Ok Larry, thanks I didn't see that one last night. sounds simple enough. I even have a heat treat oven so I won't have to bother with "my big old wife's" oven.
I'm looking at the melting point of Zinc (419.53 °C or 787.15 °F) and wondering if there would be any distortion from doing this. Also am wondering at what temperature the Zinc institute says would nominally cause the material to yield.
I can’t understand why the castings would change shape over time. Plastics change shape over time because they continually out gas over their life time, they eventually dry out causing them to shrink. Metals on the other hand, including Zinc alloys don’t out gas like plastics so I don’t see what would cause them to warp. The heat at the carburetor simply isn’t enough to relax the material.
I may be wrong, but I can’t help but think that these castings came from the factory already warped.

Back in the 20’s there were problems with some Zinc (aka: pot metal) castings growing. Many of the antique radios from the 20’s suffer from this ailment. The problem took place in between the grain boundaries and caused the casting to expand, but it was discovered that the addition of certain alloys were what caused it and after that the metals industry was well aware of the problem. This was very early in the 30’s and certainly after WWII and into the 50’s they had it well in hand.

I wish I had some castings to experiment with, I have a machine shop so I can make the fixtures with no problem.
Interesting problem, If I’m gonna keep working on these till I’m 100 I’d like to understand it better.
Denny Graham
Sandwich, IL

Denny G
Sandwich, IL