Rate increases for different bars over the stock 20.7mm 521 torsion bar
620 to 7/77 - 21.9mm, 126% stiffer
620 from 8/77 - 22.0mm, 128% stiffer
720 2wd 10/81 to 12/82 - 24.0mm, 181% stiffer
720 (all others) - 22.0mm, 128% stiffer
Nissan Comp - 26.0mm, 250% stiffer
OK I've done some research. My 98-99 Nissan Motorsports catalog provides some data on truck torsion bars.
All 521/620 torsion bars are listed under the same P/N's
99996-T4010 for their oversize 26m bar
99996-T1450 is their 2 stage bar
The same part numbers are listed under the 2wd 720. The 26mm bar is also lists for 4x4 720 up to 12/82.
They also provide the stock bar diameters as follows.
521 - 20.7mm
620 to 7/77 - 21.9mm
620 from 8/77 - 22.0mm
720 2wd 10/81 to 12/82 - 24.0mm
720 (all others) - 22.0mm
WOW, Dime Dave great info! I just got flamed a little for asking about the torsion situation on the 620 boards. They linked me to this. I ordered the T4010 for my '73 620. It's great to know that if they are too stiff I have cheap options at the wrecking yard that'll get me stiffer but under the 26mm size.
Much appreciated info!