A Viper 640 comes stock with a Gnav Vang that runs along the built in groove of the boom, which tends to bind up a little and is only plain bearing. To take advantage of a more responsive vang system, we recommend installing this Harken Viper 640 GNAV Vang Kit.
The Kit includes a cut to length track, car, end caps and all necessary mounting hardware. Reuse your existing blocks and line or replace.
The Harken GNAV kit requires you to reuse your boom-end strut bracket to mount the strut to the new Harken car and you most likely (read: almost certainly) will need to drill out the strut bracket to fasten it to the Harken car. The kit requires a bit of installation, but this is the best/only solution to work on all vipers.