The correct size magneto?

[JV Puleo]

This page from the catalog of the Bi-Motor Equipment Company catalog of 1911 lists a number of different magnetos. It is evidence it its own right that these were items that people regularly changed during the working life of an early car. What interests me is that the horsepower of the engine is directly related to the choice of magneto. This is a subject I have never seen addressed. If important (and they obviously thought it was at the time) you shouldn't put a DU4 on a car with an engine larger than 20 HP. Nearly all mid-sized brass cars were in the 30HP range so I have to wonder how many are running on this very once popular unit. I'm also curious as to what made a difference between the smaller and larger mags?

 

This catalog is dated 1911. Does anyone know of a similar listing dated around 1915-16? There are additional, readily available Bosch mags like the ZU4 that aren't mentioned here because they weren't introduced until slightly later but I'd like to know how they fit into scheme of things.

 

 

[EDIT] Further search has turned up a little more information. The Z series was introduced in 1911 and manufactured up through the early 30s. The Z4 was for engines with a 5" bore or larger, The ZU4 for engines up to 4-1/4" bore and the ZF4 for engines up to a 3-1/2" bore. The original brochure says nothing about engines with a bore between 4-1/4" and 5" so perhaps either of these would work or maybe there was a miss-print of some sort. I suspect the ZU4 worked best in a range of 3-1/2" to 5".

Edited September 20, 2017 by JV Puleo
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[Layden B]

Well excuse me for trying to be helpful!

Edited September 20, 2017 by Layden B (see edit history)

[JV Puleo]

Why would you presume that I have no idea that it is a complicated subject? I am quite sure you don't know me, or what I do or what I know. I'm quite aware of how many magnetos they made and of the multiple types. What I am curious about is why and what part of this story I am missing. As I said, I've never seen a discussion of the subject, nor have I ever read "the xxx magneto was for engines of thus-and-such a size." You've given a reasonable explanation of that but you aren't the only person here who has some specialized knowledge.

[Craig Gillingham]

There is a bit of difference between a ZU and ZF magneto, the ZF is physically smaller and has a lower shaft height than a ZU as well as a different bolt mounting pattern. I thought that the smaller magnetos were used because they suited the smaller engines more appropriately, for example, no point having a DU1 magneto on a 1/2HP engine when a ZA1 will do the same thing. I know Bosch really set the standardisation of magnetos, and most other companies adopted those standards, which is why a lot of magnetos are interchangeable.  There are always exceptions to the rule, and there are a few very specialised magnetos made to fit specific applications, however, generally, there are about 1/2 dozen 'standard' magneto mounting types. I've got a good table I can post if you're interested, that gives the specifications of each type

[JV Puleo]

Yes, please do.

[Mark Shaw]

Yes, please post the table.   This would be very helpful for many on the forum.

 

Also, please advise if anyone makes the mounting plates to adapt the DU4 mags to replace Remy in GM cars & trucks.

[JV Puleo]

What do they look like? That doesn't sound like a difficult job. I might be able to hep.

[JV Puleo]

Here is what PM Heldt has to say on the shaft height. He doesn't mention the size of the mag at all but does say that practically all
"foreign" magnetos have a shaft height of 45mm. He was writing in 1911, before the Bosch factories in the US were seized by the Alien Property Custodian during WWI and sold (thus becoming the American Bosch Company). The wording is exactly the same in the 1916 edition of his book. The DU4 measurement is 45mm. There was a time when I had a shelf full of mags... if I still had them I could compare the shaft height but now I'm curious as to what differences there are. Notice that in the catalog shown above, no mention of the shaft height is made. This would imply that they were largely interchangeable.

 

 

I've just found another reference, a British engineering manual of the time, that gives the shaft height for the ZU4 as also being 45mm.

Edited September 21, 2017 by JV Puleo
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[Craig Gillingham]

Sorry for the late reply, attached are the scans out of a British magneto book -I can't remember the title-  with the details of the then current magneto dimensions. I've used this info a lot, and by comparing the magnetos in Fig. 197 & 198, with the different styles of shafts shown in Fig. 199, 200 & 201, and cross-referencing Table IX. A Bosch DU magneto and a ZU magneto have the same dimensions, so 45mm shaft height, 50mm square bolt pattern, 3/8"BSW mounting threads, the 'H' offset is 80mm, etc. A ZR will be similar, except they are higher than either a DU or ZR magneto. I remember the book was first published just after WW1 and the only mention of Bosch magnetos is on that first page, and the only place Germany is mentioned is in that table. It mentions just about every other magneto made at the time, just not Bosch. And where it says 'German nomenclature', that's just the Bosch magneto model. There are some exceptions to the rule, as there are some specialised magnetos made for particular applications, however, generally, most of the base-mounted magnetos fall into one of these categories

 

Going back to what Layden said about larger magnetos having bigger armatures, stronger or more magnets, etc. which makes sense, as a larger engine is going to take more effort or be slower to crank over so that would be beneficial in starting that particular engine. It's not something I'd thought of before, but it makes sense. Early on, Bosch placed a lot of emphasis on their magnets, they had two or three side-by-side, then they also had magnets on top of magnets, etc.

 

 

Edited September 25, 2017 by Craig Gillingham (see edit history)