Tranny swap

Adding an overdrive (BW T-19 to ZF-5 transsmission swap)

by Bakari Kafele on August 23, 2012

If you’ve never driven a vehicle more than a couple decades old, you probably take overdrive for granted.  You may not even have a clear idea what that term means.That 5th or 6thgear, with a ratio smaller than 1.0 (meaning the driveshaft is turning faster than the engine) lowers the engine RPM speed on the highway and can make a huge difference in the fuel used to go the same distance at the same speed. Gears on a car are just like gears on a bicycle; imagine trying to ride a bike with only a small chainring and big cog, and having to spin your legs like crazy to get anywhere at a decent speed.  Lower RPMs means less internal friction, less internal reciprocal motion, and therefore less wasted energy.

If, like me, you don’t care to spend the money for a new – or even remotely new-ish – vehicle, you may have noticed that overdrive was once upon a time not always standard equipment, or even available as an option.

The Ford F-Series of trucks has been one of the most popular vehicles world-wide for decades, and though much has changed over the years, many of the internal design factors stayed the same from one generation to the next.  They were rather reliable, so a good many older ones are still on the road.  Those two factors mean that there is much interchangeability of parts among different generations, and those parts are easy to find.

The 7th generation F-series (1980-1986) had a couple of manual transmission options, all of them 4 speed. My own 1983 diesel F-250 ¾ ton truck came with a Borg-Warner T-19, in which the 1st gear was an extra-extra low granny gear (6.32) which is normally not used.  For all practical purposes it is a 3-speed.  No overdrive gear.  In fact, even 4th gear isn’t quite direct drive, at a 1.1 drive ratio.
This means shifting into top gear at 25mph, and 2400 RPMs at 55mph.

2400RPMs means each piston is going up and down 40 times every single second, which means the mass of the piston head has to stop, change direction, move a little, stop, change direction again, 80 times every second (once up, once down, for a full rotation).  This is bad enough is a small engine with light parts and a couple cylinders (like a motorcycle) but in a heavy V8 diesel engine, a lot of inertia is going to waste.
Not a terribly big deal in 1983, when the national speed limit was still 55, but post-embargo gas prices had dropped again; the lack of stock overdrive leaves a lot of potential for increasing highway fuel mileage.
The addition of an overdrive gear reduces engine speed from 40 cycles per second down to only 28, a 30% reduction.

The 8th generation Ford trucks came with a couple of 5-speed transmission options, and although built by a completely unrelated manufacturer, they were made close enough to the old specifications that they were interchangeable.  As such, the transmission swap from gen 8 trucks into gen 7 is a fairly popular and common one.

It is possible to put any transmission from a truck from 1973 all the way to 1996 into any other from that range, provided they have the same engine size, fuel type (gas vs. diesel) and drive type (4×4 vs. 2×4).  This includes going from automatic to manual (or vice versa), although of course then you need additional parts.  With the right adapters one can even cross the engine, fuel type, and driveline compatibilities.

I’m just going to go over one of the simplest and most common transmission swaps, the one which I recently did: starting with a Borg Warner T-19 4-spd (behind a 6.9L International Harvester IDI engine) and swapping it with a ZF S5-42 which I got on ebay (the process from the T-18 or to the ZF S5-47 should be identical).  The information here would likely apply or be useful to someone doing any other 7th/8th gen F-series tranny swap as well, and maybe even 6th or 9th gen swaps – but I haven’t done them, so I can’t say specifically which parts will apply.


Before I started the project or purchased anything, I spent a lot of time looking up information on it.  A lot of helpful people who have done it before have taken the time to provide information on it.
Everyone consistently said “it is a bolt-in replacement”.

Well, technically it is.
The actual bolts line up perfectly between any transmission for the 6.9L and 7.3L diesel engines, whenever they were made, whatever size truck they were from.

But “bolt-in replacement” gives the impression that everything will mate up.
Its just literally the bolts.
Almost nothing else is compatible.

I also found there are a lot of helpful people who answer the questions of people who get stuck on various truck enthusiast forums.  Any problem I ran across (and any you are likely to) has probably been experiences by someone else somewhere, asked online, and answered accurately.

The only problem is, you have to know the right questions to ask, (or rather the right keywords to enter into a search engine), to find those answers, and until you run into a particular problem, you don’t necessarily know the right questions to ask.

So, to insure no one else has to go through the same learning process I – and many others – have taken, I’ll list all the potential problems one may come across, if you happen to own an older Ford truck and you want to upgrade to a transmission with overdrive.

Instead of going through the entire process step-by-step, I’m just skipping to the potential problems – the things I wish I had known in advance – because there are plenty of other guides and manuals that can walk you through it – but even more because really, if you have enough confidence and experience with mechanics to even consider removing and installing your own transmission, it is actually a fairly easy and intuitive process.

If, like me, you’ve never changed a transmission, a clutch, or any other drivetrain parts, the tips that will follow here next week are things to know will make your job a whole lot easier.



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