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Luigi bmwR68
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ITALY
Joined: 05/22/2008
Posts: 47

Ciao a tutti.
I've find one Bosch D2 A LS 25 (about 1935 - 1937)
The arrow indicates counterclockwise...
What do you think? Can I use it for my BMW R12? ..or It rotates in the wrong side?
Luigi

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Ian R11
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VBMWMO #8148
Yorkshire uk
Joined: 10/22/2011
Posts: 204
Mag dyno

I was thought someone with R12 would answer this, but it is the essentially the same as my R11 S5 with chain driven cam and mag/dyno. From the front it should rotate clockwise so no good for R12.

Luigi bmwR68
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ITALY
Joined: 05/22/2008
Posts: 47
Bosch D2A LS25

Many tanks Ian
yes, I've the same magneto on my R11 Serie II too,
but I didn't know if is right for R12.
Luigi

(I can't uploaded more pictures, I've exceed my disck quota..)

Peter D. Nettesheim
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VBMWMO #5744
Joined: 01/27/2006
Posts: 128
Magneto

Hello Luigi,
The magneto case you have can be used for an R12, but there are some things you need to know. First, with these 80 year old machines there is no telling how many times over the last 80 years the magneto may have been rebuilt. The plate with the arrow on the front of the magneto is easily removed and replaced with one having an arrow that may point left insted of right depending on the rebuilders reconstruction of the unit. The unit can be rebuilt for either purpose, left or right. There is no difference in cost associated with this. As a matter of fact this same magneto is used even for single cylinder bikes with one ignition port just plugged.
Your magneto will need a rebuild and magneto recharge. How do I know this? I can tell from just one of the pictures in your post. You see, you have removed the magnets from the housing and just doing that immediately reduces the magnet strength by 5-10%. When the unit is rebuilt it is taken as a complete fully assembled unit and put in a special machine that re-magnetizes the field. From that point the magnets should not be removed. A Magneto recharged in this way will give strong quick starts and smooth engine operation. Without going into too much detail, the different direction is mostly associated with the generator portion, not the magneto.

Peter Nettesheim

spider rust
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VBMWMO #8708
Joined: 04/07/2014
Posts: 105
Magnet re magnetising

The Magnets can be removed and re-magnetised. I have done this with my Bosch Mag/Dyno.
But keep them apart until installation.

spider rust
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VBMWMO #8708
Joined: 04/07/2014
Posts: 105
BMW R12 Bosch Magneto/Dynamo Regulator conversion

My unit is fitted with a new electronic regulator, I need to sort out the wiring to connect it to the restored Magneto.
I did take notes and images showing where ea wire went but my horizontal filing system has broken down and the files misplaced, actually lost! Can anyone give me a diagram to connect them to?
Thankyou.
Nigel

Darryl.Richman
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VBMWMO #6285
Surf City, USA (Santa Cruz, CA)
Joined: 10/27/2006
Posts: 2185
Wiring up an electronic voltage regulator

I went out to the garage and took a couple photos of my spare D2B magneto with its electronic VR. Note that none of the wires you see in the photo are original to the magneto, and the colors don't correspond to anything used in the time period.

You can see:

  • The two white/green wires are the wires from the field coils. They go to the terminals on either side of the VR.
  • In each case, there is a second terminal that is connected to the field coil terminals. On one side, the blue wire goes to the D+ terminal on the VR.
  • On the other side, the yellow wire goes to the DF terminal on the VR.
  • The red wire comes in from the cap that covers the VR. In the cap, there are two terminals, 30 and 51. 30 is the Battery positive and 51 goes to the headlight shell. They are joined in the cap, and the red wire goes to B+ at the VR.
  • The white wire at the Masse terminal on the VR is for ground. It is screwed to the magneto body, underneath the VR.
  • Terminal 61 on the VR, which is unused, could power a generator light if your bike has one.
  • If you look at the two terminals where the white/green and blue wires are, you can see that there is another wire that comes up through the board and is clamped by the same terminal as the blue wire. This wire goes to one of the two carbon brushes located under the rectangular covers on the mag body. The other brush is grounded.
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