A very tight pinch can be caused by by a sleeve that is "egg shapped". This is not very common from the factory but it is very common from having a resize done. Pull your engine down and put the piston through the sleeve unitl it is tight. Shine a flashlight into the bottm of the sleeve. Now look at the top of sleeve and see if there is any small gaps between the piston and sleeve. I know there was batch of Novarossi and Mugen .12 engines with this issue a few years ago.