The Bench Racers Journal

[Terry Bentley]

Chapter 135

HOLY COW, there was a lot of clearance on the ignition coil. When checking air gap on the coil I was surprised to find out how much there was. I kept sliding in a thicker feeler gauge before things got tight. It took 0.019" to fill the void. OK that is nineteen thousandths, if it didnt register there at first. I dont know exactly how to write machinist talk.

Now the difference in air gap is almost 100% farther away than the goal. That is simple enough to understand. At around 0.010" thousandths air gap sure doesnt take much to change a major amount. So even changing one thousandths, would be a 10% difference. That is assuming my 8th grade math skills are up to par. I realized that sometimes applied math does not always come from book learned teaching.

I had cut down one of the old flywheels and notched the sides so the air gap could be measured. Surprised to find out how much it was. My goal is setting the gap down well below ten thousandths if it doesnt cause a problem this close. Ten thousandths is about the normal clearance on most of the small engines me and Steve have messed with.

Since this ignition assembly sits under the flywheel, there is no way to make adjustments with the flywheel installed. You cant measure or do anything. But makes sense because there is nothing adjustable, except for the points gap. At first I was going to drill out the rivets and slot the coil laminates. Replace with a nut and bolt to hold them in place.

After looking over everything, I could see this would be a bigger problem than first thought might be. The screws that hold ignition assembly to side cover also retain the coil. It is riveted and bolted. Actually it is riveted to hold permanently in place. Then the mounting screws bolt thru the coil and mounting plate to the side cover. That means both the rivet and screw holes would have to be slotted.

Slotting both parts and attempting to adjust air gap, then bolt tightly in place would be guess work as best. I would have to slot both sets of holes, leaving the coil bolts loose. Then adjust gap, tightening the assembly down in place. Remove flywheel, and then tighten the other set of screws that replaced the rivets. Now the air gap should be close enough. Measure and make sure nothing makes contact.

This turned out to be a lot more work than was described in three different hop-up articles I have read so far. I thought about giving up on this considering all of the work. Still not knowing if it would really help out enough for this much effort. Then I looked at the chopped up flywheel and an idea popped in my head.

On the cast iron flywheels the magnet is just bolted on. I had the great idea of shimming the magnets out closer to coil instead of doing the opposite. Nobody mentioned doing it this way in those articles. I wonder why it wasnt even considered.

The main article published in a magazine was written by a rocket scientist. I mean with that much education and brain function, would be a no brainer. Maybe I will never have the brain capacity of that engineer status like Frank. He always says an engineers job is to solve problems. But how to you solve a problem like Frank Price? After looking things over, this makes so much more sense than slotting the coil and mounting screws. Sure is a lot less work.

I finally hit a stumbling point when seeing an error in my thinking. There is a down side since this cant be done on the aluminum flywheels. But for now I am keeping with the cast iron flywheels that were cut down a bunch. Most of the engines at the track in our class racing have the heavy cast iron flywheels anyway. For now that is the plan.

After measuring everything a few times to make sure it was correct, I cut a strip of 0.005" thousandths brass shim stock. Very easy to cut with a regular pair of scissors. Then marked and punched the two holes for mounting screws.

After assembly, I checked the air gap again. This time I could only force about 0.015" feeler gauge in between the coil and magnets. So I cut another piece of 0.005 shim stock. Measuring this time, I was right at 0.010" thousandths air gap.

One more piece should do the job. Now I have a total of 0.015 thousandths shim stock spacing out the magnet. Only a 0.004 feeler gauge would slide in without a lot of resistance. I could force more in there or not so much resistance with slightly less, so close enough for the first time around trial run.


Flywheel face

You can see here how much was cut off this way too heavy flywheel. Being a much earlier model it was so much heavier than the newer ones. After cutting this one down, it still weighed more than the newer one before it was even machined. After cutting both of them and sitting side by side then I realized how much thicker the fins were. This is where a lot of the extra weight had been hiding.


Flywheel back side

You can see how much had been removed and the sides notched out to make access for adjustments. Sure would be cool if this flywheel could run the way it is. But I am too much of a chicken to find out. I dont want this thing to explode.

What I was not sure about is the opening you see. The back side here was pocketed. So when the face was machined down it was cut thru. At first I thought that maybe it was planned for an access plate to fit over to adjust points gap. But not in the right location for that to even work. Not that it makes any difference, but I am at a loss as to why.


Flywheel magnet

Here is the magnet. There sure are a lot of balancing holes drilled in the cast iron. This is another thing I might need to figure out how to fix really soon. Since the engine now turns a lot more rpm than originally designed, there could be a ba1ance problem. I figure from the material taken off just for lightening on the flywheels, we might have that problem anyway.


Flywheel magnet bolts

This big magnet is held on by these two bolts. When looking over everything, is when I figured out to shim this closer to coil. I cant find a reason yet why it will not work. But probably find out soon enough when we make it out to the track next time.


Flywheel cut out for adjustments.

With the sides cut out it is now easy to measure air gap on both legs of coil. So long as they are close I am not worried about having to shim up one side more than the other. That would just complicate things even more. I dont need any of that right now. We are way too close having these engines ready to go racing again.