Hi Ian333, welcome to Outdoorking. One of the problems with zinc parts intended to fit together tightly, is that the material is neither hard nor strong, and it can easily develop burrs when you try to reassemble the machine. My first guess is that you have a burr there somewhere, perhaps too small to see even, but possibly detectable with a sensitive fingertip or a magnifying glass.
Essentially though, if that is a half crank machine (the 26 cc is, don't know about the 30 cc) I don't know why you would bother.
