Ignition Failure & Faults

Distributor Upper Plate with Modern Points

The "distributor upper plate with modern points and condenser" is in use by many Model A owners. A basic design and construction fault has been discovered. Tests on both cars and a distributor stand were made on ten different units from five suppliers.

The problems were initially revealed during timing tests of four cars at two 'A' Meets. The Ignition Timing Kit described in the Jan/Feb 1992 Model A Trader, pgs. 2-3, uncovered the faults and causes of the malperformance.

The specific ignition performance symptom observed was:

  • The spark control full range could not be used 
  • The engine stopped when the control was moved beyond some setting

The owners thought the problem was just mislocation of the timing gear dimple, piston # 1 Top Dead Center.

The malfunction was instead found to be improper location of the point assembly on the movable distributor plate. The rubbing/actuator block was thus changed with respect to the cam-rotor slot.

The relation between the rotor extended brass conductor and distributor fixed button conductor was consequently not proper at the extremes of the movable plate rotation. At some extreme (full advance or retard), the two conductors were so separated that the spark could not jump the large air gap. The spark failed - the engine catastrophically stopped!

The above observations were strikingly visible using the cut-out cap, timing light, and procedures referenced in the Nu-Rex Precision Timing Kit.

The original Ford point plate, points, cam, rotor, and distributor were properly designed to work on the distributor 40 degrees advance-retard plate rotation. The distributor button is a nominal 5 degree width additionally. The movable point arm rubbing block was positioned, relative to the rotor slot on the cam, so that a portion of the rotor conductor was always opposite the distributor button throughout the advance to retard extremes. This provides the smallest spark air gap.

Supplier chosen "modern point assembly" does not duplicate the pivot and contact points relation of the Ford original design. This is not a prime requirement but it does also affect the point dwell time (the time the points are closed). The fabricators have chosen to use some existing holes in the standard point plate, presumably for ease of modification.

The existing plate hole chosen for the assembly pivot location changed the rubbing block position so that the cam slot improperly locates the rotor brass conductor relative to the cam high point for point opening and subsequent spark.

On some assemblies tested, this caused the rotor-distributor contacts to be separated by as much as 3/8 to 1/2 inch in the full retard position. The spark could not always jump this large air gap. The engine then stopped!

Nu-Rex solved these problems with a new Model A Ford upper distributor plate assembly. This new plate is stamped from original dies with no excess holes. The modern point sub assembly and condenser incorporates fixed pivot pin location for proper cam and rotor contact relations and reproducible point assembly replacement. It also features tungsten contact points and condenser constructed of modern materials and sealed against moisture for long life. The use of modern and standard components make replacement parts available at virtually any automotive parts supplier. The condenser has been relocated to shield it from engine heat effects. The package also features a 0.020 plastic feeler gauge for easy point adjustment, correct plating, and instructions for adjustment and use. This is the finest and most popular modern upper plate available. This is a genuine Nu-Rex part.