Some important numbers for restoring a coil are:
- The air gap between the core and the vibrator spring should be 4mm (5/32") for the Tungsten and 6.4mm (1/4") for the Platinum.
- The air gap between the point on the vibrator spring and the point on the contact spring when the vibrator spring is pushed down until it meets the core is 0.7mm (1/32").
- When powered from 6VDC the primary coil should draw between 1Amp and 2Amps, if you power the coil from 12VDC the Amperage figures are halved.
|This image shows both the coil we're about to work on, and the basic set of tools you will find handy for this job. In the picture are some 600 grit sand paper, rubbing alcohol, a pair of pliers, a dental scraper, a flat fine (jewelers) file and a razor blade. In addition I also use a shop cloth, a Dremel rotary tool, and some 320 grit sandpaper.
Note:I doubt I really have to tell you this, but notice that the coil is not connected to anything while I'm going to be working on it. These little buggers can give you a very nasty surprise if you touch one which is energised!
|The obligatory "before" shot. Note that there is a buildup of oil and grease on the top of the coil box and all the metal components, this can cause current to leak instead of following the path the designer required. This can reduce or prevent sparking. In all fairness, this coil is actually fairly clean - I've seen much worse.|
|Note that after undoing all the nuts, I've laid out the pieces in the order they came off and those parts which are next to each other when assembled are next to each other when dissasembled. Doing this will help you with any restoration project, I try to do this all the time.
You'll note that the contact spring (the brass item at the top) is in pretty good condition and is reasonably clean. In fact, normally apart from cleaning the points and contact areas I'd leave this one alone, but since I've got the Dremel out I'll clean it up.
The vibrator spring is in a fairly sad state, there has been some nasty arcing which has etched lines into the steel. This has probably affected the "springiness" of the steel which means this unit is either harder or softer than it is supposed to be. This arcing would have been caused by a build up of grease and dirt near the points and the electricity basically took the easiest path through the grease.
The points (the small round circle on the left hand end within the larger circle of the vibrator spring) are badly pitted and worn. I've heard of people going to extremes and replacing the points with new ones removed from standard automotive ignition points (I understand Volkswagen beetle points are particularly easy to get apart). I personally wont do this, my engines don't really do any work and the wear left in these points will at least outlast me. These points will be tungsten or platinum depending on the model and will be much harder than the junk you can buy now.
|This is the top of the coil box with all the parts removed. The iron core is the large bunch of "wire" sticking up towards the left of the picture. The leftmost bolt is only for tensioning the contact spring and does not carry any power. The two large posts on the right carry power to the contact spring, and the two smaller posts carry power from the vibrator spring.
Note that the box is quite dirty and greasy, you cannot see the wood. This is where the razor blade comes in. I scrape away all the dirt down to the bare wood.
|By holding down the vibrator spring you can see the gap between it and the iron core. The distance is specified above. What the books do not tell you is where to measure the gap from! I measure it from immediately below the point (the small protrusion to the left of the disc on the left end of the vibrator spring) to the top of the core. This seems to be a logical place to measure from and the coils do work later :)
You can see in this photo just how much the point is worn away. Electricity is a bit like kids they wear you down until they get where they want to be (it wont be too long before my kids are old enough to read this and hit me for saying that).
|I've used the sand paper (and the Dremel) to make everything nice and shiny again. Take care not to damage the various protrusions - they're all important! Pay particular attention to the contact spring. It is in two parts, the hard brass top layer and riveted to this with very tiny rivets is a softer copper bottom layer which carries the bulk of the current. Take care not to grind away the rivets and wreck the copper.
The points can be clearly seen in this photograph on both units. The vibrator spring point (bottom) is the small circular protrusion at the left of the larger disc to the left of the unit. The contact spring point (top) is the similar protrusion just to the right of the big hole at the left of the unit. Judging by the marks that were there before cleaning up the small copper nub to the right of the point on the contact spring was where the previously noted arcing occurred.
When you are cleaning the points themselves, the object is to make them dead level, flat, and smooth whilst removing the smallest amount of metal possible. If they are pitted you can use 600 grit wet & dry sandpaper with water as lubricant or an oil stone or a fine jewelers file. If you use the file finish off with the paper. If the points are just dirty but not pitted remove the dirt as much as possible with alcohol and the cloth, then use the sandpaper by folding it in half (so it has two rough sides) then sandwich it between the points by pressing them together and drawing the paper through.
|When reassembling the unit, put it back together as it came apart, paying particular attention to which nuts go where. On this unit all the posts were the same size and thread but I've seen ones where they differ. Screw down all the nuts hand tight, then align the points so that they touch reasonably evenly as seen here. Only then tighten down the nuts the rest of the way using pliers/spanner whatever. Don't get carried away with making these tight, the posts are just embedded in thin wood and "gunk" on the inside i.e. there is not much holding onto them.|
|All done, much cleaner than it used to be :). The brass and copper will quickly corrode. Do not put anything on them - the idea is to keep it all clean now to prevent current leakage.
Notice the small gap between the washer on the rightmost post and the metal stand off on the next right most post - if your points are ever this far apart during sparking and this area is dirty, the current will jump here in preference to the points.
|Job well done, a nice fat hot blue spark jumping the gap in the plug. You remember that I said earlier that this coil was not very dirty - before I cleaned it it was a yellow spark on a brand new plug - this would have resulted in very quick deterioration of the plug as the spark was not hot enough. This would have lead to incomplete combustion and misfires with sooty buildup on the plug. This happens because the yellow spark is so "cold" that the ignition of the combustion mixture happens later than it should and does not complete before the exhaust port opens.|