Figure 21: Chassis Dimensions And Clearance Requirements - Juniper MX240 Hardware Manual

Chapter 3: Preparing the Site for Router Installation
routers. Allow 2.8 in. (7 cm) between the side of the chassis and any
non-heat-producing surface such as a wall.
For service personnel to remove and install hardware components, there must
be adequate space at the front and back of the router. At least 24 in. (61 cm) is
required both in front of and behind the router. NEBS GR-63 recommends that
you allow at least 30 in. (76.2 cm) in front of the router.

Figure 21: Chassis Dimensions and Clearance Requirements

33
Clearance Requirements for Airflow and Hardware Maintenance