The dash heater controls operate plastic vent outlets on the underside of the dash, one in each footwell. When the vent is open, the air follows the passage of least resistance into the footwells. By closing the vent, this path is blocked and therefore the air is forced to exit via the dashtop windscreen vents.

The vents themselves consist of five interconnected vanes with the central vane connected to the dash control. Operating the dash heater control rotates the central vane, and with it the other vanes, between the fully open and fully closed positions.

Somehow the central vane of one of the vents has either been misplaced or lost during the constant sifting through the boxes of parts. Unfortunately the vents seem to be unique to the Series 2 and, as far as I’m aware, are not available any more.

After fruitless searches of the parts boxes and keeping an eye out at Stoneleigh spares day, I had to bite the bullet and start researching if and how I could fabricate a new vane. The problem is that without the central vane the vent is useless.

I think most plastic parts are generally injection moulded which isn’t really a DIY option. However there are some very low viscosity polyurethanes available that are suitable for moulding which may produce a good replacement. At least having two vents meant I still had a central vane to make a mould from!

A order was placed with MB Fibreglass Supplies who were very helpful in explaining the moulding process and several days later some RTV Silicone Mould Making Rubber (Polycraft GP-3481), Fast Cast Polyurethane Liquid Plastic Casting Resin (Polycraft FC-6720) and black polyurethane pigment arrived. Some white modelling clay (water clay) was also required, which can be obtained from most craft suppliers.

The first step was to produce a two piece silicone mould of the vane. Four ‘L’ shaped pieces of plywood were fabricated with a depth of around 3″ to make a mould housing. Using ‘L’ shaped pieces has several benefits; they can easily be moved relative to each other to obtain the desired mould footprint, clamping together is straightforward and they can easily be removed at the end without damaging the mould.

The mould housing is then half filled with the modelling clay and clay rubbed along the each of the corner joints to seal them. An off-cut of wood and some coach bolts was used as a mini tamping device. The vane was then pressed into the clay until the long lengths of the vane were flush with the clay (ie half above and half below the clay). Finally a number of indentations were made in the clay which will act as key for both sides of the mould.

It was now time to make the first half of the mould with the two-part silicone system, mixed by weight – 10 parts rubber to 1 part catalyst, ably assisted (hindered) by my two nieces who were on mixing and pouring duties. Being a red colour, it was easy to see when the catalyst had been fully mixed into the white rubber part. The mixture was then slowly poured into the mould housing, covering the clay and vane. The technique is to pour slowly and in the same place so that the silicone pushes out the air as it flows over the part being moulded.

The Room Temperature Vulcanizing (RTV) silicone normally cures in around 4 hours although I left it overnight as a precaution as it still felt tacky after 4 hours, probably due to the cold weather. The mould housing can then be turned over so the clay can be removed, to reveal the first half of the silicone mould with the clay indentations now appearing as small peaks.

Traces of residual clay were removed by wiping with a damped cloth to prepare for the making of the second half of the mould. Once dry, the first half of the mould was lightly brushed with Vaseline, diluted in white spirit.

This should act as a releasing agent stopping the second half of the mould sticking to the first. Some more two-part silicone was then poured into the mould housing as before and again left overnight to cure. Now for the moment of truth …. will two halves separate?

They actually separated very easily and the original vane came out without damaging the mould. The quality looked very good although the proof will only come once the new vane had been cast. The final preparation of the mould was to cut a conical channel for pouring in the polyurethane casting resin and an air vent to help prevent trapped air bubbles in the cast.

The polyurethane resin used was a two part product which naturally cures to an ivory white colour so a small amount of black pigment is required to get the desired finish. The mixture ratio by weight of resin part A, part B and pigment was 10:10:1 so the main difficulty was weighing the three parts accurately as the part only weighs 4 grams.

The resin cures in approximately 60 minutes so it wasn’t long before the first cast was ready. The initial impression was very good – even the original casting marks were faithfully reproduced. However the part was far too flexible so the nieces rudely declared it a ‘FAIL’.


MB Fibreglass Supplies were again helpful and thought the cure process had probably been compromised, most likely caused by having insufficient temperature in the component liquids when they were mixed.

A second casting was made after first heating the liquids on a radiator. This produced a much stiffer vane which seemed to stiffen even further once it had been removed from the mould and left on the radiator overnight. I now had two operational heater vents!!