Early Bosch Horn Restoration

[911T1969]

Just finished a restoration of a early Bosch horn:
Before...


After...


Anyone interested in a step by step tutorial?

[donporfi]

Please post the tutorial.
Thanks

[911T1969]

I'm in the middle of refurbishing another one, so this post will take a while. This bug to figure out how to refinish one started with mine, a none-working horn from my '69 911. I didn't want to just toss it and get some new horn off Ebay. I did go to Ebay though, and I found 2 early Bosch horns. They couldn't guarantee that the horns worked, but for $10 for both I couldn't resist. When I got them, I immediately found that they were exactly like mine, with the same model numbers stamped on the back. Well almost exactly... the terminals had set screws instead of blade terminals. Close enough. Maybe from an old Mercedes? They worked but sounded sickly.
The hardest part in doing this restoration is getting the 6 aluminum rivets. You can't find these at a hardware store. I got mine at Grainger, a tool/hardware supply store. They had to order these which took about 4 days. Maybe another place to look would be McMaster-Carr. What you need to order are 5/32" by 1/4" aluminum flat head solid rivets. Grainger item # 6LA30. A package of 250 rivets cost be a bundle...$7.50! For those of you outside the US maybe you can find metric rivets. It would be easier if the whole world was metric, but don't get me started....

[Rich Lambert]

Looking forward to this one. I've got a couple of '66 horns I'd like to restore.

[Sunroof]

Thats it? Just rivots? How about the internal diaphram and set screws?

I have four of those horns lying around now because my original horns stopped blowin and the two used (Ebay) units I bought were not much better. I took a major step as a purist and installed Hella horns that sound more like a 1983 Cadillac Eldorado is coming then a classic autobahn beeper!!

Actually, I feel abit safer with a louder horn as SUV's and mini-vans, hummers and Land Rovers tower over me and once in awhile cut me off because I am too low for their mirror sighting.

Bob
73.5T

[911T1969]

Everyone be patient with me. This thread will be in installments. I'm restoring one now. In the next post we will crack one open and take a look at all the parts...

[911T1969]

If your horn is working, but not in the best shape cosmetically, its best to open it up anyway. To properly paint it, I feel you should take it apart. To open it you will need to drill out the solid rivets from the back side. I used a new, sharp bit that is slightly larger than the rivet. It helps to have a drill press. Mine is a $40 special. If you have steady hands, a hand drill could work also. Drill slowly, checking it often. You don't want to drill into the horn body.

Once all 6 rivets have been drilled out you will punch out the remains of the rivets.
Drill a hole in a piece of scrap wood slightly larger than the rivet head and about a 1/4 inch deep. Place each rivet head over the hole in the board, and from the back side knock it out with a punch and hammer.

Lets now look at all the parts:

You will have the diaphragm, retaining ring, and the horn body. Bolted to the diaphragm is a resonating plate with various spacing washers. More to follow....

[911T1969]

Now, let's see how it works. Take a look at the photo below:

The horn is just an electromagnet with a set of points. A rectangular plate is bolted to the inside of the diaphragm.

When a current is applied to the horn, the electromagnet pulls this plate down, moving the diaphragm down. On the way down the plate hits the set of points, opening the circuit to the electromagnet. With the electromagnet now off, the plate/diaphragm now moves back up. The points close, closing the circuit, and the electromagnet is re-energized, pulling the plate/diaphragm back down...over and over and over. Where do the horns fail? The points get the most abuse. The coil is not that fragile. It would take a big current surge to fry it. You can use a multimeter and check for continuity in the electromagnet coil. Check for continuity across the points. Also, corrosion can form on the inside terminals. It seems moisture can penetrate the horn and rust will form on the inside. The paper gasket between the diaphragm and the horn body is not very durable. Electrical arcing between the points along with moisture will cause them to oxidize. I pulled a piece of doubled- over 1000 grit sandpaper between the points a few times to remove any oxidation. If the points are pitted and fried, you will have to cannibalize one from another horn, since no replacement parts are available. At this point you can scrape off the paper gasket from the body and diaphragm.

Next we will take a look at the seals in the horn. More later...

[SP2]

Excellent!

[sc_rufctr]

Excellent x2...

It's interesting to see how the horn actually works. I always though there was a small fan in there that blew through something to make the noise... Learn something new everyday here.

[911T1969]

Before I continue, I just want to clarify something with the drilling of the rivets. I didn't drill all the way through the rivet. I didn't want to risk enlarging the rivet hole. I just drilled away the portion of the rivet sticking up on the backside so the rest could be punched out.

