Most current supply distributor caps will have aluminum terminal
posts, where original issue parts generally had brass posts. The
brass terminal type caps may well last for decades if not physically
abused. The aluminum terminals are a whole different story, not
particularly pleasant, but obviously a little cheaper to produce.
With enough running time the aluminum terminal posts become anodized
from the constant electrical bombardment. The resulting aluminum
oxide coating is an electrical insulator. As the face side of the
termial is oxidized adn becomes increasingly resistive, the spark
from the rotor has to jump to the side of the terminal, and
eventually all the way around to the back.to find a better conductive
surface.This increase in the open air spark gap may well be enough to
cause running problems in just a year or two of regular use.
Some years ago (1976-77) I had this problem big time with a Datsun
Lil Hustler truck (1974 model) with 1700 engine and 55,000 miles
onteh odo.. The dizzy cap contact posts were so bad that there was no
way to make an electrical connection with a test light or ohm meter
for 3/4 of the curcumference of the posts. When I couldn't get a
repalcement distributor cap on a Sunday, I tried to repair it. The
aluminum oxide coating is very hard and was resistant to grinding
with a Dremmel tool.
I finally proceded to pick away at the face side of the contact posts
with a sharp pointed pick, and LOT of physical force. Eventually
the badly anodized surface come off in big flakes, leaving the soft
aluminum exposed on the flat side of the terminal posts (somewhat
chewed up). This restored the electrical path and got the engine
running again.
Lesson well learned. If I had a choice I would gladly pay double the
price for a disrtibutor cap with brass terminals.
Barney Gaylord
1958 MGA with an attitude
http://MNGAguru.com
At 07:09 PM 12/30/05, Max Heim wrote:
>....
>I have found that the one set of items that do require regular
>replacement are the cap and rotor. ... There is never any visible
>defect, but rough running can be cured by a simple cap and rotor
>replacement. Perhaps the parts supplied here are of a different quality.
>....
>
>on 12/30/05 5:00 AM, Paul Hunt at paul.hunt1@blueyonder.co.uk wrote:
>
> > I've only ever had to replace a distributor cap or rotor once in 40 years,
> > including several cars that have done up to 75k miles.
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