Another diff thread

[socaldatsun]

So my rear diff has been whining since I did the swap and can't remember it did that before. I have a ka24e in my 72 521. I also have a custome drive shaft so the flanges are easy to swap out. My question is if I can get a hard body diff or somethin better then what i have and bolt it up? I've seen people just swapping out the guts and doig it that way and honestly dot feel like dealing with that

[EastBay521]

Im pretty sure the 620 axle bolts up to the 521 but not 100% sure. I was wanting to swap a 720 axle into mine but figured it would be alittle cheaper and easier to just swap the 3rd member

[Charlie69]

I put a 1980 720 complete rear end in my 1966 520.  I had to move thew brake line to the driver side on the rear end.  If I remember correctly  the 1980 720 is the same width as the 78-79 620 rear end, gut was 2" wider than My 66 520 rear end.  Stock wheels with 195 75 r14 clear the bed just fine.

[socaldatsun]

I'll have to go and find something soon, my diff is whining while in the freeway

[datzenmike]

The 620 is wider than the 521 and the later 720 even wider.The D21 Hardbody is wider still. Yes the axle splines are the same (29 tooth) but axles are too long.

 

You can just swap the third member with the gears. A 3.90 would still have lots of snap with a KA. 720s had them and Hardbodys with 2wd 4 cylinders through '93 at least

[socaldatsun]

Sorry I'm barely doing research on this so still a little confused. I'm using my axels from my 521 and the guts of the 720/hb. How do I know if it's a 390? Is there an LSD option? I want the best of what I can get since I'm already there.

What year 720/hardbody should I be looking for, or is it 720 in general

[]2eDeYe]

There are numbers stamped on the ring gear. 39:11 would be how the number is shown on the gear and you divide the numbers. 39/11 is a 3.54 ratio. 

 

I have a 92 hardbody H190 that is pretty much the same as my 70's H190. The pinion flange is a bit bigger, but that is the only difference that I have seen (besides the better brakes and longer housing).

[Farmer Joe]

when i put the KA in my truck, i put a 3.70 ratio 3rd member in my 620 housing. 

 

the center chunk we found with the 3.70s was from a 720. 

 

i wouldnt swap out the whole rear axle... thats much more work than just swapping the 3rd member. 

 

if you have a KA already, i would try and find a 3.54 ratio like redeye mentions. 

 

youll have to pull the current 3rd member and see what gearing it has... 

[socaldatsun]

So the lower the number the better? I just realized that this is going to be a pain in the ass since my fuel cell and my diff only have like 3 inches of space in between :/

[ggzilla]

The higher the number the better acceleration.

 

The lower the number the better fuel economy.

 

With 4.375 it will rev up fast and you'll quickly need to shift gears. With 3.545 it will rev up slower and you'll have to wait to shift into the next gear. Though with a 2.4 it won't be bad. Just depends on what you are after - insane speed? Or leisurely driving?

[socaldatsun]

I want something in the mid range so I can still take some people off the line and still keep up in higher speeds. Also since I'm at it an LSD package would be awesome

[ggzilla]

With the stock 4.375 you can still do 85 MPH or more depending on which 5-speed you have.

[wayno]

I don't know a lot about the power/torque of KA, but if you have the stock gears that came in your 71 521, they would be 437s, so you likely have a hole shot of a hotrod, but when on the hiway your RPMs are fairly high, 4000rpms at about 70mph.

You will give up that hole shot going to 370s, it will be about like starting in 2nd gear as you truck is now, if you get 354 gears, it will be like starting out in 2nd with a load, but using these more hiway oriented gears will reduce your RPMs significantly on the hiway, not sure about fuel economy though, but you will likely do better if you drive on the hiway a lot, if you drive mostly in the city, not sure it will change anything except the hole shot, city driving is more comfortable with high numbered gearing, hiway driving is more comfortable with low numbered gears.

If you have the torque to turn 354 gears, with stock size tires, you will be turning 3000rpms at 80mph. :)

[Charlie69]

Why not use the very common 3.899 720 2 wheel drive ring and pinion in a LSD third member?  Does this gear ratio not fit an LSD?

[ggzilla]

Because its slow ...