Phase 1a
-Steps: 4
-Trucks: HTCR, version 1
-Access panels, short hood bottom corners: rectangular
-Fuel cutoff switch: in sideframe
-Radiator intakes: flat
-Panel under right radiator intake: large
-Front engine compartment: 2 doors, flush
-Panel above dynamic brake intake: 10 bolts, 2 panels
-Panels under central air intake (2 left, 1 right): 16 bolts
-Long hood end: rounded hood and light housing corners
-Engine compartment: rounded top edge
-Door under right central air intake: small hinges, single handle
-Dynamic brake intake: grid, close spacing
-Short hood: version 1 (rounded edges, tapered, no forward slope)
-Rear grabirons: bottom-mount
-Jacking pads: metal plate covering
-Left-side handrail stanchions behind fuel tank: 5

Phase 1b
-Steps: 5

Phase 1c1 - 2000.04 - 2000.11
-Trucks: HTCR, version 2

Phase 1c2 - 2000.10 - 2001.10
-Access panels, short hood bottom corners: notched
-Fuel cutoff switch: large housing under sideframe

Phase 2a - 2000.11 - 2001.01
-Radiator intakes: flared, version 1
-Panel under right radiator intake: small

Phase 2b1 - 2001.12
-Front engine compartment: 2 doors, raised
-Panel above dynamic brake intake: 8 bolts, 1 panel
-Panels under central air intake (2 left, 1 right): 8 bolts
-Long hood end: square hood top and light housing corners, rounded hood side corners
-Engine compartment: square top edge
-Door under right central air intake: standard hinges, dual latches

Phase 2b2 - 2001.12 - 2002.02
-Fuel cutoff switch: small housing under sideframe

Phase 2c1 - 2002.01
-Dynamic brake intake: vertical rods, wide spacing
-Fuel cutoff switch: large housing under sideframe

Phase 2c2 - 2002.01 - 2002.08
-Fuel cutoff switch: small housing under sideframe

Phase 2d1 - 2001.12 - 2002.11
-Front engine compartment: 3 doors, raised
-Central air and radiator intakes: fewer bolts

Phase 2d2 - 2002.03
-Short hood: version 2 (square edges, not tapered, side panel forward slant)

Phase 2d3 - 2002.03 - 2002.11
-Short hood: version 2
-Long hood end: square hood side corners
-Rear grabirons: top-mount

Phase 2d4 - 2003.01 - 2003.04
-Short hood: version 1

Phase 2e1 - 2003.04 - 2004.06
-Short hood: version 1
-Radiator intakes: flared, version 2

Phase 2e2 - 2003.04 - 2003.06
-Short hood: version 2
-Left-side handrail stanchions behind fuel tank: 4

Phase 2e3 - 2004.09 - 2004.12
-Jacking pads: open

- Other details -

-UP units Phase 1c2 and later have underframe light under right side cab.
-UP units Phase 2b1 and later lack access panels on engine compartment roof.
-Partway through Phase 2d1 production, UP units no longer have right rear speed recorder cable.
-Larger brackets for radial-steering linkages on HTCR trucks appeared partway through Phase 2d1 production.
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Michael Eby 2008-2009