This is NOT an AMC valve fault since the exhaust valves were swapped for good quality, new TRW (VW) parts.
Rather that the valve guides have been either reamed too small (worn tooling) or not fully finished. I could rock them in the guide a little, but a 9.0mm reamer will not even look at the guide...
FYI a new exhaust valve stem should measure 8.9mm dia
Always check new parts, never assume that anything has been made correctly
