04-11-10 Transmission surgery

Transmission Surgery

My foot started acting up again this weekend so I didn't get much done, but I did get the 'midshift' setup done on the transmission. Why midshift? It's not necessary by any means, but the engine/transmission is set back so far in the Cobra chassis that the normal shifter location for the Mustang transmission most often used requires a shifter handle that leans far forward like THIS to be easily reachable by the driver. The 'standard H' shift pattern is likewise angled forward. Not a big deal, but if you want a shifter that's more vertical like most are, relocating the shifter is necessary. There are some expensive Mustang transmissions like the TKO-600 you can get which are designed to give you a choice of locations, but it's getting into the $2k range plus the cost of the parts kit to change them out. There are also some jerry-rigged ways to do this on the ol' reliable and plentiful Borg Warner/Tremec T-5 transmission by making an external linkage, and some have had good luck with it. But, because it's easy and inexpensive, and because the T-5 was used in so many vehicles in the 80's and 90's (Mustangs, Camaros/Firebirds, S-10 pickups, etc. etc.) it's better just to do a midshift conversion on your T-5. I ended up getting all the parts I need except the shifter from FFCobra.com member JJ'snakepit, who is the real expert on all this, I'm just reporting on my experience following his method (Thanks dude!)

From a manual-sihfted S-10, you need the tailshaft, top cover w/ it's shorter shift shaft (you don't have to get the shift forks if you don't want to, but it's easier to swap them as an assembly), shift block, and a modified reluctor (speedometer) gear from the S-10 tailshaft. You'll also need a modified transmission mount and shifter (I'll go over those later).

First step is to drive out the drift pin from the shifter block in the Mustang shifter pocket so you can remove the block from the shifter shaft, like so:

With the shifter block removed, the eight bolts holding the tailhousing to the main part of the transmission can be removed and with a bit of leverage to pop the RTV sealer, the tailhousing can be pulled straight backward and free of the tailshaft and shifter shaft. Also, from the shifter block you already removed, make sure you save the spring and ball to install in the S-10 shifter block.

You see that the speedometer sensor hole on the S-10 tailhousing is in a different location than it is on the Mustang tailhousing. I measured 8-1/4" from the front of the tailhousing to the center of the speedo hole.

Measuring on the tailshaft itself, you can see where the yellow mechanical speedometer drive gear sits. It's not needed for the new setup, so it can be removed. Pull a little metal clip and the plastic gear will slide off the tailshaft pretty easily. The black ring to the right of it in this shot is just a seal, it'll be reinstalled later.

A lot of S-10's use an electronic speedometer sensor rather than a gear-driven cable, and therefore use a metal reluctor gear. It doesn't contact anything, but the speedometer sensor reads the teeth spinning past with a magnetic sensor, which sends a voltage signal proportional with the speed of the tailshaft (and therefore, driveshaft and rear wheels) which can be sent to an electronic speedometer, which will be pretty accurate when calibrated correctly. The reluctor gear has inner teeth sized for the splines on the S-10 tailshaft, however. This means you either have to swap the S-10 tailshaft into the Mustang T-5, or if the inner teeth are machined out of the reluctor gear and you have a bushing made up to take up the slack (which JJ included in the S-10 parts I got from him), you just slide the gear and bushing in place, tighten down a set screw with an allen wrench, centered on the 8-1/4" measurement, and you're good to go:

Here's the Mustang T-5 top cover next to the S-10 one. Only difference is the length of the shift shaft. The forks and everything else are pretty much identical. clean off the old RTV, put down a new bead, and bolt the new cover on at 6-11 foot-pounds of torque. Need to make sure the shift forks seat in the correct locations, of course. If you don't you probably can't get the cover to seat properly anyway unless you really have something hosed up.

Then before you apply new RTV to install the S-10 tailhousing, mock it up to make sure the reluctor gear is in the right place on the tailshaft, like in this shot. If so, pull the tailhousing back off, apply a bead of RTV and install the tailhousing. Apply some sealer to the bolts if they don't have it and torque them to 20-45 foot-pounds.

Then you'll need to hold the spring and ball I pointed out earlier in the bottom of the S-10 shift block and position it in the shifter pocket of the S-10 tailhousing while you work the block onto the shift shaft. You'll see the six round dents in the bottom of the shift pocket that the ball pops into as you shift though the five gear and reverse. When you line the block and shaft up properly you can tap the drift pin through the hole in the shift block and into the shift shaft to hold them together. Make sure you transfer the plastic shifter bushing (slightly orange-brown color in this shot) from the old shift block if your S-10 block doesn't have one. Then you can install your new shifter. It just so happens that an aftermarket shifter for a Viper, Camaro, or other T-56 6-speed transmission is just the right size for the S-10 shifter pocket, so it'll be easy to find.

And here's my completed T-5Z midshift ready to install along with the motor, hopefully next weekend. A couple of other things, though...the S-10 transmission mount bolt holes are in a different spot than the Mustang ones, so you may need to fab up your own mounts. But, Mike Forte just happens to have a 'hybrid' polyurethane transmission mount with S-10 upper bolt holes and Mustang lowers, than should allow it to drop right into place. Also, you may want to grind off the extra tab on the S-10 tailhousing (see arrow). It may hang lower than you want it to in the FFR chassis. It might be intimidated if you've never cracked open a transmission before, but if you have all the parts handy and the speedo gear and bushing already machined, you can probably do the whole mid-shift conversion in a couple of hours, if that.

I dragged all the dashboard gauges, switches and lights upstairs so I could get off of my feet and started mocking them up in the dashboard. I still need to enlarge a couple of the holes, one for the ignition switch and the other to make the Probe turn signal fit where I want it, and I still have to patch the extra steering shaft hole. For the hell of it I'm trying out one of Autometer's DPIC (digital performance info center) gauges.

This shot is actually from last weekend, I forgot to include it. It turns out the Cobra valve covers I was going to use will not clear the stud-mount roller rockers I have, so I'm going to have to get a different pair. There's another big change of plans for the motor as well. I've decided not to use the Mass Flo EFI system. I'll instead be using the Powerjection III setup by Retrotekspeed (formerly known as BossEFI). Much less wiring, much more directly tuneable by me, if need be, with just a laptop, and no need for burning new chips. It's also designed to be self-learning, which negates one of the biggest advantages Mass Flo had. More on that in the next few weeks as I get ready for first start. If all the parts arrive I should be picking up and installing the drivetrain next weekend.