The Healey Technical Archives

This page was last updated on $Date: 1999/01/07 01:55:35 $.

Rebuilding HF.1748 Horns

By Bob Haskell

These are the notes I've made from taking a few HF.1748 horns apart. This particular horn is found on the Austin-Healey 100, 100-6 and 3000s up til the introduction of the BJ-7.

I'm not an expert, so please don't take what I write as gospel. Refurbished horns are available for $100 each and that's probably not a bad price if your horns are in rough shape. I haven't seen anything published about the horns beyond adjustmenting them and I was curious about what was inside them. As the horns are assembled with screws and nuts, it would seem that I should be able to take several apart and get at least one back together....

As I try to get a horn back together I'll update this article. Hopefully, I be able to wake the neighbors!

Identifying

A pair of horns are installed, one low tone and one high tone. On the 100, the horns are installed on the front crossmember in front of the radiator. For the 100-6 and 3000 roadster, the horns were attached to the cowl and cross-brace assembly just to the outside of the heater and fresh air intakes.

Each horn has an id tag installed on the back side the horn using the same cheese head screws for attaching the mounting bracket. Information on the tag include the model number, the voltage, and the month and year of manufacturing.

Front Side Back Side
front view of horn back view of horn

The diaphram of the low tone horn is painted gloss black and may have one vee notch on the outer edge. The diaphram of the high tone horn is the same metallic biege as the rest of the horn and may have two vee notches on the outer edge.

Disassembly

  1. Remove the brass nut in the center of the horn on the front side using a 3/8" withworth wrench.
  2. Using a 9/32" wrench, just break loose the nut you uncovered by removing the brass nut.
  3. Remove the aluminum disk (the mass). This disk can be really pitted.
  4. Using a 9/16" wrench, loosen the thin nut that was under the disk.

    Note: the size of the threads found in the brass nut, the thin nut and the threaded tube appears to be 13/32"-40 tpi. A pretty odd size.

  5. Turn the horn over and remove the two nuts holding the id tag and mounting bracket to the horn. There should be a lock washer and a flat washer between each nut and the id tag. Remove the washers, the id tag and the bracket. There are two spacers between the flat springs of the bracket that may drop out. Twist/tap the two cheese head screws out.
  6. Inspect the mounting bracket. It is made with two flat springs sandwiched between steel bars. The two flat springs are made from spring steel and are susceptable to cracking along the bars and/or pitting.
  7. Flip the horn back to the front side and remove the four cheese head screws that hold the ring to the horn body. This is easier said than done. Some penatrating oil and/or heat may be required here. The problem is glavanic corrossion - steel screws threaded into the aluminum body. New cheese head screws are available (I got mine from Metric & Multistandard Components Corporation) and Recoil makes the thread inserts if the aluminum threads get thrashed. The cheese head screws are British Standard Fine (BSF) thread - 1/4"-26.
  8. Carefully remove the outer ring. The ring is quite frequently cracked through one of the bolt holes or circumfrentially in the corner.
  9. Pry the diaphram from the body. There should be a gasket on each side of the diaphram. Salvage what you can to use for a pattern for new gaskets.

The inside of the horn should look something like this:

view of the horn wiring

Internal Wiring Diagram

The electrical components consist of a electric magnet, a set of points, and a capacitor. The coil for the electric magnet is made of 460 inches of 22 B&S gage ( 0.0254 inch dia. ) enameled magnet wire. The capacitor has a value of 0.1 microfarrads and is wired across the points to reduce arcing when the points are opened.

Drawings

Assembly

The ring may have the word LUCAS cast in. LUCAS should be located 180 degrees from the mounting bracket.

Painting

Adjusting

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