Simanaitis Says

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“ARE BREAKER POINTS in tennis matches?” No, my little one, they have to do with pre-transistorized automotive ignition systems. “What’s a transistor, Grandpa?”

Internal combustion requires something to ignite an engine’s fuel/air mixture. If it’s a diesel, the heat of compression alone is enough. But, as its name suggests, a spark-ignited engine requires a separate means of setting off its gasoline/air mixture: the spark plug.

Breaker-Point Ignition. And, before computerized ignition, breaker-point ignition systems produced the timed impulse to bridge a spark plug’s tiny gap. I’m using the past tense because modern cars do this all electronically, much to their benefit.

The breaker point gap at left opens and closes in response to cam action upon the pivot at right. Image from Sonett72 at English Wikipedia.

Breaker points were a type of electrical switch. As described in Wikipedia, “The purpose of the contact breaker is to interrupt the current flowing in the primary winding of the ignition coil. When this occurs, the collapsing current induces a high voltage in the secondary winding of the coil, which has many more windings. This causes a very high voltage to appear at the coil output for a short period—enough to arc across the electrodes of a spark plug.”

A Delicate Mechanism. Maintenance of a car’s breaker points was a delicate matter. Too small an opening gap, and the primary circuit could overheat. Too large, and the secondary circuit’s spark could be too weak to cause ignition. And, even if it were sufficiently strong, the spark would arrive too soon causing dreaded detonation or ping.

The Feeler Gage. A typical points gap was around the thickness of a badly worn dime. However, badly worn dimes vary in thickness. Also, specified points gaps varied as well: 0.014 in., 0.016 in., 0.021 in., etc.

A Thickness Gage, with range from 0.0015 to 0.0250 in. Image from

A feeler gage was called for. This gizmo is still useful when measuring precise gaps or clearances. It consists of perhaps 25 thin metal strips arranged in a fan. By selecting one or several, measurements can be assessed from its thinnest to combinations of its strips. Every mechanic, home or professional, had a feeler gage.

The Distributor. Breaker points resided within the engine’s distributor, which, as its name implied, distributed high-voltage spark to the appropriate cylinder at the appropriate time. The points were actuated mechanically by a cam containing as many lobes as the engine’s cylinders.

This schematic evidently shows breaker points for a six-cylinder engine; note the hexagonal cam.

A 1908 Alternative. In The Grand Prix Car, Volumes 1 and 2, 1954 Edition, Laurence Pomeroy describes the engine of the 1908 Italia Grand Prix car and its interrupter cams.

Illustration by L.C. Cresswell in The Grand Prix Car, Volumes 1 and 2, 1954 Edition.

The Italia had four massive cylinders (total displacement 12 liters!) arranged in iron-casting pairs. Pomeroy writes, “The exhaust valves… were operated directly from a side mounted camshaft and the inlet valves through push rods and rockers, the latter mounted in the plane of the crankshaft. The camshaft also drove vertical shafts driving a cam on the top of the cylinder head which impinged on short spring-loaded rockers operating the wipe contacts of the low tension ignition system inside the combustion space.”

Another Historical Note. When Chrysler introduced its computerized ignition system in 1971, the press release said, wonderfully enough, “New Chrysler has pointless ignition.”

I wish I had said that. ds

© Dennis Simanaitis,, 2019


  1. Duane
    September 7, 2019

    I had a 1957 Chrysler 300 C with a dual point distributor.

    • simanaitissays
      September 7, 2019

      I remember it well: the shoe polish paint touchup and my sleeping under its dashboard at Cedar Point.

  2. Steve
    September 7, 2019

    Two sets of points on a Triumph Bonneville (and other British bikes). Three sets of points on a 3-rotor Mazda.

  3. tomtheaustingroupllccom
    September 7, 2019

    So now, all cars are pointless, eh?

    Lovely post.


  4. Bob Long
    September 7, 2019

    Dennis, if cars are pointless, I guess this was a pointless post? Made me smile and want to dig through the tool box to see if my feeler gage is still in there.

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