also if anyone knows of or has a transmission for sale let me know. Were they all 3 speed Saginaw's with Borg Warner overdrives? And are there different models, one for sale on ebay said is was from a Chevelle wagon. What year(s) should I be searching in the junk yards?

Blaine Dumkee
Fort Smith NT
Canada

Follow the T-85N links on this site: Burb Club

the OD truck 3spd should be a bolt-in, Blaine, but I wouldn't count on later car trannys fittin the same [tho they might], and then there's the electrical and switchin to set up.

I've got a truck one I might be usin on a 57 panel that's in the works, but the other possibility is the better ratio ring and pinion, which is supposedly a drop in deal [3.38:1], there's a place in Van, BC that has the correct set in stock - their website\'s here .
not cheep [~$566CDN w/ taxes - that's about 375US fer you southern folk], but cheeper than from the US and could be cheeper than the tranny shipped from eastwhatevr.

good luck w/ it - I'm into gettin better highway speed and mileage for my task force trucks too . . . . .

Bill

I would also confirm Squeeze's comments about a direct replacement. I replaced the the stock 3-speed in my 57 Chevy pickup with an OD 3-speed. It was a direct replacement, driveshaft, mounting, etc. I also have a lower ratio rear, so I am very happy with my setup. The "Burb" link is a good one. In addition I got a "compiled" manual from a contributor to this forum (don't remember who) that was helpful. The factory manual also has info on the OD unit in it. Last, with some digging, I did find some alternatives for wiring it. At the moment I don't know exactly where. Basically if you dig around you can find some good info.

There were a couple of 3-speed ODs that will work. The older T85N is referred to as the Borg Warner, the later (65-72) is the Saginaw although both versions used a B/W overdrive unit, and even though some parts are shared they won't interchange. By interchange I mean you can't bolt a Saginaw OD tailshaft to a early 3-speed & vice versa. But a complete later OD trans will bolt up to your 235 and has the advantage of a synchro 1st gear. Car and truck versions of the Saginaw were the same (to my knowledge) so that shouldn't pose a problem but the shift rods on the tranny side were different from the earlier unit, so some fabrication may be needed.

The factory scenario on both these ODs is hard to duplicate these days. It was actually a fairly elegant design, with a governor to prevent lugging (certain death to overdive internals), a kickdown to give a "passing gear", free-wheeling, clutchless shifting, and a momentary ignition cutout for smooth engagement of the OD. Very slick and almost seamless. But the parts for the entire setup are hard to come by these days, thus the need for alternative solutions like that mentioned from the Burb club page. This article is well done and reflects a thorough understanding of the way the OD was designed. But, a simple toggle switch to control the solenoid is really all that's needed. My experience is that split-shifting through six speeds is fun for a while, but as the goal is top gear cruising, I just engaged the solenoid for the OD once up to speed in high gear.

True overdrives went out of fashion on American cars in the '70s, once Borg/Warner perfected the manual OD 5-speed in a lightweight, compact but efficient package. Much less costly to produce and the end result is the same, although features like free-wheeling and clutchless shifting are nice to have, once you get used to them.