This Shay lawn mower with its Villiers engine, that was destined for the local tip, had last run around 20 years ago when it was put at the back of the shed and forgotten. I was asked if it was of any use to me.... well, it has a classic engine from the 50s/60's from a very famous maker, how could I resist.
Once it was home I gave it a good clean up regapped the points and spark plug, put some fresh fuel in and gave it a tug..... nothing, in fact I could only just get a jolt from the coil but no discernible spark.

An evenings searching on the web gave me all I needed to know, these engines usually die due to the copper wire in the coil rotting and turning into green copper oxide.and the spark getting weaker and weaker until it no longer fires.

Step 1: Search for a New One

For somewhere around $75 (£50) I could get a "new old stock" coil..... so that would be 50 years old then.
Or I could get a brand new coil that will need to be modified.

Step 2: Rebuild or Remake?

I had a close look at the existing coil and realised that the laminations go right through the coil so the design is completely wrong for modifying. I decided instead to start from scratch. The first thing was materials, an old grey iron slideway would give me the bits I required.

The required part was soon sawn off ready for machining.

Step 3: Iron End Pieces

The iron block was machined to the required width.
A brass template the correct shape and size of the end pieces was cut and the holes drilled and tapped in the iron block.

Step 4: Cut the Curves

The curved parts were roughed out with a hack saw then milled the lumps off.
The final shape was done by hand with a file.

Step 5: Jig

At this stage I had ordered the replacement coil $30 (£20) but it had not arrived so I built a jig that was a tight fit for the old coil so that I had some reference dimensions. The iron block is finished but for the holes to insert the coil armature and separating (marked with Yellow lines) before final assembly.

Step 6: The New Coil

The coil was, as expected 2 mm longer than the existing one so I had to modify the jig and, to get it to fit the engine, 5 minutes with a scraper and a file had it fitting into the flywheel housing.

Step 7: Final Assembly

The iron end pieces (still in one piece) were set up in the mill and the holes were drilled to insert ends of the armature.
End pieces were than sawn apart ready for trial assembly.

Step 8: Fitting and Testing

Once the fit was adjusted I mixed up some epoxy with iron dust (from machining) and stuck it together. Ground was checked to both coils with a meter and it was ready to go, here it is alongside the old one.

Time to fit, connect and test it on the motor! I put the flywheel on, spun without any scraping noises, and a spark was seen at the end of the HT lead. I then fully reassembled the engine.

I put a bit of fuel in the tank, tickled the carb and closed the choke.

First pull a couple of fires so off with the choke.

Next pull and it was running! As there is no exhaust fitted I turned it off after a few seconds but I think I can safely say that my efforts have been amply rewarded.