Whats the purpose of having the adaptor, cap and rotor changed on a Duraspark II ignition system?

I've seen pictures of both... but there's no explanation for why one exists over the other.

 

The wider cap is said to prevent arcing ..

 

To convert to a wider cap, all you need is:

1) adaptor for cap
2) wider cap
3) change out the rotor

Correct?

 

Yep .. That's correct

 

Perfect! Thank you good sir!

 

You are correct, as long as the distributor is a Ford unit.

Redroad is correct. The wider spacing on the terminals of the wide cap is to help prevent cross fire or arcing. The Duraspark systems operate at a much higher secondary voltage than the older point style systems.

For what its worth... Ford used the small caps on some of the early Duraspark systems but switched to the wide cap about 1975.

 

I upgraded to a wide cap and msd tfi coil in another vehicle. Going from the stock set up to the wide cap with a hot coil and 8mm wires was a nice change. Bump the key and it fired right up. Loved it

 

The wide cap is generally a good idea if you go with a TFI coil. With the stock '70s coil, you'll be fine with either cap style, as the voltage won't be high enough to arc across to the wrong cylinder anyhow, but as posted above, all that's needed to switch is the cap adapter, new cap, and new rotor.

 

This is all very interesting information because the Napa distributor I wanted to order today (1982 F150) was a standard size cap... which to me was a bit of a shock.

I was expecting it to be the wide cap.

The 1983 302 Cougar engine I've got sitting in my garage is a wide cap... why wouldn't a full size truck also be a wide cap?

 

As long as the cap & rotor match, you're basically OK.


Ford switched to the wide cap with the DSII, basically just in case the voltages got high enough to create arcing problems in the smaller cap. But as it turns out, people don't have problems running the earlier small cap with a stock DSII system - the factor of safety was just such that they were overly cautious in making that switch when they did. But, later (~86, with EFI), they switched to the "TFI" ignition system, and with the stronger coil there, the wide cap actually is necessary in practice.

My '84 Mustang had the wide cap. I used the wide cap on my build, thinking I may one day switch to the TFI coil. But I had the same coil and ICM as present on my old engine, with a small cap and never had a problem.


The distributors are actually the same. There's an adapter that installs on the regular distributor in place of the small cap to make it work with the wide cap, and the rotor for the wide cap is taller to make up for the extra height of the adapter. You can switch any time.

 

Very cool info.

Was playing around with my points distributor last night and found that the rotor inside is a very simple system. Quite easily interchangeable with another rotor... if I was keeping my points lol.

 

I have a '75 that has electronic ignition and the small cap. The cap has female terminals. I have an MSD "Cap-Adapt" for later model (80s) DS2 distributors, with the female terminals. I should be able to use that on my '75 small cap distributor, correct?

 

The distributor is the same all the way through the DS years, and even the TFI years as far as cap mounting goes.