Dash and Plenum

Mitch Krause

I was looking at things, had imagined that the plenum would attach to the dash support and then the dashboard slip over that. Tried that with everything loose and was having fit problems (dash would not go over plenum and fit below spider). All could be solved if the spider was off and then dash installed, but then makes getting it out near impossible. I then read that the plenum should be bonded to the dash and installed as one piece. (RCR GT40). Did some searched and didn't find much on this. So question has anyone done this? Thoughts and advice? It looks like there should be about a 1/2 inch bracing put at the ends of the plenum (near the outside of the dash) to keep pressure down on it and make it seal. Anyone have advice or help?
I think it depends on what year plenum you have. I believe the early ones were bonded to the underside of the dash, the newer ones are attached and sealed to the tub. Mine fit below the dash and spider, however, you will have to massage the plenum a bit (i.e. remove some topside material and mold up a better seal (mating) area to connect to the defroster vent.

There's way too much detail in this area to describe here, so I can't stress enough to verify everything several times before you start drilling holes. The placement of the spider (wrong or right) has a cascading effect on most everything else you do going forward...
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Randy V

Staff member
Lifetime Supporter
Your plenum was the first design that RCR made and came out a couple of years after I got chassis 45 from them. It was meant to be attached to the top of the monocoque, but like you figured out, the dash would need to be installed and then the spider.
My plan was made ((before)) RCR made a plenum - I had planned to make the dash removeable. It is cut to clear the A-pillar and rollcage to slide straight back - except - now the plenum is in the way. I had decided to go ahead and screw/bond the plenum to the top of the mono and then cut off the leading edge of the dash that fit under the spider. I had not yet done this before you bought the car. Making the dash mate to the bottom of the spider would have been done with metal guides bonded to the bottom of the new leading edge of the dash. Those guides would slide under the old leading edge of the dash (now trapped by the spider)..

Not sure if this makes sense - happy to discuss on the phone.

Oh - your spider is(was) located exactly where it needs to remain for all the bodywork to properly align.

Mitch Krause

Thanks Tom and Randy,

I was contemplating different ways, didn't want to cut the plenum on the sides to let it slide in with the dash, as that appeared like it would get much to close to the holes in the dash, and also have sealing problems.

Randy, I will give you a call.
FWIW, and from my experience so far, I think the removable dash is over-rated. If you have to remove the dash, you probably have bigger issues, and removing the spider will gain you all the access you need and save you a ton of time (and aggravation) in the long run.

With that said, you can access most if not all of your electricals through all the gauge holes, switch plates, and underneath. Just do a good, reliable wiring job (add extra wires for future expansion..), think ahead, think worse case scenarios, and place your relays. etc., so they're easily accessible. Make your gauge holes just big enough so that you're able to press in your gauges and avoid using the screw on rings with the small gauges (tach and speedo OK). Due to the thickness of the fiberglass dash, you probably can't use the rings anyway...

This all saves you from butchering your dash to try and make it removable. Also take into account that on a hot day, that black dash sees a tremendous amount of heat. Leaving it one piece will minimize the distortion and warping you will most likely get after cutting it into pieces...

There's a way to secure the front of the dash securely by attaching (or bonding) floating nut plates behind the front dash lip and securing them with counter-sunk screws at the front of the spider. I used 5 AN countersunk 8-32 screws along the front radius of the spider (hidden when front bonnet in place). It draws the dash forward nice and tight! Drill through the spider into the dash first to mark your holes...
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