Air dam push type retainers

What is the correct size of the push type retainers that hold the air dam on?

I haven't found one single place that mentioned the size, including the OE diagrams. Have you by chance tried removing one to measure it? I know they sell these things at any ACE Hardware
 
I think I need to remove one and measure the hole. So far I've bought some 18mm that are too small. On one of the other Wrangler forums one post said they are 20mm and another post said 25mm. Now I know how people end up with so many extra parts. ;)
 
I think I need to remove one and measure the hole. So far I've bought some 18mm that are too small. On one of the other Wrangler forums one post said they are 20mm and another post said 25mm. Now I know how people end up with so many extra parts. ;)

Yep, I think the only way to find out for sure is measure. My ACE Hardware has tons of those things. I buy a lot of them just to have around, since you never know when you might need one!