In 1776 the partnership erected two engines, one for Wilkinson and one at a mine in Tipton in the Black Country. Both engines were successfully installed, leading to favourable publicity for the partnership. Boulton and Watt began to install engines elsewhere. The firm rarely produced the engine itself: it had the purchaser buy parts from a number of suppliers and then assembled the engine on-site under the supervision of a Soho engineer. The company made its profit by comparing the amount of coal used by the machine with that used by an earlier, less efficient Newcomen engine, and required payments of one-third of the savings annually for the next 25 years. This pricing scheme led to disputes, as many mines fuelled the engines using coal of unmarketable quality that cost the mine owners only the expense of extraction. Mine owners were also reluctant to make the annual payments, viewing the engines as theirs once erected, and threatened to petition Parliament to repeal Watt's patent.