Logo
FULL SITE MENU

cap - contact marks

B

bobct

Advanced Contributor
I replaced both my caps/rotors yesterday and noticed that the "burn" marks on the contacts were almost on the edge on one of the caps compared to the other side which were pretty much centered.

I seem to recall that this meant the timing was off. Is that true or an old wives tale?
 
With the mechanical type advancing mechanism, there will some lead and lag between the tip of the rotor and the distributor cap tower contacts when moving from out of BASE advance, into the progressive and while reaching TA.

Here's part of the deal:
Let's say that your TA is 28* (doesn't matter about RPM here).
Let's also say that BASE is 8*.
That leaves only 20* of mechanical advance.
(BASE advance is a result of the distributor housing position prior to any progressive, and the TA is a result of all advance as seen at the crankshaft)

Minus BASE (for the moment)...., the 20* seen at the crankshaft is only 10* of advance within the distributor.
Split that 10*, and you have 5* that will be seen at either side of the rotor alignment with the cap wire tower contact.
(that would be if you used absolute center of rotor tip and tower contact)
Now add the dimension of the rotor tip, and the diameter of the tower contact.
Under atmospheric pressure, a good spark will easily jump this gap.

Bottom line, if the factory indexing between the triggering unit, cap position and rotor are dead-on, then there should be no problem.


.
 
Please explain in more detail :)

Thanks, that was helpful. So the 5 degrees could easily be off a hair which would result in an uncentered contact point. I guess that could become an issue if the tolerance moved any further and potentially starts missing the contact. Pretty sure I would feel that.
 
bobct said:
Please explain in more detail :)

Thanks, that was helpful. So the 5 degrees could easily be off a hair which would result in an uncentered contact point. I guess that could become an issue if the tolerance moved any further and potentially starts missing the contact. Pretty sure I would feel that.
Click to expand...
Ha ha! :D

Ya know, I don't recall that I've ever actually explained that to anyone before.
But it's all part of the distributor, rotor and cap fit and indexing design.

The EST systems don't have that issue.
 
Back
Top