A valve stem sealing unit (65) for forming a seal round a valve stem (41, 43) of a poppet valve (19, 21) in an engine (1) having a body (5, 7, 13) and operated by a working fluid, the valve stem sealing unit (65) including: a housing (67) defining a through passage (79) running from a first end to a second end, the through passage (69) being arranged to receive a portion of the valve stem (41, 43); a first seal (85) arranged to form a seal between the valve stem (41, 43) and the housing (69) to prevent egress of the working fluid from the first end of the housing (69); and a second seal (89) arranged to form a seal between the housing (69) and a body (5, 7, 13) of the engine (1) to prevent egress of the working fluid from the second end of the housing (69).