After making some new 3/8-16 BSW threaded rods to screw into the brass flags, I reinstalled my WT28 horns.
One of them sounded fine but the other sounded like it was blowing under water.
The way these work is an electo-magnet pulls up a diaphragm with a shaft on it, which pushes up the contact points, breaking the connection, and the diaphragm moves down again to repeat the cycle. The moving diaphragm moves air to make the hoot.
So here is how to adjust and fine tune the horns.
Use a points file to clean the contact points. My file is shown in the points.
There is a tiny hex screw with a locking nut that adjusts the gap between the plunger shaft and the points. My wrench is on the nut.
Mine had too small a gap and the diaphragm was not lifting high enough to make a good toot.
There was paint on the nut, indicating that nobody has touched it since Tuner No. 11 set it at the Lucas factory.
Probably the points were somewhat worn down, which changed the gap.
In case anyone is interested in the Part Codes, Patents, etc here are a couple of close up photographs of the internal and external stampings on my twin horns (now tuned up nicely thanks Rob).
The photo is one of the pair on my 1948 Mk IV, which I believe are originals fitted in the car from the factory. If the 8 and 9 are date codes and not a general stamping then year then month is more logical to the car build(1948) and delivered 1st December.
Just checked - both horns have same internal and external stampings on them. However the brass fixing brackets are slightly different to those in Rob’s drawing with the ends more rounded (threads the same size).
Very interesting variation on the mounting, those bent rods. Thanks for posting those pix, David. It would suggest your more rounded or chamfered ends may be intended for clearance with the horn body as mounted in that direction.
When I visited the Jaguar Hertiage Museum in the UK to investigate my car when I first started the restoration I was told about some features for the optional extras and SE model (like mine) for the UK market cars. My notes mention the twin tone horns (but no mention of the fixing brackets differences), single piece leather to the seats (as opposed to cheaper small rib pieces), air con, chrome trim to outside door windows. I have seen these twisted brackets on a few other 1940’s MK IV cars at classic car shows in the uk, but not sure why they are twisted as the single screw fixing allows you to rotate the horns. They do look a bit more fancy than a straight bracket, but only if you look closely!!
Just looked at the video of this car. Yes it has the same two thin bar mounts as David’s horn photos above. Not sure about the tapered tee piece. Perhaps they were modified to suit.
Ha, ha.
I saved the parts, though I don’t recall anyone thinking these horns were original equipment on a Jan '38 car. But a PO liked them and I like them.
I wonder if there was a SPC published in '38-'40? All I have is the post-war SPC, and it only lists these WT29 horns.
As I recall, Ed was rather insistent that I should look for some HF728 horns. I did, but didn’t find anything at a good price.
Looking good.
Thanks to Ian Mullins I just received two pairs of the mounts cast in brass. Just need to machine the holes and clean up the castings and one pair should be ready to go on my car.
Rob. Since you provided the excellent detailed drawing I utilised would you like the other pair?
You could hang them on the tree next Xmas or re-gift them
