LS-3,LS-4, same problem.

There are different types of valve stem extenders. Plastic, walls too thick. I make sure to get the metal ones.
You can find the metal extenders that have threads to the end.
Some are built that the threads start about 1/4 in or more up in the barrel. Those don't work.

Metal extender with the thread that starts at the beginning work well. In the US, be picky, but you can normally find a package of 3 or 4 extenders at Auto Zone.

Bill T