The following describes the replacement of the stock ’73 240Z 3.364 R180 differential to a ’82 280ZX 3.9 R200 differential.
- Find a 3.9 R200:
A popular diff to get your hands on would be the 3.9 R200. This particular unit was used in the ’80 280ZX (2X2) MT and ’81-83 non-turbo MT 280ZX. I managed to find one parted out from an ’82 280ZX. If your not sure what you got because the guy you bought it from didn’t know either, pop off the rear cover and find the stamping on the ring gear. This gear had the following stamped on it:
- JH 39:10 DQ 1 11 25
The important info is the “39:10” bit. It states that the ring gear has 39 teeth and the pinion gear has 10 teeth. Divide 39 by 10 and you get the final drive ratio. If your a non-believer in those that stamp numbers on gears, count the teeth yourself.
- Figure out how it fits:
In order to properly bolt up the 3.9 you’ll need the following parts:
- ’75-78 280Z moustache bar
This allows the larger diff to fit by facing the bar the opposite direction (it looks like it’s turned around when you compare it the 70-73 bar) as well as having a series of bends where it meets with the rear cover. You want to use the bar from a ’75-78 so the bolt patterns line up. All Z’s from 1970 to 1978 use the same locations for mounting the drivetrain parts. Because the ’75-78 280Z cars used an R200 (3.545) the outer case is exactly the same as our later R200 (3.9). I also used hard urethane bushings from MSA.
- ’75-78 280Z R200 half shafts
You will need to find half shafts that were originally built for an R200. I found a set from a ’75 280Z that were in good shape. The R200 in the photo below has a dramatic offset towards the drivers side to accommodate the larger ring gear. Because of this, the half shafts are not the same length. The driver side half shaft is slightly shorter than the passenger side. You can’t tell which is which just by looking at them, so extend both shafts and set them side by side to determine which is the shortest.
- Mounting parts
Besides the parts listed above you can use the existing parts that came with your ’73 240Z to assemble the new R200. The front mount, insulator, and rear control arm hanger are completely interchangeable. Note that if you have a ’70-72 240Z the rear control arm hanger will need to be replaced with a ’73-78 version. The later hanger has a curve in it to allow for the diff to sit back further. The early one is straight and will not accommodate the bigger R200. On a minor note, the hanger assembly is bolted on in front of the R200 moustache bar, whereas the R180 setup has it mounted behind the moustache bar. This is something that is very obvious when you puzzle it back together.
As for the front diff mount I bought one from a ’76 280Z, because initially I thought that the ’73 version would not work. Keep in mind that BOTH will work, but if you look closely the ’76 mount it uses thicker steel and has a larger radius to accommodate a bigger exhaust pipe. Since I have a 2.5″ pipe it made sense to use the ’76 mount instead. Also, you can use the ’73 insulator as long as it’s in good shape. This part should definitely be changed if it’s worn out. A bad insulator will give you a clunking sound every time you drive.