Published 

Section 

Change

R1 

2021-11-02 

Reassembly – Part 2 

1st two pictures corrected

Original 

2021-10-08 

Original publication

Thierry Van Nuffelen 

Italian publication covering Bosch horn Low / Hi sound

Gordon Raymond / Terry Rushbrook 

Alternative material for gasket fabrication

Multiple internet sources 

Method of replacing original capacitor

Title 

Pub. 

Date 

Coverage 

Source

Giulietta Workshop Manual 

637 

1959/06 

750 

Author

Giulietta 1300 Spare Parts Catalog 

776 

1961/11 

750 & 101-1300 

Author

2000 Spare Parts Catalog (Table 78) 

728 

1960/11 

Cast Iron 2000 

Author

Bosch horn article 

N/A 

1960s 

Bosch horn internals 

Thierry Van Nuffelen

Horn catalog page 

N/A 

? 

Bosch & Hella horns 

Internet

(750) Workshop Manual, publication 637, 1959/06 

While this is written for the Marelli horn, the concepts also  apply to Bosch. 

The main components of the Marelli horn are: • The diaphragm which is caused to vibrate by the  electromagnet and which emits the sound; 

• The electromagnet designed to cause the diaphragm to  vibrate. 

How it works: 

• The diagram in figure (at right) clearly demonstrates the  operation of the horn; by closing the circuit in practice  effected by pressing on the horn ring on the steering  wheel) a magnetic field is created by means of the  electromagnet which then attracts the armature 4  connected to the diaphragm 10. 

• By moving towards the electromagnet, the armature  operated circuit breaker 6 in such a way as to open the  electric circuit; the magnetic effect then fails and the  armature returns to the initial position. 

• When the armature thus returns to its original position,  the circuit breaker contacts close and restore current to  the circuit. The cycle is then repeated. 

• The very frequent repetition of the opening and closing  of the electromagnet circuit cause the diaphragm to  vibrate and the horn sounds. The tone of the horn  depends on the frequency of the oscillations.

The (750) Workshop Manual illustration (above) indicates the +12 volts connected to the electromagnet Coil  and the ground is connected to the Stationary-Point. 

A few Bosch samples supported the concept (see photo). 

The author is not convinced it matters, but uses that  convention throughout this document.

Low / Hi Sound 

Mark on Bakelite 

Alfa # 

Grill Mark “A” 

Grill Mark “B”

Low 

12/1 

101.03.65.126.00

294 323 

295 342 

290

295 

….. nothing

Hi 

12/5 

101.03.65.127.00

296 

298 

362

296 

….. nothing

Tone 

Alfa # 

Bosch # 

Bakelite Emboss 

Bosch 10 Digit # 

Bosch  

Description

English  

Translation

Low 

101.03.65.126.00 

HO/FDG 12/1 

0 320 106 001 

Tief (290 Hz) 

Deep

Hi 

101.03.65.127.00 

HO/FDG 12/5 

0 320 106 004 

Mittel (362 Hz) 

Medium

(Cast Iron) 2000 Spare Parts Catalog 

Old Bosch Catalog

1365.76.448 

HO/FSA 

12/9 

12/9 

“Tief” (deep)

1365.76.449 

HO/FSA 

12/10 

12/10 

“Mittel” (medium)

One pair of horns had diaphragm/armature  plates with silk screened / white paint markings

(After paint removed):

Low/Hi 

Bakelite Mark 

Thickness 

Diaphragm/Armature

Height 

Diaphragm / Armature

Low 

12/1

.511 

.60

.503 

.62

.522 

.68

.512 

.54

.505 

.68

Hi 

12/5

.603 

1.50

.630 

.91

.614 

1.65

The Bosch catalog page shown in Low/Hi Tone section contains an illustration showing the position (ἀ) of the mounting bracket relative to the electrical  connection terminals. 

The catalog then lists a different position (ἀ) for  each vehicle / horn combination; therefore, do not  just match the illustration during reassembly. A  photo of an unmolested Alfa horn is included in  Reassembly Part 3 & Adjustment section. 

