On Wednesday, December 13, 2017 at 2:39:07 PM UTC-8, Ivan Vegvary wrote:
Installing valve seals ...
Since the cap is 0.005" larger than the seal, I have 0.0025" to make up with glue all the way around.

My first thought: high-temperature heatshrink tubing, to bulk up the OD of the
head your cap goes onto. There's heatsnrink made of teflon (PTFE)

http://cdn.techflex.com/assets/pdfs/catalog/hfa.pdf