Conversation so far:
What was James Watt's first engine?
The first, for example, had a cylinder with a diameter of some 50 inches and an overall height of about 24 feet.
When did he make his first engine?
In 1776, the first engines were installed and working in commercial enterprises.
Did any other models follow that one?
Over the next six years, he made a number of other improvements and modifications to the steam engine.
What modifications were made?
A double acting engine, in which the steam acted alternately on the two sides of the piston was one.
Who else did he work with?
Boulton.

Question: When did he work with Boulton?
Self-contained rewrite: When did James Watt work with Boulton?