Translation of word on illustration: 

“Sketch for the position of the leaf spring holder”

Coil broken wire repair 

22-gauge solid copper “hook up wire” (Radio Shack or similar)

Grommet replacement for Bakelite  harness connector

Options: 

• 3M strip chalk 

• Stepped grommet (hardware store)

Bakelite broken 

• JB Weld Plastic Bonder (2-part epoxy for plastic) 

• Dupli-Color PAE101 Semi-glass black acrylic enamel

0.22 microfarad “film” capacitor 

eBay – January 2021, 12 Panasonic brand for $10.07

Nylon spacer ½” x 1” for #10 screw 

Hardware store – less than $1

(2) Copper 3/16” x .375” rivets 

McMaster-Car 97440A355 – January 2021, 50 for $10.78 + shipping

• While probably only the  

“Diaphragm/armature plate” has  a specific orientation; play it  safe and mark each layer’s  

original position during  

disassembly. 

• The inner holes in “megaphone”  line up with inner holes in  

“Outer cover” 

• Remove six screws on the rim of  the horn. 

• Carefully separate the layers, so  that the two thin (0.2 mm) paper  or plastic gaskets are preserved.  Some horns have multiple  

gaskets between Main Cup/Coils  and Diaphragm/Armature.

Now look at the side of the  diaphragm/armature plate that  faces the electromagnet. Observe  the protrusion on the armature  (metal bar attached to the  

diaphragm/armature plate) on the  side that faces the points and  capacitor. 

You will probably see a slightly  worn or shiny spot. This is where  the bar hits the Moveable-Point to  break the electrical connection  which makes the horn work.

Hint: clearly mark the contact side  with paint

It appears that some armatures are slightly offset on the diaphragm plate for the point  contact. Record what you find: 

□ Offset 

□ Not offset

Although there are gaskets, the  seal is imperfect. Therefore, dirt  and water will eventually reach  the critical interior. 

It appears Bosch anodized the  interior of the main shell and the  armature side of the diaphragm  plate, but that was not enough to  prevent rust after water entered.  As rust forms, the vibrations turn  the rust to a talcum powder  consistency that floats onto the  electromagnet’s points. The  powder then burns onto the points;  eventually causing them to fail.

CAUTION: During the next step take care not to break the wire between capacitor and electromagnet as you complete the disassembly.

Study the diagrams so you know  what to watch for as you continue  the disassembly. 

• Remove only the screw labeled  #3 and the other pieces  

associated with the Movable Point (see below). 

• CAUTION: on some horns this  is an odd 3.5 x 0.6 screw; while  on some horns it is a common 4  x 0.7.

These numbers will be referred to  throughout this document.

Examine Bakelite harness  

connector for cracks or missing  pieces.

What you hopefully will not find:

Look for arcing at all electrical  connections. It can be caused by:  • Deteriorated insulators 

• Corrosion

This wire is covered with carbon deposits  

from arching.

Look for broken wires. 

(example in photo, this wire  should have connected with  capacitor holder.)

Connect ohm meter leads to: 

• Clip #2 

• Screw #7  

If your meter has residual internal  resistance, remember to subtract the  value to obtain actual reading. 

If the resistance is high, clean all  connections and repeat test. 

If the resistance is “infinite” (open  circuit), investigate if either wire  entering coil is broken (Footnote 1). If  that is not the case, the coil is probably  burned out and a new horn will have to  be found.

CAUTION: do this just briefly so the  coils do not overheat. 

• Using a 12-volt power source:  • Connect negative lead to Clip #2 • While holding a piece of steel above  

the electromagnet coils, briefly tough  positive lead to Screw #7 

• The steel should be strongly 

attracted to the electromagnet. 

• If the steel is only weakly attracted,  try cleaning the connections. 

• If still weak, move on to capacitor  tests. 

CAUTION: keep your fingers out of  the way … this will hit like a hammer  if it is functioning properly.

Name 

Symbol 

Conversion

picofarad 

pF 

1 pF = 0.001 nF

nanofarad 

nF 

1 nF = 0.001 µF 

1 nF = 1000 pF

microfarad 

μF 

1 µF = 0.001 mF 

1 μF = 1000 nF

millifarad 

mF 

1 mF = 0.001 F 

1 mF = 1000 μF

farad 

F 

1 F = 999.5 mF

• Two versions of this paper & foil tubular  capacitor have been observed. 

• There are matching retainer variations 

• The version without a cap is probably the older  version

Note: you must hold meter contacts  to the capacitor until the meter stops  changing its reading. It takes a  while for the capacitor to be fully  charged. 

• Black lead to wire Clip #2 

• Red lead to top of capacitor 

If not close to this rating, replace  the capacitor.

Meter Connection 

Spec. 

Expectation

First, “zero” your meter

• Set meter to highest ohm range  (for example “Rx10k”). 

• Red lead connected to “V/Ω”  on meter 

• Black lead connected to  

“COM”. 

