Nut size for 87 catalytic converter
 

I just went to put the exhaust on the RX7 I just bought and it is missing the nuts for the cat. Does anyone know what size and thread they are? Thanks for your time.

I found them at the hardware store. 12mm x 1.25

If you use the hardware store ones on the downpipe and near the cat you may have a clearance problem. They are M12x1.25 from the factory but they have extra small heads on them, being 17mm instead of 19mm.