Version 1 pistons are not spring loaded meaning when they are extended they do not retract. Version 2 pistons are spring loaded meaning when  they are extended they retract back into the piston. All Bowflex Treadclimber pistons look the same but they are manufactured differently internally . Version 1 and Version 2 pistons only apply to Models TC1000, TC3000, TC5000. All other models use specific pistons for that model.