Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Hi - I had exactly the same problem.
The slider was fitted too precisely and then jammed.
I had to open the slider box and take out the slide and cut off a bit with a file.
Now the D-valve can touch the port face fully.
The Maxwell Hemmens steam engine are very precisely made and they
have no "play" - better to loosen a bit.
Good luck and congrats for this nice one.
Arnold