Rotating the tires and counting drive line rotations, it came up at 4 1/2:1. No one around here had ever heard of such a thing in an r180. To end a bunch of local arguments, I pulled the cover off . The numbers on the ring gear are 37 X 8, which works out at a 4.50.

Can anyone tell me from the picture what I actually do have, such as a brand name, or a type or how I can find out? (It is a 180 isn't it?) 4.5 seems pretty deep for a driver, but it will have an 83 5 speed in front of it (this is all in my 72 240 with an L28).

I am open to opinions on what I should or shouldn't do

Thanks Jon. Now I am curious if it would have been oem in something from Nissan or would have been purchased over the counter. (if oem in something, I can look for another for the other car:cool:)

Have you considered the possibility that it might be an R190 ( called the 'R192' in Japan )? Same design as the R160 and R180, but bigger. Originally fitted as OEM equipment on the PGC10 and KPGC10 Skyline GT-Rs and the PS30 Fairlady Z432 and PS30-SB Fairlady Z432-Rs, and also as a 'Sports Option' in Japan and Datsun Competition part in the USA. The R190 / R192 will fit in place of the stock R180 without any other mods ( unlike the R200 ) and can easily trick you into thinking it was an R180.......

I did see a reference in the text about Jmortensen's but didn't realize it was his.

It definately is a limited slip of some sort, both wheels do move the same direction.

I have been away from rear end stuff since the mid 80's when as a kid we were modifying chevy stuff to get the biggest tire made under our trucks. At that time, the carriers were often rated by the ring gear they could support. A 3 series carrier wouldn't accept a 4xx gear ratio and a 4 series carrier wouldn't accept a 3xx ratio, etc. I don't have a lot of disposable cash for this project and don't want to buy a 3.7 ring and pinion (for example) to find out that it won't fit this carrier.

There is so much info on the net (if it is on the net, it must be true, right?) that part of it overlaps and contradicts the rest of it. I know from the brief description and the haphazard pictures it is difficult to determine much, but I was hoping someone recognized the thing, where it might have came from, and so on. I've never met nor corresponded with jon, but have read enough of his posts to determine he knows what he is talking about (can't say that about a lot of folks.....)

Thanks. What you're describing on the American cars is called "carrier breaks". One carrier might fit from 3.08 - 3.90 gear ratios, then there is another that fits from 4.11 - 5.38 or whatever. They do this because as the gear ratio gets numerically higher, the pinion gear head gets smaller, and if you use the same carrier, the ring gear is farther and farther away. By using different carriers, the ring gear can be thinner and still mesh up with the pinion correctly. The R180 and R200 don't use carrier breaks, they use a really thick ring gear. Yours is a perfect example. That is a thick gear!

You could swap gears from another diff, but if it is an R180 they changed the ring and pinions in 76 or so. I believe the ID of the ring gear got larger, I can't remember, info might be in that huge diff post. I think zcarnut on Hybrid Z once posted about using a thin spacer between the two. Regardless, you wouldn't want to put the ring gear loosely on the smaller carrier. The ring gear needs to fit tight.

Alan is right, it could also be an R190. Tape measure on the ring gear will tell the tale there. That is a "racing" gear ratio, and the R190 was pretty common back in the day for club racers via Datsun Comp. I've seen a few around, even one that had a detroit locker in it. If you have an R190, I don't think you'll find gears for it or another diff with the ratio you want. Probably best to sell it to someone else who has an R190.

Follow up:

Spoke with Les Cannaby this evening. It is an r180 with a nismo lsd. Now to decide what ratio I need to change it out to. It is almost unanimous that 4:62 is too deep for much of anything streetable. So, 3.7's or 3.9's? Going to search for a gear calculator and see what comes up. Everybody has a favorite. Car is going to be an occasional weekend driver, mostly around town, some rural hilly two lane mountain roads. Probably running a 225/50/16 or so. Any thoughts?

I'd probably lean towards the 3.7 for relaxed high cruising, but that's just me.

On the other hand, the early S30s for the ROW (Rest Of the World) that came with 5 speeds came with 3.9.

The '82 ZX 5-speed and 3.90 R180 I have in my '73 make for fun driving on the twisty roads in the nearby foothills & mountains. And it is also fun to drive in town or on the highway. In 5th gear at 4000 rpm the speedo shows 100 mph.

You can easily sell that 4.62 ring and pinion to 240Z and 510 racers for about $300.