Now let's look at the seals. The biggest one of course is the paper one that was just scraped away from the rim. It's also the one that failed in my horn. I couldn't find a good replacement. I did find a fiber/rubber material at Auto Zone, but it was 1/32 inch thick - too thick. If you use this material it will raise the diaphragm, and more importantly it will raise the stricker plate that opens the points. In the end I used a silicone sealant. It was carefully applied around the rim and around each rivet hole. I didn't want it to ooze out onto my new paint job!

The next set of seals are between the terminal block and the horn body. To remove the block, look inside and unscrew the two 4mm nuts holding down the coil wire and the points bracket. Remove the nuts and washers. Pull the terminal block off from the back side. You can also slip out the fiberboard insulator on the inside. My insulator board was OK so I reused it. New material can be found at Radio Shack. Where the block mounted to the horn you will see to rubber washers. Mine were in bad shape so I made new ones. Get two rubber grommets 11/32OD X 1/8ID from the hardware store. Slip one rubber grommet over a small bolt; I used a small screwdriver.


Now with a new, sharp single edge razor blade, slice off one edge of the grommet.

Your grommet should look like this. Discard the portion on the left and keep your new seal on the right. Now make a second seal the same way:



I bought 2 new terminal bolts, since mine were rusty. I used 4mm x 16 bolts with new washers and nuts. When you reassemble everything it will look like this:



Note that the smaller diameter portion of your new terminal seals will face the holes of the horn body when you reassemble the horn. This will prevent the terminal block from shifting and causing an electrical short.

The next horn seal is on points adjustment screw. This is the small screw head you see on the back side. This screw adjusts the height of the points and to a certain extent the pitch of the horn. Unscrew this screw. Be careful. Note the small spring under compression. You don't want to loose this. The points bracket will now be loose. Under the spring is a concave washer. Note the concave washer faces the back of the horn. Under this metal washer is a rubber washer. Mine was in great shape, so I'll reuse it. I tried to hunt down a replacement for this seal, in case yours was in bad shape. I couldn't find a good one. If you need a new rubber washer, I would try to find a metric one, because the SAE ones just would not fit properly. My set screw was rusty, but I kept it. It has a special head that you will not be able to find elsewhere.


The last seal is between the rectangular points striker plate and the diaphragm. Loosen the nut on the other side of the diaphragm and take everything apart. Note the order of all the washers.


You will see under the plate are two brass washers, a thick aluminum washer, a paper insulator and the seal. the seal is not shown in this photo though:

I replicated this seal by carefully slicing a 7/16OD X 3/16ID grommet. That's it for all the seals. I just improvised when replacing the bad ones. You might find some better solutions.

Now you have a horn that is completely disassembled. It's your job now to get it all back together. See ya!
Just kidding. Together, we will step by step reassemble this mess. At this point I threw everything that was rusting into a bucket of Evapo-rust. I love this stuff. I used it to clean out my motorcycle tank, and it removed every bit of the rust. It won't damage paint. and is environmentally safe. Just Google it. At this point you will be preparing everything for painting. After leaving everything in Evapo-rust overnight, I washed and dried everything. The next step is to use a paint stipper and remove traces of the old paint. Be very careful removing paint from the horn body. The parts still inside cannot be remove easily and will be damaged by the paint stripper! You might consider wet sanding the horn body instead. Your choice. I won't be posting for a few days, but we will continue soon. More later...

[Chuck Jones]

I wish I had read this earlier....I ended up tossing my original horns and buying the Griot's Italian Air horns....and although I like the 135 db rating of the sound....and they sound "european", the fact remains I tossed an original part and replaced it with an aftrermarket part.

[67targa]

Got my Saturday planned thanks for the detail!

[mjw]

Thanks for this great post Craig!

I've had no low tone (335 Hz Bosch) on my 1973 911T over the last couple of years. I've had no luck locating a replacement, everyone is selling the version with the integrated trumpets these days. Well darn it, that's not what came on my car back in 1973.

Upon reading this post I decided to drill out the six rivets and take a look inside. Once inside I grabbed my trusty multimeter and started checking for continuity. Sure enough everything measured fine except the points. I used some doubled over 1000 grit sand paper (per Craig's description) and low and behold got my continuity back.

I put everything back together and circuit went open again! All that was required was an CW adjustment of the Phillips screw on the back (it is simply a points gap adjustment); I went untill I got continuity across the main terminals ... and then went another 1/8th turn for good measure.

One other note, I used (6) #6-32 panhead screws and locknuts for my reassembly, rather than re-rivet, just in case I need to get back in there for service again.

One less mystery in this 36 year old machine, I love it!!

Cheers,

Marty

[jrsully]

Anybody use this resto method on a 71 red bosch trumpet horn.?

[copbug]

Craig, I can't find the rest of this article -- including the re-assembly portion. Did it get finished? I have some dissected horns awaiting advice.