We have an old Chateau valve in a shower. I got an adapter from the local shop that should allow use of a Posi-Temp valve trim kit (I have a Genta LX). When I try to put the adapter on over the Genta trim tube, the adapter doesn't line up with the valve. If I remove the trim tube and place a stop tube on the valve, the adapter fits perfectly - the diameter of the stop tube seems to be what is needed for the trim tube which is significantly larger. Help - what am I missing? What do I need to do to make this work (other than retreat to Chateau trim)?