The first run of engine "R1" took place at Rolls-Royce's Derby factory on 7 April 1929 with "R7" running the next day. Many mechanical failures were experienced during bench testing including burnt valves, connecting rod breakages and main bearing seizures, while considerably more trouble than expected occurred with valve springs; at one time two or three would be found broken after a 10-minute run, but the continual redesigning and testing of components reduced all these problems. Unknown to Royce himself, the engineers had also fitted "Wellworthy" pistons that were better able to withstand the 13 tons "pressure" of each firing stroke.