One whole day of the week is missing. The rain begins on Wednesday night, but the narrator has the rain beginning Thursday night. All of Thursday is missing.
When Thomas Two-Stroke is inspecting the car, he surreptitiously breaks something, then claims he has found the problem and shows them the offending piece of the engine. When the two Englishmen ask what it is, he replies that he doesn't know what it is in English, but gives them the supposed name in Welsh. Translated literally, it means "what's it called" - basically, a thingamijig.