Rotor-tip to Cap gap?

Even if you could measure it, how would you adjust it?

 

Thought I would post this from the Buickstreet forum. Ignitionman had this to say:

--------------------------------------------------------------------------------

This is information either not, or not adequately addressed at the V8Buick site, and concerns two topics concerning distributor rotors, tip to terminal clearance and phasing. A "checking cap" can ber easily made from an old cap, with a large hole drilled into it adjacent to one or more of the wire terminals, so a timing light will light the rotor tip when testing is done, to show rotor to terminal alignment and phase.

Question 1: Rotor to terminal clearance. This post appears to be some sort of joke, posted as that, but I will address the subject, so for those whom wish to actually know, info is provided.

Yes, there is a gap between the rotor firing bar tip to each wire terminal, they do not strike each other as the rotor turns. This gap also has a spark jump, just like a spark plug electrode set has, and this spark jumping is also resistance in the whole ignition sceme of things.

In later GM vehicles with window style caps, there are two different gap specifications available, standard, and emissions.

Standard gaps are set to a spec of .100 and have a +/- tolerance of .030. These rotors have no marking on the end of the firing bar. These gaps have the least resistance of the two designs.

Emissions rotors have a shorter rotor firing bar length, to give a larger gap to bridge, to force the coil to make more spark energy, by having to overcome the larger terminal to tip gap. This was done to have the ignition coil work harder to make more volts from the coil, which ended up marginally increasing spark energy, but ended up just overloading the coil. Attempt was to make ignition output higher so it would help lighting off ever leaner mixtures, doesn't work well. Gaps for these rotors are usually set to .200/.250, and the rotor firing bar is identified with a letter "E". The large gap rotors are not the best for performance applications.

Not all small gap rotors are unmarked, example, NAPA RR167R is made by Echlin, is a short gap rotor, non-emissions, but the NAPA RR169R is a large gap rotor, has the E as well, so...best to measure the tip length and select the longer rotor tip when obtaining a new rotor. In the case of the RR167R, the E is for Echlin, the RR169R, emissions, somewhat confusing if you don't know what is what.

Question 2: Rotor to terminal phasing. This is answered in two sections, mechanical and vacuum curves.

Mechanical: This is the phasing of the rotor tip to the coil terminals in reference to the operation of the mechanical advance. When rotor phasing is correct on a mechanical advance only distributor, the center of the rotor tip will aim directly to the center of each spark plug wire terminal around the cap. This same phasing is also done for mechanical advance on vacuum advance distributors, as discussed below.

In essence, firing points on the points/magnetic pickup should correspond with the centering alignment of the rotor to terminals centers. Adjustment is usually not possible, as production of the parts, namely the points cam or magnetic pickup reluctor relationship to the rotor mounting is fixed and not adjustable. MSD does have a rotor that will adjust, for some distributors.

If rotor phasing is performed on a mechanical advance distributor only, the rotor tip should align centered with the terminals, as viewed with a timing light and checking hole at one terminal. If the distributor is a vacuum advance distributor, the alignment will be in the retarded phase when viewed with the light through the hole in the cap, as tghe points plate/pickup will be in retard phasing.

When mechanical advance curves are checked with the checking hole method in the cap, the rotor will appear NOT to move in the hole, it will show firing in only one position, this is normal.

To correctly check phasing on a vacuum advance distributor, check it the way described below.

Vacuum advance. Vacuum advance moves the points/pickup through an arc of degrees, by physically moving the plate the points/pickup is mounted to. Vacuum advance always has a set number of degrees of plate movement, and is always divided in half for that movement, center to retard, center to advance. Example: if a vacuum advanc egives 18 crankshaft degrees of advance, the rotor will not be centered in the checking hole until the vacuum advance is CENTERED in its travel, or at 9 degrees of movement.

When checked with a timing light through the checking hole, vacuum advance not engaged, the rotor will appear a certain distance retarded in the hole, when vacuum advance is pulled all the way in, the rotor will appear advanced the same distance from rotor tip center in the opposite direction of vacuum advance disengaged.

To check the rotor phasing with a vacuum advance distributor, first find the amount of degrees of vacuum advance with a tioming light and degrees on the balancer. Divide in half to get center point of vacuum advance, run engine, set vacuum advance to half travel, read rotor phasing in checking window, should then be centered in on the terminal.

Example: vacuum advance is found to give 24 crankshaft degrees, set vacuum advance to 12 degrees to check phasing, read tip to terminal alignment. Adjustment is done by slotting the vacuum advance holes and physically moving the advance on its mounts to center the rotor tip. When viewed in the checking hole, a vacuum advance, when actuated during a phasing check, will appear to move the rotor tip in the hole, this is normal.

Note: Usually, either the rotor tip or cap terminals will have ends wide enough to allow for a slight production mis-alignment, without creating issues/problems. In the case of the large cap GM HEI, rotor tip is usual width, cap terminals are wide. This is done to compensate for a vast number of different vacuum advance cannisters and many different numbers of degrees available from same. Slight misalignment with these cap sets is allowable. Ford Duraspark caps/rotors have the same features, but the cap terminals are narrow, rotor tips are wide, accomplishes the same end, just reversed.