These rings are used with our VQ adaper bell housings. Our bell housings are machine to fit the VQ35DE transmission input shaft bearing. As the transmissions became newer models, the input shaft bearing became smaller. So we make these rings so our bell housings will work with the new transmissions too. If you are not sure which trans you have, you can measure the input shaft bearing. Here's some sizes:
Car Trans Engine Bearing Size
2003-2006 350Z CD00A VQ35DE 100mm/3.937"
2007-2009 350Z JK41B VQ35HR 95mm/3.740"
2010-2020 370Z 1EA0B VQ37VHR 90mm/3.543"