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What is it with 720 4x4 transmissions?


720inOlyWa

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I have a 4x4, 86 720 that I have been fixing slowly. It really is a cool truck. I have never owned a 4x4 or off road capable truck before and I am looking forward to playing with it- gently. Before then, I plan on digging into it more deeply this summer. The reason it is parked now is because when I got it running and drove it on a test drive, it sounded like all hell broke loose in the tranny when I grabbed 3rd gear.  I limped back home in first and second and parked it and began looking into a transmission rebuild.

 

In the mean time, I keep an eye out locally for a dead 4x4 with a good transmission, but there is no such animal. in fact, many of the 720 4x4s listed on crags list have transmission issues.

 

I have pretty much resigned myself to a professional rebuild because I am not going to guess on a transmission. My question is this: Do you think these trannys are dead because of previous owner neglect / abuse, or because they are inherently weak? Is it because most 720 4x4s end up being commuter vehicles, a job not ideally suited to 4x4 trannys? Is there some bone headed move that ruins a lot of them?

 

I do wonder what your experiences tell you. The 2wd (long shaft) tranny seems to roll forever.

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The short shaft came out around 1984, the only difference between the 2wd and 4wd short shaft is the 2wd tail housing has a speedo drive gear access hole while the 4wd doesn't.

My guess would be that all the 4wd trucks for sale are for sale because the transmissions make noise and they want to sell them before they break, or they were parked for the same reason and you ended up with one of them, there are lots of late 4wd Nissan trucks with quiet transmissions, they just are not for sale.

The weak spot in these transmissions is the bearings, specifically the counter shaft/clustergear bearings, one cannot say power has anything to do with it because I have acquired a few 61hp 720 diesel trucks because of the longshaft transmission making noise or it is outright broken, there are also lot of diesel trucks with good quiet transmissions, they just are not for sale.

When you have your transmission rebuilt, have them use the best bearings money can buy, I buy strictly Japanese bearings for mine, I have never had a bearing I put in explode so far, but I have changed out several bearings over the years, short shaft, long shaft, there is no difference except for the length

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Forget the b series I went through 4. Swapped to a c series all good 12 years. 2wd trans work in the 4x4 just have a higher first gear. You will have to have the primary driveshaft length changed and a easy plate adapter for trans mount made from scratch. Also modify shifter hole, all easy stuff. I even just changed out the oil to GM syncromesh per ratsun recommendation and everything is even smoother

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An FS5W71C (2wd) from a Hardbody is a good idea if dissatisfied with the 720 5 speed. It will bolt to your engine but may be a little shorter. Should be easy enough to lengthen your intermediate driveshaft to the trans case. The gearing will be a little lower? You have a 3.549 first and the 71C will be 3.321.

 

Keep your release bearing collar and install on the 71C clutch arm.... that's about all.

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