Restoring a Rotating Armature Magneto

Table of Contents
(Note: I added this Table of Contents after uploading the final installment of this thread. Clicking on an underlined link will open a new window with the individual post)

Table of Contents, Background, Introduction

Initial Inspection and Initial Tests

Disassembly and Detailed Inspection

Disassembly and Detailed Inspection (Continued)

Disassembly and Detailed Inspection (Continued)

Repairing Damage Done to Armature by Previous Restorer

A Sidebar About Screw Threads

Repairing Damage Done to Armature by Previous Restorer (Continued)

Preparing the End Cap
-- Removing the Bad Condenser
-- Testing the Armature's End Cap

Tests and Repairs of the Electrical Components
-- The Slip ring
-- The Coil
-- The Condenser
-- Sidebar About Replacement Magneto Condensers

Tests and Repairs of the Electrical Components (Continued)
-- Reassembling the Armature
-- Testing the Reassembled Armature
-- Brushes

Tests and Repairs of the Electrical Components (Continued)
-- Brush Spring Pressure
-- Contact Breaker Points

Mechanical Components
-- The Bearings

Mechanical Components (Continued)
-- The Cam

Reassembly, Remagnetizing, and Extended Stress Tests
-- End Float
-- Contact Spring Pressure
-- Magnetizing
-- Sidebar about Magneto "Chargers"

Reassembly, Remagnetizing, and Extended Stress Tests (Continued)
-- Strobotac

Reassembly, Remagnetizing, and Extended Stress Tests (Continued)
-- Long-Term Tester
-- Elevated Temperature Test

Final Tests
-- Low Speed Test
-- Distributor Tester
-- Disassembly, Inspection, Remagnetization, and Reassembly
-- Timing a Magneto Using an Inductance Meter
-- Performance of the Magneto on the Road

-- The Major Problem Areas in Restored Magnetos
-- Tools
-- Magneto Repairers
-- Armature Winders
-- How Much Should it Cost to Have Your Magneto Rebuilt?
-- Cosmetics
-- Final Comments

Appendix I: Post-WWII Magneto Condensers

Appendix II: Replacement Condensers for Post-WWII Magnetos

Appendix III: Anatomy of a Post-WWII Lucas K2F Magneto

Appendix IV: Diagnosing a Failed Aftermarket Slip Ring

Appendix V: An Alternative Replacement Condenser

Appendix VI: Adapter to Attach Points Plate to Condenser

Appendix VII: Rebuilding a Lucas KNC1 Competition Magneto

Appendix VIII: Rebuilding a Lucas Magdyno

This "bike project" will be different than most on this site, in that it will be about just one component. However, the magneto is a component that is of interest to owners of a variety of classic American, British, and Continental machines.

In addition to Harley-Davidson and Indian (and others), Bosch magnetos also were used on a number of very early British bikes as well as ones made on the Continent. Also, all Lucas and BTH rotating armature magnetos are direct copies of the Bosch design, which was appropriated by the Allies at the start of WWI. For these reasons, my restoration of a c1920 Bosch ZEV might be of interest to a variety of motorcyclists. Because of this, although the "original" will be on BritBike Forum (, I have given permission for it to be cross-posted on and However, those are the only locations that have permission to post copies of this material.

While I won't be able to reply to individual emails, people who are reading this on other than BritBike Forum can send questions or comments to [email protected] and I will address them in future posts if they are of sufficiently broad interest. To save people time, no, I am unable to recommend someone to restore your magneto. Sorry.

Recently a good friend purchased a 1923 Harley-Davidson Model F twin to use for an event. He shipped the bike to someone to rebuild for him but, despite it having a freshly rebuilt Bosch ZEV magneto, he asked me to check it out while the engine was being rebuilt. Unfortunately, the magneto was non-functional when it arrived at my house even though it appeared to have had 0 hours on it since it had been "professionally rebuilt."

Although I finished rebuilding it a few weeks ago, only now am I finding the time to start converting my notes to prose. I'll post this in installments as I complete each segment, and will try to write it as if it were a "real time" narrative, but it probably will end up in both present and past tense.

Because my friend had specific plans for his motorcycle, I was working to a hard deadline to get this magneto fixed, so I didn't always interrupt my work flow to photograph every step. However, so there wouldn't be gaps in the visuals, I later made additional photographs to reconstruct the missing steps. I've noted these where ever they occur, such as the following of a single-cylinder Bosch ZE1.

[Linked Image]

[Linked Image]

The ZE1 shares the same construction and many parts with the V-twin ZEV, so the magneto I rebuilt looks almost identical to it.

Although I try to work only on my own magnetos, I've disassembled about a dozen "professionally restored" magnetos over the past fifteen years. Without exception, what I found inside each of these failed magnetos in no way deserved the term "professional." I'm not going to name names, but two of those failed magnetos were rebuilt by people who many riders of classic British motorcycles would recognize (I don't know/remember who restored the others). However, it's important to add that I've never looked inside a restored one that was functioning properly, so what I've observed cannot be taken as representative of the work of all magneto restorers.

My goal in restoring any magneto, including this one, is simply to return it to the condition it had when it left the factory. No better and no worse. In some cases improvements are required because original components or materials are not available, or there is some intrinsic defect that has to be addressed. For example, I use a modern resin on the coil rather than whatever old resin was used 50+ years ago, because those old resins had several undesirable characteristics.

The steps I will describe in subsequent posts are the ones my experience tells me are necessary to return a magneto to its original performance. To do less would be to produce a magneto that is less reliable, has a shorter life, and/or produces a lower output. As I think will be clear from my subsequent posts in this thread, the magneto repair and testing equipment I routinely use to accomplish this are well beyond those employed by people who do this for a living. Whether someone could find customers willing to pay what it would cost to restore a magneto as I will describe is not an issue, because I am my only customer.

I'm guessing that this magneto restoration tale will take a dozen installments to tell, not counting responses to any questions, and it might take two months to get to the end. So, please be patient.