620 differential misery

[Valvebounce]

Hey team,

so I've been chipping away at building a 620 ute the last while. I scored an impossibly rust free rolling shell ('78) with frontal damage. And for an absolute song I Scored a rusty af '73 620 to save it with.

the guy I. Bought the '78 off kept the diff. But I had the '73 one, so no drama.

I had the housing sand blasted and powder coated. And I managed to source a 3.888 diff head for it. 

I've reached the stage of doing the axle bearings, (converting to deep groove from cup and cone) before buttoning it up and rebuilding the brakes.

now.along my travels I picked up a lot of spare parts. The backing plates on my diff were bent to hell. Some f-ktard must have beat on them trying to extract the axles at some time. And I finished up buying the whole rear axle out of my '79 for a dozen beers. (After id already had mine blasted and coated) so I'm trying to fit an axle. And guess what?

It seems to be too long. So I grabbed another four axles off another friend. They all seem too long too.

finally I work out which axle was the other one from the '73 unit. (It was bolted back up into the diff I'd bought for the box of beer.

and it's the right length!

Somewhat baffled, I start running the measuring tape over both my nicely powder coated '73 housing, and the '78 one. By this point, I'm starting to sweat that my nice housing might not fit my '78. I sat the housing on the leaves, and the spring pads line up. 

With further measuring, it appears as the the '73 housing is around 25-30mm narrower (outboard of the spring pads) than the '78.  Perhaps the early wheels ran a different offset? (The '73 came on a gnarly (read pretty ugly) set of widened (badly ) datsun 6 stud steel wheels. (I swapped them for a box of beer just to get rid of them)

 

My Haynes manual makes zero mention of rear track width. Does anyone know If it changed thought the 620's production run?  Or has anyone else encountered this difference in 620 differential widths before? 

It could well work heavily into my favour in the long run. Give me another 14mm at the outside for more wheel dish/tyre. (Assuming I don't get rubbing issues on the inside face of the sidewall)

[G-Duax]

Wait, what ???

You took out the tapered roller bearing, and put in ball bearings on the axles  ?

 

The H190 truck diff has a spacer in the center just to hold the axle bearings in required pre-load.

If you put pre-load on a ball bearing, it will fail quite quickly.

 

Just what are you trying to gain by going to ball bearings ?

(besides failure)

[datzenmike]

The differential (third member) should swap between years. Seldom does anyone swap axles between housings, usually everything gets swapped. There were differences in 620 track over the years. The disc brake trucks were definitely wider than the earlier ones.

[Valvebounce]

7 hours ago, G-Duax said:

Wait, what ???

You took out the tapered roller bearing, and put in ball bearings on the axles  ?

 

The H190 truck diff has a spacer in the center just to hold the axle bearings in required pre-load.

If you put pre-load on a ball bearing, it will fail quite quickly.

 

Just what are you trying to gain by going to ball bearings ?

(besides failure)

 

I have removed the thrust block from the centre of the diff. Actually, my 3.888:1 head never had one. Whichever application it came from originally must have also used deep groove bearings. I retro fitted one, but then removed it again when my axle wouldn't slide all the way in. (It still wouldn't go all the way home) 

Interesting about the differing track width.  

With the deep groove bearings I've had to machine up a small spacer to sit in the backing plate between the outer of the bearing and the spigot on the end of the axle tube on the diff housing. The deep groove bearing is approx 1.5mm narrower than the outer of the cup.and cone was. (Just to be dead certain that the bearing cant migrate in the backing plate) It also means I can dispense with the axle bearing preload shims, which will no longer be needed. 

 

After having slept on it, I've  decided that a narrower axle can only be a good thing. (Better flexibility with wheel.and tyre fitment) it's a lot easier to space a wheel out to gain inner clearance than deal with guard rub at the outside edge.

mostly I'm just pleased that Ive finally got to the bottom of why none of my axles fit.