• Connect Red to Black

0 

Adjust to 0

Charging capacitor: 

• Black lead to top of capacitor • Red lead to Clip #2 

• Hold for 20 seconds 

• Disconnect Black lead only 

WARNING: Do not touch  capacitor with hand between  steps.

Meter action: • Starts at ∞ 

• Briefly  

jumps  

toward zero 

• Then fall  

back to ∞

If this pattern  of movement  is not  

observed, the  capacitor must  be replaced.

WAIT 2 minutes

Checking if capacitor has leaked: • Return Black lead to top of  capacitor

Meter should  not move.

If meter  

moves, it has  leaked. 

• If movement  is slight it  

may still be  

useable 

• If as much as  1st time,  

replace it.

Note: this is the “Adjustment  Screw” that is used to set  

armature to Moveable-Point clearance. 

• Measure the distance the screw  protrudes above the Bakelite.  The values I have found are  typically around 3 mm. This  one was 2.5 mm. 

• Take a picture, that way you can  also count exposed threads. RECORD VALUE: 

• Height _________ 

• Tread count _____

Note this screw comes in from the  back of the horn Main Cup. 

Note: the screw head should be  covered with a wax seal. Take a  picture of the seal, so you can  duplicate for you concourse  winning projects. 

If the seal is missing it is likely  the horn has already been repaired  at least once.

Study the diagrams so you know  what to watch for as you continue  the disassembly. 

• Turn Screw #8 to remove Nut  #1 & Bakelite Nut Holder

• Remove Screw #4 

• Remove capacitor top contact • Remove Stationary-Point holder • Remove all components  

associated with Screw #8

This step is not required to repair  the horn, only to provide access  for fully painting the Main Cup. 

• Remove Screw #7 

CAUTION: don’t let exterior piece fall and break 

• Carefully pry up the interior  Bakelite piece. 

CAUTION: 2 special rubber  grommets are under Bakelite

• Now that everything you can remove is out of the way, clean the rust out of the inside of the housing as best  you can. Use a scraper, sandpaper, or whatever you are comfortable with. Be careful not to damage the  electromagnet coil or its connecting wires as you clean. You are not going to be able to get it all, but do what  you can. 

• Spray a heavy dose of electrical contact or electrical motor cleaner over the entire interior. Tip over the  housing to drain off the excess cleaner. Then let it air dry. 

• Clean with isopropyl alcohol 

• Clean with well water or distilled water (not city treated water) 

• Dry with hair dryer 

• Note: because the wires can’t be fully protected, I did not use the usual acid wash before painting.

Paint the interior without getting it on the wires.  Consider POR 15 for its ability to stabilize rust.

Disassembly of the Diaphragm/Armature Plate was not  done in previous steps. You may wish to do this for a  more complete paint job, but damaging the molded  paper (or plastic) disks (G & I) may provide enough  reason to leave it alone. 

CAUTION: 

• Horns have varying thickness shims at C & E • Horns have varying numbers for shims at point “E” RECORD VALUES: 

C: ________________ 

E: ________________ 

See Hardware List section for detailed measurements.

This type of damage can be repaired with JB Weld  Plastic Bonder 2-part epoxy and then painted. A  “mold” was created by simply wrapping cellophane tape around the part.

After curing it is a good match to the black color, but it  turns to gray if sanded. 

Painted “semi-gloss” black

Note: if you have an old “point file” in you tool box, leave it there; because it will destroy the points. 

• A hobbyist vise, for example PanaVice, is a great way to hold the point while polishing it. • Spray the points with electrical contact cleaner and then use a relatively soft brass brus to take off dirt and light  glaze. 

• Wrap a strip of 1000-grit sandpaper around a metal strip (approximately ½” wide by 1/10” thick). • Rub primarily in circular motions. Be careful to retain the flat surfaces of the points. • After a little polishing, check the results. If you can see definite burned spots that are not going to be removed  with 1000-grit, use the 800-grit paper. Work with the 800-grit until you  

can get rid of the most of the burn spot. 

• As you polish, watch that you retain an even shiny circle on the point;  

which tells you it will be flat and level. This example (right) is starting  

to have too much cut on one side. 

• You may not be able to get rid of 100% of the burn spots and still have  

sufficient point surface left. 

• Return to the 1000-git to remove sanding scratches you just inflicted on  

the point. 

• Finally polish to a flat, mirror-like surface with the 2000-grit.  

Remember; take off no more than you absolutely have to. 

