Easy 6-Speed Transmission Tunnel Cap Install for Your Classic Car

WRITTEN BY STEVEN RUPP FOR HOT ROD MAGAZIN ON JANUARY 20th, 2023

How to expand your trans tunnel to house a modern six-speed manual transmission.

We ask a lot of our classic cars. After all, they were never designed to support the parts we're trying to transplant into them. Back in the '60s, transmissions were small, and with only three or four gears, they didn't need to be any bigger. Today one of the most popular transmissions for transplantation into our early Camaros is the Tremec Magnum six-speed. The six gears, two of them overdrives, offer the perfect blend of performance and highway manners. The big problem (pardon the pun) is that it's far larger than what GM envisioned when they designed the Camaro's transmission tunnel. You can also run into space limitations with a larger automatic transmission swap.

The simple solution is to do a little metal surgery and make the tunnel roomier, a task made simpler thanks to the transmission tunnel cap offered by Chris Alston Chassisworks. This preformed panel takes a ton of fabrication out of the equation and allows someone with basic tools, some skills, and a welder to make their tunnel "Magnum six-speed friendly." For this project we headed over to our favorite shop, Best of Show Coachworks (BOS), to get our 1968 Track Rat Camaro project ready to receive its drivetrain. This work is being done on a 1st-gen Camaro, but the process would be about the same for whatever hot rod project you're wrenching on.

Here's the stock floor in our 1968 Camaro. Aside from a few holes and some mounts for an aftermarket ratchet shifter, it was in good shape, but just a bit too small to house our new Tremec six-speed transmission.

We were able to make the marks needed under the car and start removing the top of the factory tunnel, making sure not to go bigger than the tunnel cap we sourced from Chassisworks. The transmission was supported by the Chassisworks tubular crossmember (PN 5916-F10-05, $256) that's designed to work with our subframe, or with a stock subframe using adapter plates. Here you can see what we removed from the factory tunnel. The new cap will raise the tunnel about 1.5-inches. Not a lot, but enough to clear the new six-speed.

Checking The Fit

With the old trans tunnel out of the way, we were able to raise the new Tremec transmission into place, check fitment, and locate where the shifter would poke through. Using an abrasive disc, we also cleaned up the edges in preparation for welding. If you don't need six gears then you might want to look at Tremec's TRX five-speed trans since it's quite a bit smaller and may fit your factory transmission tunnel.

This is the transmission tunnel cap from Chassisworks (PN 5922-F10, $111). It came preformed and ready to rock, complete with a mating flange on the downward edge. The cap extends from the firewall back to the seat crossmember.

We went ahead and marked where the shifter would poke through and made the necessary hole.

Next up we clamped the new tunnel cap in place to check fitment. As you can see, we needed to fabricate a panel to transition to the firewall. We also decided to cut a small curve into the front of the Chassisworks cap to make the firewall transition piece easier to form.

We then used a pair of snips to remove the marked section from the Chassisworks tunnel cap.

Tricks For A Clean Result

With that done, we drilled some holes around the perimeter and used Cleco fasteners to hold the new tunnel cap in place. Cleco fasteners, and the required pliers, are inexpensive and come in handy when doing any sort of sheetmetal welding. If you look closely you can see how BOS notched the corners and bent the side edges of the tunnel cap so they fit inside the factory sheetmetal, creating an overlapping lip. Though not completely necessary, it makes for a nicer finished project.

With the tunnel cap secured by five Clecos and a couple of pairs of locking pliers, we started tacking it all together using our Millermatic 140 MIG welder.

A tack every couple of inches was more than enough to do the job. If you're an extra-miler, you can perimeter-weld the cap in place, but it's not absolutely necessary.

Next, it was time to fabricate the panel to tie the tunnel cap into the forward section of the trans tunnel. For this, we had someone hold a piece of heavy paper under the car so we could trace the opening with a marker. Once we had the paper template made, we traced it out on a piece of sheetmetal.

If we made the panel the same size as the opening, we wouldn't have any overlap to weld to. The solution was to trace the template onto some steel then use ¼-inch tape to add a flanged edge to the panel.

Fancy Tools … Or Not

The panel was then cut out with a pair of snips and curved to match the tunnel. You can use something fancy like an English wheel to make the curve or just fake it with something curved, like the acetylene bottle we used.

We then used a pair of pliers to slightly bend the curved edge of the panel so that it would better mate up with the tunnel cap and the curved section cut out earlier.

The front edge was tacked down and a hammer was used to force the far edge down so it could be tacked in place.