I did this exact swap in an 84 6.9 about 3 yrs ago. The T18 4 speed is almost exactly the same length as the ZF5
. So it will bolt right in. Even the slave cylinder is the same. The driveshafts and transfercase shifter will all swap over absolutely zero issue there. There are, however, 3 minor issues.
1) The crossmember for the 4 speed is no where near being correct for the zf 5. You can either A) get a 5 speed crossmember and drill new holes in the frame because the frame will not have the correct holes for the 5 speed (as the 5 speed was not even an option in 86) or B) You can make your own cross member. This is what I did. I made it out of a piece of scrap channel iron and some angle iron. This took me maybe an hour.
2) The input shaft on the 4 speed is 1", the ZF5
is 1.25". So you will need to get a flywheel/clutch for a 7.3. You can piece together a flywheel clutch combo for the 6.9 flywheel (using an aftermarket pressure plate and clutch disk for the zf5
), but it is kinda a pain and I don't remember what part numbers you need. I would just get the 7.3 flywheel/clutch set up.
3) The cab floor where the shifter goes through is slightly off. the shifter will work, but the transmission shift tower sets at the very back of the cab floor hole. This isn't a problem. The problem is that the transmission tunnel is about 2" shorter than the zf5
tunnel. The problem with this is that when you go to jack the 5spd up in place, it will hit the bottom of the cab. That transmission hump/tunnel piece will unbolt, and swap out, but the problem with doing that is that your hole in your carpet and the hole in the floor will no longer match. So what I did was I put a floor jack with a short 4x4
block and I jacked on that transmission tunnel/hump right where the shift tower of the zf5
would hit. By doing this I "bent it up approximately 1-1.5". This gave me the clearance I needed and the carpet matches the hole and the floor and you can't tell at all from the inside. This allowed me to use the old T18 4 spd. shift boot. I did have to cut the hole of the boot slightly larger as the shifter for the zf5
is about twice the diameter of the shifter on the 4 speed. The best part of this option is if for some strange reason you ever wanted to go back to the 4 speed. It will bolt right back in it's stock location and everything will still be stock and look completely unmolested.
If you take your time and don't get carried away this is a very straight forward easy swap that can easily be done with 2 people on a Saturday (or even half a Saturday...........).
Best of luck