Rebuilt Distributor Checklist Question

Model A Distributor Failure: What to Check Before Installation

From the Going Places, Columbus, IA Chapter newsletter:

This month I had two incidents of Model A distributor failure and am grateful to the assistance rendered by our President Tim Diehn. It turned out that the rebuilt distributor I purchased wasn’t properly reamed or lubricated. Another lesson learned.

— John Emmering

A rebuilt Model A distributor can look ready to install, but small issues inside the unit can quickly lead to poor performance, erratic point gaps, misfiring, or outright failure. Before installing a rebuilt distributor, it is important to check the shaft fit, lubrication passages, rotor clearance, and final assembly details.

The following checklist can help identify common distributor problems before they leave you stranded.

Check Proper Reaming and Shaft-to-Bushing Fit

One of the most important checks is the fit between the distributor shaft and the bushings. The shaft should rotate freely through a full 360 degrees without binding, but it should not feel loose.

Check for side play by pushing the cam side-to-side. If the points gap opens and closes while doing this, the bushings may be too loose or may not have been properly reamed. Ideally, the shaft-to-bushing clearance should be tight, around .001” to .002”.

Also check the end play. The shaft should have very little up-and-down movement. End play should be under .003”.

  1. Check the Rotor-to-Body Gap:  The rotor-to-body contact gap should also be inspected before final assembly. A proper gap helps prevent unwanted contact and electrical issues inside the distributor body. As a general guideline, the rotor-to-body gap should be approximately .025”.
  2. Inspect Upper Plate Stability: The upper plate should be stable and should not wobble. A loose or unstable upper plate can cause erratic point gaps, which may lead to inconsistent ignition performance. If the points gap changes unexpectedly, do not overlook the upper plate as a possible cause.
  3. Confirm the Distributor Is Properly Lubricated: Lubrication is another critical area to inspect, especially on rebuilt units. Rebuilt distributors may have blocked oil passages, which can prevent oil from reaching the shaft and bushings. Make sure the oiler hole leads down to the shaft, particularly to the top bushing. If possible, confirm that the shaft and cam screw are drilled so oil can reach the upper bushing.
  4. Distributor Lubrication Points: The base oiler on the side of the distributor body should be filled with motor oil. This provides lubrication to the shaft and helps prevent premature wear. For the cam lobes, apply a small amount of cam lube or Vaseline. Be careful not to over-lubricate. Too much lubricant can work its way onto the points and cause misfiring.
  5. Top Shaft: The top shaft also needs attention. Add a few drops of motor oil to the top of the cam screw to help ensure proper lubrication.
  6. Final Assembly Checks: Before putting the distributor into service, complete a final assembly check. Set the point gap to .018” to .020”. Confirm that the shaft has minimal end play, ideally under .003”.
  7. Finally, perform an electrical check to ensure the point stud does not short against the distributor body. This simple test can prevent ignition trouble after installation.

Lessons Learned

A rebuilt distributor should never be assumed ready simply because it has been rebuilt. Proper reaming, correct shaft clearance, open lubrication passages, stable plates, and careful final assembly all matter.

Taking the time to inspect these details before installation can prevent distributor failure, misfiring, and unnecessary troubleshooting later.