A while back I bought a used set of Ground Control coilovers for my Miata. They were ordered for Bilsteins as the seller had an R Pkg. My question lies in the fitment of other brand shocks, specifically Koni STR.T? I'm guessing the only real fitment difference would be if different brands use a different diameter shock tube. Can't find much helpful info. I e-mailed Ground Control and have yet to hear back, but someone here must know the answer 
I'm pretty sure the only difference is in the ring that the sleeves sit on. If the konis are bigger, it's just a matter of have the hole in that ring bored out a little. If the bilsteins are bigger, then you need a Koni sized ring that could be machined or possibly bought from ground control. Ground control doesn't like to sell individual pieces unless it's to replace a part of one of their kits. If they are no help, contact fatcat motorsports. They sell similar sleeves and may have a ring in the same dimensions you need. They are much more willing to sell individual pieces.
There are at least two different sized tubes out there. I don't know which one uses the slightly different tube size offhand, though. I believe the position of the ring varies as well depending on the application, but I'm not 100% sure on that one.
A related question: can the STR.T handle the spring rates you're thinking of?
Hmm, I didn't even think of positioning stuff.
I'm not sure about the STR.T and my spring rates, that was my next step 
I want to say they're 440 front and 275 rear.
Edit: Google is telling me that they will not like those rates.
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