• Again, clean with electrical contact cleaner

As found 

After electrical contract cleaner and  soft brash brush

After polish (small scratches, but  also hand of photographer).

Stationary-Point

, but also hand of photographer)

Movable-Point

Mistakes

Here is what happened if you get in  a hurry and use too aggressive paper

Mistake of using 320-grit 

Stopping too soon, still some  scratches and pits

CAUTION: use screwdriver to  hold the bottom connector from  moving, while wiggling the  capacitor free.

Common hardware store nylon  spacer. Shortened to recreate  original capacitor length

Add solder to end of copper rivet  to provide softer electrical contact. 

CAUTION: verify two rivets do  not touch when inserted into the  plastic tube.

Bend 0.22 microfarad capacitor  leads to keep capacitor close to  nylon support. 

Make 1st lead close to one end of  nylon support, so capacitor is out  of the way when installed.

Solder the capacitor to both rivets.

If the capacitor hold-down clamp  is of the “single hole type”, solder  a 3 mm brass screw into the hole  in the clamp to improve the  connection.

If Diaphragm plate was disassembled: 

• Reassemble according to your recorded values for the  “C” and “E” spacers. 

• Pay attention to orientation of components:

REMINDER: at least some armatures are slightly offset  on the diaphragm plate for the point contact. You were to  make a note about this during disassembly.

If Bakelite Harness Connector was removed: 

• Install good grommets (or 3M strip chalk) into holes in Main Cup. 

• Insert Screw #7 (18 mm long) through new stainless steel lock washer, coil wire hoop connector, and interior  Bakelite piece; and then screw into grommet until it just protrudes slightly outside. 

• Hold outer Bakelite piece in place and then loosely tighten Screw #7.

HINT: if the neoprene seal is not in near perfect  condition, replace it. 

Loosely install Screw # 8 with all its washers, spring,  Stationary-Point Holder, Bakelite cup, and nut. CAUTION: be sure Bakelite cup is fully seated into  the Stationary-Point Holder slot.

If Capacitor was removed: 

If reusing the original capacitor: 

• Install it in the same direction it came out. • If metal-cap type, cap is on top 

• If no-cap type, 4 indentations is on top (3  indentation on bottom) 

Otherwise, install new film-capacitor unit

Note: there is a  

bump in the  

shell (arrow in  

photo) for  

positioning the  

bottom of  

capacitor

Install the Capacitor Clamp 

• Insert Screw # 4 (20 mm long) through new stainless  steel lock washer, Capacitor Clamp, Stationary-Point Holder, and interior Bakelite piece. 

• Hold outer Bakelite piece in place and loosely tighten  Screw # 4. 

CAUTION: there is a “bump” in the shell for  positioning the bottom of Capacitor 

• Fully tighten Screw #4, while holding Capacitor in  place 

CAUTION: Be sure the top of the capacitor or its  wires do not touch the Stationary-Point holder or  main horn case. 

CAUTION: Verify the coil wire under Screw #7 is  not touching any metal parts. 

• Fully tighten Screw #7 

• Verify capacitance (see below) meets specification by  connecting meter leads to Screw #4 and the wire to  the lower capacitor contact.

Install Screw 5 and Screw 6 with new stainless steel  lock washers.

Meter with capacitance setting: 

Verify value matches specification (0.22 μF or 220 nF)

“Analog” ohm meter: 

1st rising resistance for 2 seconds as capacitor loads; then “Infinite”.

Meter Lead 1 

Meter Lead 2 

Expected Results 

Possible Problem

• The following tests are completed with the points OPEN. Since the Movable-Point has yet to be installed, this is  true. 

• To use these tests after the Movable-Point has been installed, separate the points with a piece of plastic. • Included are typical readings I have found. As necessary adjust your readings to compensate for the inherent  internal resistance of your meter. They have been adjusted for author’s Fluke 78.

#2 (Clip)

#1 

“Infinite” 

“A”

Hole for #3 

“Infinite” 

“B”

#4 

As above

#5 

As above

#6 

0.9 ohms 

“B”

#7 

0.9 ohms 

“B”

Hole for #3

#1 

“Infinite” 

“A”

#4 

0.1 ohms 

“C”

#5 

0.2 ohms 

“C”

#6 

“Infinite” 

“B”

#7 

As above

#4

#5 

0.1 ohms 

“C”

#6 

“Infinite” 

“B”

#7 

As above

#5 

#6 

As above

#7 

As above

#6 

#7 

0.1 ohms 

“C”

Possible Problems / Solutions

A 

Excess resistance 

Broken Bakelite Nut Holder or disintegrated fiber washer

B

Excess resistance 

Corrosion on screw threads

Infinite reading

Indicates a broken wire at one or more of these locations: 

• Screw #7 to electromagnet coil 

• Coil to capacitor bottom connector 

• Capacitor bottom connector to Clip #2 

• Or if you are really unlike a burned-out coil.

