Pre-79 F-Series: Converting a Small Cap 7mm Spark Plug Wire System to a Large Cap 8

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By 1998 Tom Hogan

This is a VERY simple conversion to install the late
model large distributor cap onto an earlier model engine. The
subject truck is a 1976 with a 390 FE motor. It has the stock
small distributor cap and matching spark plug wires. The reasons
for doing this conversion are:

  1. Reliability. The terminals are farther apart
    in the large cap and this helps prevent cross fire in the cap.
    The larger wires also conduct better.
  2. Performance. The larger separation in the cap
    will allow the use of more powerful coils without cross fire.
  3. Appearance. The conversion is clean and looks
    very trick.
  4. Why not?

You will need to pick up 4 parts:

  1. Spacer. This is the adapter that allows the
    large cap to be used. The large cap is actually flat and snaps
    onto the spacer. This can be had at a wrecking yard cheap.
  2. Cap and Rotor. Matching set, get one for a late
    model 302 or 351W. Can be had at the wrecking yard if you can
    find a good set.
  3. 8.5mm Spark Plug Wires. The larger cap will
    only work with the larger wires so your old wires won’t fit.
    If the donor vehicle has a good set snag them.

You will notice that all parts can be had at a wrecking
yard if they are in good shape. This may not be too far fetched
because some people will give a dying car a tune up to try to
keep it running and when it does die the new parts go to the wrecking
yard on the car. There are tune up kits that include the cap,
rotor and wires but normally the spacer is not included so that
is the only one you should get at the wrecking yard.

Installation is straight forward:

  1. Note spark plug firing order and write it down!!
  2. Remove the old distributor cap and rotor.
  3. Install the new spacer. There is a rubber plug
    in the base of the distributor that will cause a little interference.
    The spacer has a square tab that fits in the cut out that the
    rubber plug is in. The plug can be compressed by the spacer so
    it will fit or you could trim the plug. I don’t recommend trimming
    it because you could cut the wires that go to the magnetic pickup.
    See the arrow in the photo.
  4. Secure the spacer in place with the snap springs
    that held the old distributor cap.
  5. Install the new rotor.
  6. Install the new cap.
  7. Install the 8.5mm spark plug wires.
  8. Step back and enjoy your handy work.

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