I removed the old vents for patterns and here is a picture of the best and worst. I could just about glean enough information to use for my replicas.

The first job was to make a die to form the "gills". This was a piece of scrap steel. Well two bits actually because you need to sandwich the aluminium between them. This is what is going on in the picture below. One of the dies is very slightly smaller than the other to enable me to get in close and bash the aluminium tightly over the correct sized former. The small one has the X on it so I can tell the difference. It also identifies the outside of the die because my filing is not quite uniform!

Not very clear in the photo of the die is the small slot just about half an inch up from the bottom on both ends. This represents the place where a bend in the aluminium will be. Because I made up the die from the scruffiest bit of scrap I could find the bottom edge was not very regular and all my measurements had to be taken from the top edge. Someone less mean than I could probably do this more easily by starting with a straight edge! A regular bottom edge would also help in setting up the aluminium visible around the outside of the die, whereas I did by eye each time.

This is forming the "gill" by simply knocking the surplus aluminium edge over the large die with a small hammer. Note that I have already snipped the sharp corner off. Don't try and bash it all at once - bend it part way all the way around and then go back again.

This assumes you are using nice soft aluminium of course. As I have already said, these small projects lend themselves to scraps you have hanging around, so you may need to soften older material which has age hardened. For general purposes (and without recourse to the slightly superior method of using acetylene soot) the best way to soften aluminium is to use soap as an indicator. Decent basic hard soap is getting so hard to find but I find Fairy Household to be quite good. Failing that, the small tablet you get in a Travel Lodge is not bad, but alas, even they are going over to liquid soap these days. Just scrawl on the aluminium as if you were using a crayon. You do not have to cover it all; here and there will do. Now apply heat using a decent sized blow torch. As the temperature rises the soap, previously pretty much invisible until now will start to turn brown and finally black. Job done. The melting point of aluminium is a mere 660 degrees C. so do not apply any more heat after the soap turns black unless you wish to move on to metal casting for beginners. Leave the aluminium to cool by itself and don't pick it up until it is cold.

After the aluminium had been knocked around the die, using a junior hacksaw I cut little slots in the edge, referencing the slots cut in the edge of the die. This marks the place for a bend to form the foot for the gill. Then, mounting the die flat in a vice I filed off the surplus aluminium until it was flush with the die. I have to confess - you remember I marked the smaller die with a "X", well at first I marked the larger die and then promptly filed off most of the "X" with the first one I made. This is why you can see a yellow X in the photo right, but it is not the mark; it is a mark. Ahem.

This is the gill extracted from the die, with a score mark between the slots to indicate where I am going to bend it later. Which as you can see was carefully done by putting it in a vice and hitting it. Again, if I had taken more care (and time) making my dies I could probably have bent the aluminium while it was held between them. There was a certain amount of making it up as I went along and in the end the tab for the foot (which was intended to simplify spacing the gap between the gills) turned out to be slightly too generous and had to be trimmed down to size.

At length… And I mean at length because I could not face doing all of them in one go, you end up with a whole pile of the little beggars, as seen in the photo. This is enough for two vents. Sigh…only twelve more to make to complete the set. One of them is my prototype which is why it already has holes cut in it.

The prototype holes were completely filed by hand and I fiddled around with various alternative ideas when I realised doing the whole lot with a file was just not on and inventing a punch press was equally ridiculous. Eventually I decided to cut them out using a fret saw. Fortunately I have a power one of these which I originally bought for cutting out thin plywood parts when making seat carcases, but it has turned out to be really useful for many bus preservation jobs.

I made an aluminium template to mark out where the holes would go. At first I thought I had made a mistake by bending over the foot of the gill before I reached this stage. Then I realised the piece was easily supported with a block of wood to hold it flat for cutting and more importantly, since the bend was already in place it enabled me to gauge the cutting to leave a very slight lip just above it which made the whole thing much stronger. You can just about make this out in the next photograph. I did try bending one after the holes were cut and it distorted.

Fortunately, not all of the gills require so much cutting. The small hole is the air inlet which faces towards the front of the bus. There are four of the other 2 hole gills, two each side of the centre section and the final one at the back of the whole assembly is a blank.

The base plate was made simply by copying the size of the steel original. I did not have a hole cutter of the right size so this was roughly cut and then filed to the correct size. Often the right tool is available to buy but at such a price that you would have to make dozens to make it pay, so I have a good selection of files which are useful for all sorts of stuff.

You have to fasten the gills at one end only and then fasten the centre section next. Due to the curve on the gills, the centre cannot be fitted if all the gills are in place.

The centre section was bent in a small bending machine mounted in a vice and then while the edge was still held in place the curves were put in "freehand" by bending round a steel bar, which was clamped on and packed to the right height. There were a couple of rejects before I got everything symmetrical and the radius quite right. Once one was ok I made a cardboard template (good old Kellogs) so that I could mark out on a flat sheet correctly each time. Of course if I was a proper tinsmith I would have made the template first, but calculations are all very well but somehow don't always translate to the real thing easily. In fact, if I was a proper tinsmith I would have got it right first time even without calculations.

 

This is the finished article. There is a 3 inch diameter tube that goes through the hole in the roof of the bus and the ventilator base plate. This must stick up proud or water will find its way inside the bus from rain driving into the air inlet. I made the tube by bending aluminium sheet and sealed it in place with automotive mastic, which is all the fastening it needs.

The original Rawlings ventilators were spot welded together. Spot welders are not usually found in the home workshop and one capable of handling aluminium is even less likely. I turned down an offer to TIG weld the gills in place, partly because it meant someone else aligning all my bits and in line with my motto of "made it all myself so far" settled for using solid aluminium rivets.

The base plate was prepared using a 120 degree countersink and the small excess of rivet ground off, leaving a smooth underside to the ventilator which will sit down nicely on the roof of the bus. The interior gills had three rivets each but anything with an outside edge only had the minimum required to hold it in place because of course eventually pop rivets will be used all around to fasten the complete assembly to the bus.

So there we are. They were made out of scrap off cuts and do have a bit of a hand made look. They don't have "Rawlings Vortex" embossed in the top of course. Better than my original thought which was to plate over the offending holes in the roof. And I reckon it is going to be quite some rivet counter who can find fault with them once they are fixed in position up on top.