C 

Excess resistance 

Corrosion on screw threads

Install the “Moveable-Point” 

Loosely install the Moveable-Point and all the  associated pieces. 

HINT: rotate the point so it is beside the Fixed-Point  will make installing the screw much easier. 

CAUTION: make sure the Bakelite ring goes through  the center of both Fiber Washers.

4

• Push down and rotate Moveable-Point to perfectly  align it with the Fixed-Point. 

• Fully tighten Screw #3. 

WARNING: If you can still shift the Moveable-Point  sideways, either the Clip #2 or one of the Fiber  Washers did not completely slip over the Bakelite Ring.

Adjust Screw #8 to its original height above the  Bakelite holder, which you recorded during  disassembly. 

This will provide a good starting point, but probably  not the final position.

Verify wire clearance 

• If using replacement disk capacitor, be sure its wires  do not touch Fixed-Point Holder, Coil, or Main Cup • Be sure wire from Capacitor Holder is not contacting  

the Main Cup (as was the situation with horn in  photo). NOTE: all horns do not have the protective  sleeve over the wire. 

• Be sure the coil wire attached to Screw #7 does not  contact the Main Cup.

Meter Lead 1 

Meter Lead 2 

Expected Results 

Possible Problem

The following test is completed while pushing down on the Moveable-Points to OPEN points

#5 

#6 

“Infinite” 

Incorrect assembly  of Moveable-Point

• The following tests are completed with the points CLOSED (their normal state). 

• Included are typical readings I have found. As necessary adjust your readings to compensate for the inherent internal  resistance of your meter. They have been adjusted for author’s Fluke 78.

#2 

Footnote 2

#1 

Infinity 

“a”

#3 

0.2 ohms 

“B”

#4 

0.2 ohms 

“B”

#5 

0.2 ohms 

“B”

#6 

0.9 ohms 

“B”

#7 

0.9 ohms 

“C”

#3

#4 

0.1 ohms Footnote 3

“D”

#5 

0.2 ohms 

“D”

#6 

0.9 ohms 

“B”

#7 

0.9 ohms 

“B”

#4

#5 

0.2 ohms 

“D”

#6 

0.9 ohms 

“B”

#7 

0.9 ohms 

“D”

#5 

#6 

0.9 ohms 

“B”

#7 

0.9 ohms 

“B”

#6 

#7 

0.2 ohms 

“D”

Possible Problems / Solutions

A 

Excess resistance 

Broken Bakelite Nut Holder or disintegrated fiber washer

B

Excess resistance 

Clean points by drawing a piece of paper through the points. Verify your  polishing retained a flat contact surface.

Excess resistance

Indicates a broken wire at one or more of these locations: 

• Screw #7 to electromagnet coil 

• Coil to capacitor bottom connector 

• Capacitor bottom connector to Clip #2 

• Or if you are really unlike a burned-out coil.

C

Excess resistance

• Clean points by drawing a piece of paper through the points. Verify your  polishing retained a flat contact surface. 

• Corrosion on screw threads

D 

Excess resistance 

Corrosion on screw threads

Complete the assembly (aligning all the  screw holes) 

• Gasket 

• Armature Plate with paint mark toward  the Points and the armature parallel to  the Fixed-Point Holder. 

• Gasket 

• Megaphone where the inner holes in  “megaphone” line up with inner holes in  “Outer cover” 

• Outer cover 

• Insert and tighten all screws.

• Connect ohm meter leads to Screw #5 and Screw #6 

• If the initial reading of the meter shows: 

• resistance, then turn Screw #8 counter-clockwise until the reading is “Infinity”. What this does is raise the  Point Holder until the Armature forces the Points OPEN. Then turn Screw #8 clockwise until it shows a  resistance (Points CLOSED); plus turn it another 5 degrees. 

• “Infinity” (Points OPEN), then turn Screw #8 clockwise until meter is shows a resistance (Points CLOSED);  plus turn it another 5 degrees.

CAUTION: use ear protection! 

