Welcome to ModelEngineMaker !If you have problems registering or logging in, please use the contact menu option to request assistance.
1/8" would look a bity chuncky, I only used 1/8" on my 1.5times one. I have also done a couple of other similar sized engines and use 3/32" or 2.5mm on them and its about the right proportion with the piston rod which is generally larger than the valve rod.What about a split bush or even a drill chuck held in the lathe chuck to hold the 3/32" stock
The clamp is one piece that's a press fit into the actual bore of the flywheel.