Differential Palooza: Assessing old diffs

[mtngoat]

Good morning, 

the Datsun parts re-org and cleanout is continuing apace at my place. We did our transmission assessment in the previous palooza...now it's diff time!

 

I haven't counted them all up yet because I haven't dug them all out yet, I can see several in the large heavy parts pile I haven't mined down to, yet. Others are loose. 

 

I have at least three straight axle diff pumpkins and at least two IRS units, Z or 510 sedan as yet undetermined. 

 

I never spent much time messing with the diffs on my various rigs, they just worked...or didn't. Didn't swap any ratios or anything. I'm not sure where all these came from! I did have a sedan and a Z for a while, so the IRS versions make sense...but where did all the others come from?

 

I do recall something about picking up a taller live axle version to bump up the gearing on the blue 2000, which was a 1600 conversion and I've never been happy with the low ratio in the rear given modern highway speeds. I want lower cruise RPMs, I'm not much on the point it and punch low gearing torque performance range. I did not install it, and I'm sure that's one of the loose ones. 

 

Questions:

Can diffs sit for decades (out of the rain) and still be usable?

What do I look at/for to assess their condition? 

How do I tell which fixed axle diffs are for what? Was there a standard size? 

Same question on the IRS units. 

Edited November 26 by mtngoat

[datzenmike]

2 hours ago, mtngoat said:

...

 

I have at least three straight axle diff pumpkins and at least two IRS units, Z or 510 sedan as yet undetermined. 

 

 

Questions:

Can diffs sit for decades (out of the rain) and still be usable?

What do I look at/for to assess their condition? 

How do I tell which fixed axle diffs are for what? Was there a standard size? 

Same question on the IRS units. 

 

I assume the straight axle diffs are H-190. 

 

Car H-190s have 23 spline side gears, trucks are 29 except the very early trucks like the 320, they were 21 tooth spline side gears. With care you can swap side gears to fit the axles you plan to use without affecting the set gear lash. 

 

Gear ratios might help ID what they were in.

 

Roadster....... 3.889 (4.11 optional)

520............... 4.875

521............... 4.375

510 wagon... 3.90

Roadster, 510 wagons and 520/521 truck H-190 housings are aluminum. Light but not as strong as later steel ones. 

620 2wd trucks 4.375 and 4.625. The '79 was 4.11.

720 2wd trucks range from 3.364, 3.545, 3.70, 3.889, 4.11, 4.375, 4.625

720 4x4 were 4.375 and 4.11. Sometime around '82 the companion flange was changed for a larger driveshaft. These won't fit the other H-190s and the flange can't be swapped.

D21 Hardbody and beyond 2wd 4 cylinder used the H-190.  

 

Also ran...

The '82-'83 S110 200sx and the '84 S12 also used an H-190 differential. 

 

 

 

IRS units...

 

The 510 used an R160 differential while the Z cars used R-180 and some later ZX used the R-200. This is the diameter of the ring gear. If they all look the same they are likely 510 R-160s. These came in 3.70 and 3.889 ratios. If 3.364 or 3.545 then you have a Z car or Maxima R-180. The 810 car had a 3.70 R-180 and a few ZX cars had 3.889 R-180s. 

[RoadRace]

On 11/26/2025 at 12:01 PM, mtngoat said:

Questions:

Can diffs sit for decades (out of the rain) and still be usable?

What do I look at/for to assess their condition? 

How do I tell which fixed axle diffs are for what? Was there a standard size? 

Same question on the IRS units. 

diff can sit indefinitely if they are sealed and out of the elements.
spin it  - if its smooth and quiet, thats a really good start

post pics, that helps

[mtngoat]

Thanks for the tips guys! Will post pix as the sorting occurs.