Well, to put it as simply as possible, Yes, I think it will work.
I measured the distance between the pinion bearing shoulders [where the bearing cups sit against] & they were the same for both carriers, then I measured down from the side bearings to the pinion bearing shoulder, & they too were the same, so I have concluded that we can fit the entire internals from an iron carrier H165B into the alloy carrier from the earlier H165.

There are many visual differences in the external apearance of the two carriers, but from what I can make of it, they measure the same where it counts.
Note the additional ribs on the softer alloy carrier to give it the strength it needs.

The pinion bearings have different part numbers, but the bearing cups are the same external dimentions as each other & that's all that really matters.

The pinion on the left is from the alloy H165 diff & the barrel shaped spacer is the prime distance piece that separates the two bearings. The washer on top of this piece is actually the adjusting shim & this is selected for thickness to keep the two bearings a fixed distance apart so that there is a measured amount of preload on the bearings, but no more. The pinion on the right is the H165B item & it used a compressible sleeve to achieve the same result & this is one of the main differences between the two versions of the diff.

With this later design, you can not simply replace the oil seal in the same way that you can with the earlier type.
With the later design, the whole differential must be removed & the diff case with ring gear attached, taken out of it. Then the pinion must be removed in order to replace the seal, & fit a new compressible sleeve. Then reassemble the whole thing.
They call that progress.


I do not yet recomend that anyone rush out & do this just yet as I can't absolutely guarantee that it works untill we build a differential assembly from the parts that I have.


The next step is to attempt to assemble my 3.7 to 1 ratio H165 internals into this carrier.
Then again, should I polish up the tops of the ribs first, & what about an LSD while i'm at it?
Nahh, I got my own car to work on.

EDIT
I believe that the internals from the iron carrier H145 diff in 1200's will fit into the alloy carrier H145 from the Datsun 1000 too. This would make an alloy H145 WITH A 3.9 to 1 ratio which was never a factory offering.

Todays Update
Well, in order to measure the iron carriers internal dimentions, I needed to remove the pinion. This should be simple enough.
You just hold the drive flange in a vyce, undo the big nut on the pinion, pull the flange with a puller, then gently knock the pinion out with a soft hammer or push it out in a press. Easy peasy.
Oh dear lord, if only it were so. Well, the nut came off easy enough, but that was the last easy thing it would let us do.

The flange did not come off with a few bumps from a copper hammer.
It would not come off with a sturdy puller either, so we decided that it must be rusted on. Perhaps a little push in a press would dislodge the pinion from the drive flange & the internal bearings in one go, so we set it up on a 65 Ton press, placed a 1" diameter slug of steel on the threaded section & started to press it.
At more than 30 tons, it became obvious that the short steel slug was bending, so we gave up & saw that the threaded section was bent also.

Next, we added a little warmth to the drive flange, still convinced that this was the problem. We warmed it untill it glowed red while adding some pressure with the press. Still no action.
<---- see picture.

Next, we removed the drive flange with an oxy torch. This should do it, so when we got it off we hit the pinion a few times with the hammer. Nothing.
So we cooled it all down & stuck it in the press again. At about 20 tons, we knew it was not coming out.

Next, we cut the inner cone down two sides right through. Surely this would do it, but no, even the press wouldn't shift it, but we were working on much lower pressures now. If it didn't come with a few tons, then it wasn't coming at all.

Next we cut the bearing cone all around its circumference to blow away the rollers & this finally allowed the pinion to come out, but with the remains of the inner bearing cone still firmly attached.

Once it was on the bench, we were able to break up the remains of this cone to reveal the real reason for all this drama.

This picture shows the pinion with a small piece of the inner bearing cone at the bottom of the picture.

While it isn't visible in the picture, the splines on the pinion are actually twisted ever so slightly. This is what was preventing the drive coupling from coming off, but we think that it would have come off in the press OK if we had tried to remove it by itself.
The mark in the pinion, where the bearing sits, & the corresponding mark on the bearing remnant is the cause of all of our grief. The bearing locked up at some time, probably just after it ran out of oil when the drain plug fell out, & the pinion spun inside the bearing, welding itself to the cone. At this point the diff, somewhat unsurprisingly, siezed. It was this pinion shaft welded quite solidly to the bearing that would not allow the damn thing to come apart.

Fortunately, the diff carrier is undamaged, although the paint is burnt, & is suitable for measuring as was originally intended & I will do this in the next couple of days.

To top it all off, I weighed the two carriers on a rough & ready spring scale. The alloy one came in at about 5-1/2lb [2.5 kg] while the iron carrier [without caps] weighed about 14-1/2 lb [6.5 kg] so at about 9lb [4kg] the weight saving was not as much as we had hoped for, but it's too late now, I will complete the measuring & let you know the results.
If it's all good, I will put the alloy carrier aside, discard all the iron diff parts & continue on with my own car & just hope that the bearing caps show up.

I think the 910 bluebird disc will be much heavier than the ute drum by as much as 1-3kg per side. Haven't actually weighed but have held one of each, and from memory the disc set-up was noticeably heavier.

Could Topgear be making the same mod to NOS lyberty viscous units to fit the h165?
I personally prefer a clutch pack unit.

Chris Ill keep searching

Now all I need to do is fit it up with the 3.7 ratio that I have, fit it to my ute rear axle housing, then slip it under my son's 1200 coupe.
Well, thats the plan anyway.