The Reversatron RFP (R0/a = 50 cm/8 cm) has been operated without a conducting shell. RFP discharges and high aspect ratio spheromak discharges both terminate prematurely, because of global modes with n = 1 and n = 2, where n is the toroidal mode number. The n = 2 mode is dominant as seen in some ideal MHD simulations of the spheromak at high aspect ratio. A simple theoretical model shows the RFP to be unstable to the n = 1 tilt or shift modes, depending on the vertical field index. The time scale for growth of the n = 1 mode in the presence of a resistive wall is shown to be (R0/a)2 τv, where τv is the time for penetration of the vertical field. For the Reversatron, (R0/a)2 τv is shorter than the duration of the discharge.