Below is what Bob sent me December 25, 2018
I should clear up a couple of details. The photos I sent you labeled Delrin, dog bone, stainless are of the three caps you own. If you look closely you will see a series of grid lines on the green colored back ground. These images are taken from an optical comparator display. The comparator uses a light and lens system to enlarge the item and project a shadow on to a screen. In this case the part was enlarged 20 times. Reading the grid against the shadow is how I took my measurements of the thread depth and angle. I used my phone camera and simply took a photo of the screen for each part.
My conclusion was the Delrin part was not cut deep enough leaving the large, oversize flat at the top (crest) of each thread. The thread angle and spacing were very good. The stainless part showed an irregular thread profile with poor angular dimensions.
The info you received from Greg Long is very helpful. He measured several caps and found a range of 2.030” to 2.045” This gives an average of 2.038”
George Teebay measured his cap and found it to be 2.032”
Paul Jacobs did not offer a measurement but stated his Delrin caps were made to fit his radiator neck. His (now yours) Delrin cap measures 2.040” as shown on my drawing.
One last bit of information I came up with is based on the dimension I measured off your radiator neck. This measurement inside the neck is called the minor diameter of the thread. I came up with 1.971” Doing some math, trig and geometry you can come up with a calculated major diameter (this is what you measure on the cap) I calculated a 2.047” diameter. This is larger than all of the caps but is still hitting pretty close. Going off the minor diameter and working backward is not the best way to go for several reasons.
In measuring your radiator neck, I did NOT take a series of measurements. Greg mentions his neck is 1.995” and brings up the question of roundness. I did not check roundness on your neck and should have done so.
Conclusion and Confusion: Somewhat surprised to see your neck at 1.971 and Greg’s at 1.995. This difference of .024” is a lot. If Greg’s is more typical (large) it would explain why Paul’s Delrin caps have NOT given him any reason to question size. Getting a measurement from Paul on the size of the neck he uses to qualify his parts would be interesting. We need to check your neck for roundness.
We know Paul’s Delrin cap does NOT fit your neck. Your neck is too small (or out of round) OR the Delrin part is too big. Betting your neck in NOT out of round, we should be able to machine down the thread on Paul’s Delrin part to get it to work.
Going forward—picking a dimension close to Greg’s average of 2.038” should work for your small neck and anything larger.
Paul’s comments on the “last photos—-” showing a damaged thread—-must be the photo of your stainless cap. He must have missed the caption titles and assumed all three photos were of his Delrin?
Greg is correct—the threads on the Delrin part are NOT deep enough (especially to fit a small diameter neck like yours)
Greg’s suggestion on making stainless caps with flat tops, etc. is interesting. I would like to see the photo of the cap profile he mentions.
The gas cap photo you show is interesting as it looks just like the Delrin cap. You mentioned your gas cap is not threaded and will not help us check any dimensions for your radiator cap.
This email is detailed but does summarize what we now know. Will check In with you when I get back in town.