I've been looking at pictures and scratching my head over this.  Maybe someone can solves the puzzle for me...

I think everybody knows that the design of the S30's moustache bar (MB) was changed from the original 'flat' version to a new 'curved' version at the beginning of MY-72.  This was part of the re-design done by Nissan to re-locate the diff further to the rear so that the halfshafts would no longer need to operate at a 5 degree fore-aft angle.

The 71 and 72 FSM's both use the same diagram of the Diff/Rear Suspension  (see below).  It shows the MB mounted with its end loops sitting on the front side of the bar.  The MB is shaped so that it 'droops' down from the body.  That droop means that the MB can't be mounted 'upside down'.  If you're going to flip it (for whatever reason), it only be 'flipped' around its vertical centre axis, so that the MB will now mount with its end loops sitting on the rear side of the bar. 

Note also that the diagram from the 72 FSM shows the MB sitting in front of the hanging vertical brackets for the lower diff crossmember. 

Now look at the smaller diagram, taken from the 75 (280Z) FSM.  It shows the MB mounted so that end loops are positioned on the rear side of the bar.  Why?  What happened to the rest of the drivetrain design to force this change of mounting orientation for the MB?  And is the MB for the 280Z a completely new design, or is it just the later-style MB from the 240Z, installed in a flipped orientation?

Just to confuse things further, now have a look at the two photos I've imported from other threads. 

• The one with the blue MB is Blue's old 280Z project car. 
• The other one is believed to be a stock 72 240Z with the MB correctly mounted so that the end loops are on the front side of the bar.  Can anyone confirm that this photo shows the correct MB orientation for a '72?

So, why is it that Blue's car has the MB 'flipped' (compared to the diagram from the 75 FSM) and, as a consequence, now sitting behind the hanging vertical brackets?  This would seem to result in the Diff being moved a lot further back than the factory had intended (by maybe an inch or more?), and maybe even contacting the lower suspension crossmember under certain circumstances.  BTW, this isn't the only photo I've seen with the MB sitting behind the hanging suspension brackets.  Did these owners make a mistake and put the MB back in the wrong way?... and I ask this with all due respect, because I make this kind of mistake all the time!

Edited January 30, 20169 yr by Namerow
typos fixed

Perhaps you are unaware that the 280 got a larger bar that was mounted on the other side of the two legs from those on a 240.

I had the same issue and had to finally make up a sheet I keep in the garage to use whenever I switch diffs  (R180 -R200) on the race car.

9 minutes ago, gnosez said:

Perhaps you are unaware that the 280 got a larger bar that was mounted on the other side of the two legs from those on a 240.

I had the same issue and had to finally make up a sheet I keep in the garage to use whenever I switch diffs  (R180 -R200) on the race car.

Larger in which dimensions?

The 280 M-bar is thicker and wider than the 240 bars. Definitely heavier duty. The holes for the R200 mount studs are larger as well. I'd have to compare directly, but the amount of curvature (distance from rear of R200 diff to centerline of m-bar chassis mount pins) may be slightly greater as well. 

I know can use the 72-73 Mbar with an R200. The holes in the bar need to enlarged (12-14mm or something?). It is a bit of a stretch, but it fits.

4 hours ago, Namerow said:

So, why is it that Blue's car has the MB 'flipped' (compared to the diagram from the 75 FSM) and, as a consequence, now sitting behind the hanging vertical brackets?  

Are you sure that picture came from 1975?  I opened up my 1975 file and it shows the typical R200 setup.  Regardless, you can see that "1" in your picture is an R180 (the bolts around the input flange).  That's the simple direct answer.  R180 goes one way, R200 the other.

Not sure about different bars.  Carpartsmanual shows the same bar, after July 1973.  Maybe that's where the beefier bar appeared, since they still used R180's after then, but don't distinguish a different bar (Just realized the from-to on their page is a bit weird).

http://www.carpartsmanual.com/datsun/Z-1969-1978/axle/rear-suspension