New transmission tunnel mould

So, after making the transmission tunnel top panels at great expense, I realised that they weren’t going to work well for me. I’d made and cut them so that the panel edges mated with the curve of the top tunnel chassis rails, as you can see on the left.


However, the holes in the chassis rails that you drill through to are far too near the edge of the panel to give proper, structural location. I’m not in this for the bling so whilst I could bolt the panel on, it the holes would be near the edge and probably wear and oval. It would be lighter, but wouldn’t add any structural value. As Warren says: “if you’re going to put anything on there, make it structural, or don’t bother“.

So, my first thought was to weld some brackets inboard to the top tunnel rails so I can drill through much further into the panel and get a secure location. This would also mean having to weld up the holes in the original rails which with TIG isn’t a doddle. It’d be easier with MIG but that would also mean injecting a load of filler in. Either method would require grinding the welds flat again.

Then I thought… why not chop out the top rails and replace them, adding the inboard brackets for a quicker route to location. But at this point it dawned on me that I should be thinking in 3D and I should create a wrap-around tunnel secured to the floor. Then the voice of Warren-Wan-Kenobie whispered in my ear and said take the rails off and make the tunnel totally structural.

So, that’s what I’m going to do. I’ve been working on the mould for a while now and here is the history. When you read this, you should read the titles in a Gordon Ramsey voice:

tunnel 1Clad the tunnel. I feel like I live in “world of clamps”. As it stands, each separate panel is cut in 5.5mm ply and clamped to the rail. This is when you understand why many car manufacturers use soft ally. The intersections of the chassis rails are all in different planes. With Ally, you can just drill it and tap it in place with a soft hammer. With composites there’s absolutely no option like that. The value in the part is it’s strength and rigidity. So, whatever I make must be a good fit from the start. You should be able to see the bolts in the top securing the top panel to the chassis rails to give accurate location for the rest of the panel.

tunnel 2Check the Mould. Here you can see the top of the tunnel with the side panel in place. What is missing from this picture is the subtle compound curves in the large vertical panel to the right. This is also the first opportunity to check if the mould will suffer from mechanical lock. It looks like it will, so a single piece mould will not be an option, and I need to go 3 piece.


top shot

top shot

Change Your Plan. So, it was when I was looking at the top panel originally and the compound curves with their implications for mechanical lock that I decided (with Warren’s guidance) that it needed to be a 3 piece split mould. This picture shows a much larger top panel than the first picture because I scrapped the original top panel and made a new one with two integrated flanges, minimum width of +100mm over the panel size. It means the side panels can have a mating flange to come up to this top flange and will split well. No chance of mechanical lock. You can also see one (of many) side reinforcing ribs to hold the wood in the compound curve shapes.

tunnel 3Reinforce like a git. Here you can see the side panels in place, and the mating flange up to the top flange. I’ve also put all the reinforcing ribs in to hold the compound curves. The process is simple if a little time-consuming. Clamps hold the panels to the chassis. Then you get the hot-air gun onto the panel to heat it up quite a bit to try and melt the laminate adhesive. Once done the adhesive should re-set and help hold the shape.

I then cut the reinforcing ribs and hot-glue them to the chassis. Hot-glue isn’t anything like strong enough hold these in for good, or up to the stresses pulling full-vac in the infusion process. So, the hot glue only locates the ribs before two or three layers of 300gsm chopped-strand-mat are used to glass them in place using poly resin. Once this lot has set the panel won’t lose it’s shape.

tunnel 4

Get ready for the finish. Here’s the final pair of sides bolted onto the top side. You can just see some of the bolts I used to bolt top to bottom flanges. When taking a part from the mould (unfinished) the flanges will have to be sealed with bathroom sealant.



tunnel 5Take another look. Here’s the tunnel from another angle. It’s still not right – the long flange pointing vertically up is totally in the wrong place – I should have made this horizontal but couldn’t do so whilst the tunnel is in the car (because there’s a floor in the way). Sorting this and making the mould ready for pulling the tunnel a part is subject to another post – preparing the mould.

Best tool ever

So, I was watching An MG is Born and I saw them using an air chisel to separate a panel from the surrounding framework where it is either spot or stitch welded.

When I’ve done this in the past on the footwell, it’s been a nightmare of screwdrivers, hammers, buzz-saws and angle-grinders. This time around, it was way easier. The passenger footwell still needed to be cut out (the large bits) but the separation around the welds was way less painful. Just choose the right bit and get in there with the chisel. Best of all, it was only £12.99 from screwfix.


It’s also perfect for amateur dentistry.

Polishing a Mould or Part – Notes

it needs to be revolving polisher

wash mop, and constantly spray the surface.  use a garden pump-sprayer to get a fine mist.

always spray surface and spray cloth. polished needs to be wet to work.

let it use its own weight.  put the polish on the part then the polisher on the dollop. then push polish all over the part. use pump sprayer a lot.

with carbon fibre, always wet sand, then if the water goes grey, you’re through. this is not a good thing.

Making Moulds – how not to do it again

So, in order to prevent a mould distorting under vacuum (especially when you make it from thin absorbent fibre-board) you need to stiffen it, generally by bonding it to a backboard. Previously I bonded the other mould to a backboard with bog. It worked well and made everything solid, but used a load of bog in the process (well over a kilo).

So, next approach was to try bonding with expanding foam, a technique which does work.

What to do – squirt in small amounts at regular intervals so it just makes contact whilst keeping pressure on the top so it doesn’t wobble around

What not to do – squirt a shed load on the backboard, lay the top on and leave it overnight.


This is what it looks like, all distorted and buckled, way beyond the application of bog to fix it. This is what it looks like just before it goes into the skip.