Transmission tunnel modifications

Following on from my last post on the gearbox crossmember changes, here is a quick write-up on the other important job in this area – increasing the height of the front of the transmission tunnel to accept the taller R380 gearbox.

Here is a starting view of the area in question. As well as cutting the tunnel itself, there are also 2 vertical membranes to cut through, so it was a bit intimidating to know where to start – there was definitely a reason why I put this job off for so long!

From my trial fitting I had a good idea of where the gearbox was fouling the top of the tunnel, but Roger Williams’ book also includes some useful details on this modification, so I started by marking out my cut area. I then got straight on with cutting out the relevant sections of the vertical membranes.

With the vertical parts removed it was somewhat easier to then cut out the main piece of the tunnel itself. “Easier” here is a relative term – access to this area is best described as “limited”, so this job took a good couple of hours to complete using a regular angle grinder cutting disc, a mini cut-off wheel on a die grinder, even a junior hacksaw at one point, and I had to approach some sections from above and below to reach them. Eventually I had the necessary piece removed though.

At this point I decided to do another test fit of the engine and gearbox to verify that this would solve the clearance issue. You can see from the main picture at the top of the post that the gearbox fit is now excellent – the gear lever position is spot-on thanks to the shortened remote assembly. I confirmed that I had removed enough metal, and also checked my measurements for the height of the replacement piece that I will be fabricating. Whilst the gearbox was still in I also took the oportunity to dig out the centre console and check that it will fit OK once the new piece is welded in. Most of the new metal is going to be behind the front of the radio console so looks like it will not create any issues, except perhaps with fitting a new tunnel carpet… but we’ll find out more about that a long way down the road.

Happy with all that I got on with making the taller piece to cover that massive new hole in the floor. The book also includes a diagram with exact measurements for this, so I pretty much followed that to the letter. It was a little bit fiddly forming the front wedge section, but overall not a particularly difficult piece to make.

After a bit of final fettling I got on with welding it into place. The top of the tunnel is not completely flat in this area so I found it easier to make up some small angle brackets and plug weld them onto the sides to help even out the gaps, also helped a bit with the limited access for the welding torch. Took me quite a few hours to fully weld this in on every seam, but I might have got a bit carried away, probably a bit over-welded – definitely pretty sturdy 🙂

It will need a bit of dressing and a bit more welding from underneath to make sure it’s fully sealed, but I’ll save that for another day when I have better access to the underside of the car.

Update, April 2021: It’s 10 months later and I finally have the shell mounted on a rollover jig, so I went back and finished welding the panel in from the other side. First I trimmed back any excess metal around the edges to match the existing profile of the tunnel.

With this kind of access it was then easy to give the front edge a few taps with a hammer to meet the new panel lip, before I fully seam welded the remaining joints.

Update, July 2023: George from AVID Automotive has been in touch to let me know that they are manufacturing the transmission tunnel extension panel required for this conversion, and it’s available in their eBay shop here. A useful time-saver if you’re undertaking this conversion yourself.