Topic: Distributor Cap Hold Down Clamp Rivet

[J_Speegle]

Couple more from others I've posted before. Unfortunately from the end of the pin but still shows color/coating

[rockhouse66]

I think I mentioned earlier that I see similar color in a photo I have of an 0H dated NOS Boss 429 distributor, but whatever that coating is must not have survived for long in the real world.  It is a pretty cool looking detail and I have been coloring the pins a goldish color on the 1969 and 1970 distributors I have restored.  Remains debatable as to whether this is concourse correct though IMO.

[Bossbill]

According to a number of posts at finishes.com one cause is
"The yellowish discoloration could be due to the parts being processed in an alkaline non cyanide zinc solution that uses older (non polymer) brightener technology. This would be typical. The "fix" is a 0.5% nitric acid pre-dip before the chromate, use of a "very blue" trivalent chromate and close control of the chromate pH."

I've also seen references that say "the most common cause of a trivalent passivate yellowing is too much iron in the passivation tank."

I'm not saying these are the causes here, but yellowing of zinc appears to be an issue mentioned many times using a variety of zinc processes.

[Scott302]

I traced the "B-12145" part number back as far as 1928.  It is possible the part started as brass and at some point the spec changed to allow for zinc plated steel.  I was not given that detailed info from the blueprint.  I think it is safe to say that the rivet was steel in the mid-60's and onward. 
Bossbill's info is interesting.  I'd be curious as to the time it would take for the yellowing to occur in the situations noted.
Regards,
Scott

[J_Speegle]

I'm not saying these are the causes here, but yellowing of zinc appears to be an issue mentioned many times using a variety of zinc processes.

I think that may be why a couple of my examples turned a yellow though still not convinced (see below) - can be felt and wiped but the vast majority have the coating as shown

Also note that none of the other zinc plated items attached at the same time do not show the chalky yellowing so think it has to do with the coating rather than the zinc.