I'm getting ready to assemble this Z120 engine for the TO-20 I bought back in the spring. I had a set of orings that came with the Sparex sleeves I bought. I also had a set that came with the Fel Pro gasket set I bought,so I thought I would compare them. I did measure them but you can see there is a difference between the two. Of course .009" may not sound like a whole lot but when you figure in the Fel Pro is .009" thicker the whole circumference of the oring that is quite a bit of difference. They are different in the rubber that was used also. I think I'll go with the Fel Pro orings.