mike maule Posted January 3, 2013 Report Share Posted January 3, 2013 I am looking to make my 1979 620 into a dually. I have read about bearing failures and the parts are hard to find. I have found a rearend out of a 1984 720hd dually in a salvage yard. How hard is this to swap into my 620. The guys at the yard are no help in giving me info on how far apart are the spring perches are(winter sucks). I think this could be my biggest problem.I have a drive shaft builder close if u-joints are different or I can not find a combo joint to fit. Quote Link to comment
datzenmike Posted January 4, 2013 Report Share Posted January 4, 2013 620 and 720 springs are the same distance apart and the same length. The 720 axle is slightly wider than the 620 but if running duals you don't care. The only other thing is your '79 (likely a 4 or 5 speed) will have a 4.11 rear end ratio. The '84 Cab/Chassis dual wheel with have a 4.375. If you can find a different year 620 pull the speedo pinion out of it and swap into your '79 and the speedo will stay accurate. You need a 20 tooth (red plastic) pinion. Quote Link to comment
wayno Posted January 4, 2013 Report Share Posted January 4, 2013 The dually rear ends will bolt right to your leafs, as they bolted right to my 521 leafs, the rear brake line access might be differant, but you can just use your 620 lines on the 720 axle I beleave. The dually axles came with two differant gear ratios, the 4.37 that datzenmike mentioned, and the 4.625 ratio which I beleave came in the motor homes, I know this because I have one, I was sorta unhappy about the ratio, but I beleave I can switch the 437 ring and pinion into/on to that rearend carrier, as my other carrier is shot. The nissan dealer also told me I could purchase a 411 ring and pinion that would fit these rearends, but that was several years ago. Here is my 521 dually with an 85/86 dually rearend. I suppose I should mention that when you buy it, before you hand any money over, you should grab the pinion flange and lightly turn it back and forth, if it has more than an 1/8 inch play, I would say walk away, you should also open the back plate up and make sure the shaft that goes threw the carrier holding the smaller spider gears is tight in its hole, absolutely no gap what so ever. I'll try and get some photos of my shaft in the next day or so, as the photo would explain better than I can on what to look for. Quote Link to comment
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