• Connect negative terminal of 12-volt power source to assigned horn terminal (see image below). • Briefly touch a wire from power source positive terminal to the other horn terminal. 

• Hopefully, you heard a very loud noise. If not, reverify the above step with ohm meter.

Seal the screw with a drop of hot wax.

Install bracket. Note position of mounting bracket relative to the  Bakelite harness connector on this unmolested 1964 Spider’s horns.

The 750 Workshop Manual shows the +12  volts connected to the electromagnet coil  and the ground connected to the points. 

This pattern has been observed on several  Bosch horns. 

The author is not convinced it matters, but  why not assume Alfa, Marelli, and Bosch  engineers know what they were doing. 

• Ground wire (-): to electromagnet coil  • +12-volt wire: to capacitor

Item 

Purpose 

Description 

Count 

Size 

Pitch 

Length 

Finish

1

Fix adjustment screw to point  holder 

Nut 

1 

4 

0.7 

N/A 

Black

3

Fix capacitor wire and points  to holder 

See “Moveable Point” below

4

Fix points holder to Bakelite  (harness connection)

Slotted, cheese head screw 

1 

4 

0.7 

20 

Chrome

Lock washer 

1 

4 

N/A 

N/A 

Chrome

5 & 6 

Fix harness wires to horn (@  Bakelite)

Slotted, cheese head screw 

2 

4 

0.7 

5 

Chrome

Lock washer 

1 

4 

N/A 

N/A 

Chrome

7

Fix electromagnet coil wire to  Bakelite (harness connection)

Slotted, cheese head screw 

2 

4 

0.7 

18 

Chrome

Lock washer 

1 

4 

N/A 

N/A 

Chrome

8 

ADJUSTMENT SCREW

Slotted, oval head, counter  sunk screw, with serrations  on counter sunk side to  

prevent movement from  vibrations

1 

4 

0.7 

32 

Black

NA 

Fix cover to Main Cup 

Slotted, fillister head 

6 

4 

0.7 

6 

Black

N/A 

Grommet – interior harness  wire Bakelite 

Rubber 

2 

See diagram below

Stationary-Point

Moveable-Point

4 mm Version 

3.5 mm Version

A 

Fiber insulator 

6.02 x 11.96 x 0.83 

5.96 x 12.0 x 1.0

B 

Bakelite insulator 

4.16 x 6.00 x 2.84 

3.71 x 5.96 x 3.20

E 

Fiber insulator 

Same as “A” 

Same as “A”

F 

Steel washer 

4.50 x 8.18 x 0.68 

3.75 x 8.85 x 0.47

G 

Screw 

4 x 0.7 x 6 

3.5 x 0.6 x 5

Diaphragm/Armature

Item 

Purpose 

Description 

Count 

Size 

Pitch 

Length 

Finish

A 

Fix all components to plate 

Phillips, thin cheese head,  partial threaded screw 

1 

6 

1.0 

30 

Black

C 

Shim washer Footnote 4 

Disk 

1 

6.2 x 14.7 x various 

Brass

E 

Shim washer Footnote 5 

Disk 

Various 

6.2 x 14.7 x various 

Brass

F 

Spacer 

Machined shape 

1 

See diagram 

Footnote 6

G 

Gasket 

Molded shape 

1 

See diagram 

Footnote 7

I 

Spacer 

Machined shape 

1 

6 

1.0 

N/A 

Footnote 8

J 

Gasket 

Machined shape 

1 

See diagram 

Footnote 9

K 

Nut

Nut 

• Most 10 mm wrench • Some 11 mm wrench

1 

6 

1.0 

N/A 

Zinc

“C”  

Examples found: 

• 0.58 

• … lost list of others

“E” 

Examples found: 

• 0.65 

• … lost list of others

“F” & “G”

“I” & “J Version 1 (paper) 

I” & “J” Version 2 (same as “G”)

Miscellaneous

Wiring Harness Connector 

Details of grommet at right

Grommet

Mounting Plate 

(“Leaf spring holder” in Bosch terminology) 

These measurements are from HO/FDG horn 

Item is not listed in Spare Parts for HO/FDG, but it is for  predecessor horn (HO/FSA) with a Bosch # of HOBE-4-G-3Z  (no Alfa #)

Ground Strap 

These measurements are from HO/FDG horn

Ground wire not listed in Spare Parts

Mounting Plate

N/A 

Fix mounting plate to horn 

Nut (14 mm wrench) 

1 

8 

1.25 

N//A 

Black

Lock washer 

1 

8 

N/A 

N/A 

Zinc