Sorry folks, but you all got it wrong:
The singular form is either "Werknummer" or "Werk-Nummer" (the latter one is a rather outdated variant). Plural would be "Werknummern" or "Werk-Nummern"
In German language, if you combine two words you either put them directly together without any gap or you use a hyphen. Something like "Werk Nummer" or "Werk Nr." is always incorrect.
"Engine number" would be Motornummer or Motor-Nummer.
Cheers,
Christian
EDIT: Just saw that Roland did got it right - sorry Roland!
