I called Stance back in the day about this and they gave me a weird figure. They said their shaft tapers, so I knocked it off the list. I don't see how or why, but I asked two separate people and they both said it started as one size and then got larger at the bottom... Again, I don't think they understood the question because that's the only coilover I'd ever heard of doing that... and we all know these are made in the same places.

Ksport would never answer me.
Those cheap MDU coilovers, Megans, and other $500ish coilovers all use the 6.5-7.5mm shaft (which is tiny).
HKS was something like 22mm, but depending on which version you had they ranged from 17-22 (but I remember they never said any were 20mm). I had HKS at the time and had to sell them due to this.
BC racing was either 16 or 18 or somewhere abouts there. This was almost a year ago, so I'm a little shady.

Most all Japanese import coilovers ran small. The cheaper the coil, the smaller the shaft diameter. These originate from the UK and were prototyped on older VW's which used large shaft diameters like 22mm - very hard to find on *** cars. This was one of the reasons Adam started making different size collars.
Again, for simplification and cost effectiveness, Function Form are the only coils I found that had a 20mm shaft diameter and had a spring diameter less than 90mm (although some rubbing occurred on the inside of the cup). They are the most "bolt on" coilover to use with these cups from how they originally came.

 

Yeah, anybody can PM for install help. If it gets too extensive I'll give you my number.

 

God damnit. Read.

I'm just talking to myself in this bitch.

The way the cups are now, they will not fit BC Racing. You will need reducers. BC Racing was 12.5mm now that I think about it, but you need to call or email BC to verify.