Positraction unit is interchangable with 521 Pickup

[Minh]

I saw this piece of info on EBay...

(Steve - or anyone else - can you confirm this. If so here something to add to the Part Interchangability Page)

http://cgi.ebay.com/ebaymotors/ws/eBayI ... 2441389149

"This is a new old stock Datsun positraction unit. I believe it is for a 521 pickup truck but may fit other models as well. The inside diameter of the ring gear is approximately 4 5/8 inches and will have 8 bolt holes. The axles have a 23 tooth spline and are approximately 7/8 inch diameter. This unit has been sitting on a shelf for years and does have some minor surface rust. It will probably need to have the side bearings replaced before installation.
Please take into consideration that this unit weighs about 16.5 lbs. before packaging.
I have found a faint part number (_8420 16320) on this unit which identifies it as a posi for a 1600/2000 roadster with 4.375 to 5.874 gear ratios. The 4.375 ratio is stock for the 521 pickup truck."

[S Allen]

I have heard the rear end bits for the 521 truck can be put into the pumpkin of the roadster but I have no first hand knowledge of this. Sid, do you know?

Steve

[spl310]

My understanding is that the side gears have to be changed as the splines are different. The truck axles are bigger iirc. The person to ask is Brian Hollands - he has been doing his homework on these things. I am just an innocent bystander....

[oilleak]

This is the same diff I just took out of my car - we'll not the same one, same p/n 38420-26320. It's a nissmo diff from the early 80's. If it's got 23 tooth splines it's for a roadster not a truck. That's important as the LSD side gears are just as nla as the diffs themselves. The problem with this diff is that you can't run anything numericly lower than a 4.38. don't know why Nissan did it that way but there are two different offsets to the diffs. One is for 9 and 10 tooth pinion gear sets, one is for 8 tooth pinion sets. This is for the 8 tooth sets - 4.38, 4.63, and 5.12 to 1. The seemingly dumb part is that the other diff offset allows you to run all the gear sets, you just need a spacer for the ring gear to run the 4.38 to 5.12 sets? Weird.
This diff looks to be in good condition. The friction and spring plates (internal parts that wear) are also nla for this one but the later peices are still available and can be made to fit. It just takes a few minutes on a grinder to get the plate's ears down to the right size. This is what I had to do to set mine up. These are not "bolt in" mods. It's not hard to set the breakaway torque - instructions on Gordon's web site - but you'll need someone who knows how to set the pinion height etc up on these, just like an open diff - no different.
The 4.38 works pretty well on the street although it's a bit buzzy on the highway. I'd be turning 4500 rpm at 70-75mph with it. I like the 4.11 much better on the highway. 4.38 is also illeagal in SCCA stock and street prepared Auto-x catagories. Not that anyone would know. If you don't do many long highway trips, the 4.38 is a nice ratio (Head snapping 1-2 shifts let me tell ya) and can still be found in some of the early (521, 620) truck diffs. Some of the R&P's from the truck diffs may require a little effort to fit as at some point they switched from 7/16's to 10mm ring gear bolts but it's not a big problem. If it wasn't the 4.38 offset, I'd snap that puppy up.