PIGDOG,
I believe you are right on the splines. The H165 came out of a late 60's Datsun 411. The H190 came out of a 1970 Roadster. Both have 23 splines. The H190 axles are 2mm's bigger so the splines are not as deep. Im afraid to install the H190 because I have to custom make a driveshaft (chaching) and I feel that I will break it anyways. Plus, the roadster diff is worth a pretty penny. I know that I can sell it but I need something to replace it.
Racing starts next month and I am no where finished with this project.
What do I do?

roadster diffs all have tiny axles. im guessing the h165 was a roadster one too and not a 1200 ute one. the 1200 ute and all other later h165s have the 23 spline axles(like what your h190 has). whereas the later h190s have 27 or 29 spline axles im pretty sure, they are a fair bit bigger

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Only because I broke the axle using a A15. I am now moving up to a Z24. I am afraid that it won't last for more than a week.

if it put up with one season of drag racing with that set up it isnt too bad.

why not try the 2mm larger set up till you find the pick up ones

OK than. I'm going to have to do some trading now. I need one of those bigger axles.

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They are not. The pickups have bigger axles than the Roadster.

I'm not so concerned about gear ratios, I'm more concerned about using an axle that is only 2 mm larger in diameter than the one I just broke. If someone has axle diameter chart to share, that would be great. My axle is out of a roadster and I would like to know if all H190's are the same. It has the 10 hole center diff pattern but I thought the axle would be a lot bigger. Oh, I forgot, it only has 23 splines on the axles. That I don't understand.

To make more gear ratios available, you can use any H190 center section if you use switch the side gears. There are three different H190 side gears, for different types of axles.

If you could find the H190 out of a 520 utility, they have much larger axles than the ones out of the Roadster, due to the weight of the ute & it's expected load,versus a light car that carries 2 people. Plus they're an alloy centre & a 4.6:1 ratio. And they're only slightly wider than the original diff housing, drum to drum. The only downside is they've got 6 studs instead of 4, but you can get the stud pattern changed on the axle & the drum. I don't think you'd break an axle, as I think Mark Alford uses one in his SR20DE+T powered Sunny(210) race car.

Need some info from the club.
Any of you folks out there using an H190 diff in your car and if so, what motor and HP are you running? CA18DET or SR20DET?

I am putting a supercharged Z24 motor in mine with a 50hp shot of Nitrous. I had the same setup in my car but with a built A15 motor. I was using an H165 rearend that had the spider gears welded. I broke the axle after one season of drag racing. I purchased an H190 rearend which I will be welding up as well.
I finally got around to removing the H165 from the car and paired both axles up. By eyeballing it, the H190 appears to be only 2 milimeters in diameter bigger.
The H190 diff came out of a 1970 Roadster.

Will that be good enough for my new setup or will I break that one to?

Thanks for any help.