Aerodyne Boot Lid Droop

The boot lid on a 401 or 403 (called a “door” in the instruction manual!) is held open by a two-piece telescopic tube with a large compression spring inside. The boot lid is opened by pulling a knob from inside the passenger compartment that can be accessed by folding down the rear centre armrest. Pulling the knob releases the boot lid catch, and the spring, according to the instruction manual, is then intended to “retain the door in any intermediary position”. By comparison, on my 403, pulling the knob caused the boot lid to open violently, swing up to about two thirds open, then settle after several oscillations to something around halfway. Apparently this is not unusual, and is most annoying as in practice anything less than fully open is really no use.

 

I remember some time ago seeing a post on the now defunct BOC online Wiki where someone had successfully replaced the spring with a gas strut like the one used on a modern hatchback, so I decided to have a go myself. The make and model of strut is now no longer available, but I have been able to source a near identical one. I have now completed the job, and thought that others might be interested in the detail.
Most gas struts have a fixed resistance to compression, often designed for a specific purpose (for instance a particular make and model of hatch back or SUV). However, struts are available that have a deliberately too strong a resistance, but also have a bleed valve, so that one can carefully let out gas until the correct resistance is achieved. The strut I used came from a company called Metrol. Fortunately, they have no problem selling a single item. The part number is NS-V-8-100. It has a 150mm body length, a 100mm stroke and its maximum diameter is 18mm, so it easily fits inside the original two-piece telescope. It has M6 threaded ends. Unfortunately, its total extended length is somewhat less than the boot lid fully open distance which is about 300mm. So, a length of 6mm rod is needed (100mm is more than enough and can be shortened after the whole assembly has been adjusted). I used an internally threaded coupling with a length of 18mm to join this extension rod to one end of the new gas strut. One other thing to note is the bleed screw, which takes a 2mm allen key. So a clearance hole for the allen key needs to be drilled in one of the telescope pieces so that the gas can be let out with the whole assembly in place on the car. Also, a pair of 6mm locknuts is needed on the extension rod, to fix the total length of the assembly.

Once the new strut with its extension had been installed in the telescope, I then adjusted the total length of the assembly (by trial and error, altering the position of the locknuts) until the boot lid was just clear of its front edge clashing with the paintwork underneath the rear window, thus providing the maximum opening. I then locked all the nuts down with Loctite.

The next job was to release sufficient gas. As installed, to close the boot lid would have caused damage. However, starting to release gas would run the risk of letting too much out. The original Wiki post suggested adding some weight to the boot lid, to provide a warning that there is not far to go! I used a house brick, suspended from the steel tube under the boot lid, just above the lock mechanism. Obviously, one stops releasing gas as soon as the lid starts to fall under the weight of itself plus the house brick. One then checks to see how much force is required to close the boot lid without the extra weight of the brick, letting a tiny bit more gas out if necessary.
I am very pleased with the end result. On pulling the release knob, the boot lid opens in a very controlled manner, and gently moves to fully open, with no overshoot or bounce; and the whole assembly looks original apart from the small hole for the allen key.

Short video below

Short video

Mike Brooks