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So I have a 1974 260z that I'm putting a sti r180 rear differential into. I've got a mustache bar and control arms from techno toy tuning, along with wolf creek cv axles. When I try to put the front mount of the diff in place it flexes the mustache bar and the differential seems to be tilted too high in the front. When comparing to the stock parts the rear cover on the diff has the studs in the same place and the two differentials look identical. The hole locations and offset of the original and techno toy mustache bars are correct but he factory steel bar seems to have a bit of twist. I can't tell what I'm missing here and wanted some input.

 

post-181-0-99407900-1481870165_thumb.jpg  post-181-0-04369400-1481870174_thumb.jpg  post-181-0-15890400-1481870180_thumb.jpg

Edited by RB26powered74zcar
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A fellow seattlite, hello!

 

Hmm a little hard to tell. It almost looks like that front mount is a bit too tall. Are you running an RT style mount on top? That would be advisable if you are going poly in the rear.

 

With an R180 the stock mustache bar sits forward like mirror image of how you have it. I'm not sure if the TTT bar accounts for that or if they purposely put the R180 bar that way to pull the diff back to fix the axle angles.

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That clears up a lot with the edit. 

 

The bar is supposed to be perpendicular to the chassis it shouldn't have a twist in it, it may have gotten a twist due to a worn front diff mount over time, but it should more or less be flat in the back. Hard to imagine the rear mustache bar flexing especially that TTT unit. The guess would be your front mount if you are reusing the stock one is probably incorrect? Hard to see, but it looks like the front mount is loose in the last picture, but the diff is level. Would it be correct in assuming that if you bolted up the front mount it would jack the diff up by about 2-3 inches?

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i dont have the front mount bolted up at this point, if i jack up the front of the diff to bolt it in its twists the rear bar. dont have the stock strap upper for the front any more, its a steel brace with rubber insert. i guess ill try putting the stock bar in and see if it works. i measred the stud height on the 2 differentials and they are the same. ill update as soon as i find out more. 

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 dont have the stock strap upper for the front any more, its a steel brace with rubber insert

Seems like this might be your problem.  If you have a factory stock rubber mount try that before swapping bars.  And take some decent pictures of the front mount with light.  Your pictures are terrible.

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I wonder if you stacked the weight on top of the mount or something of that nature.

 

http://enginewhisperer.com/wp/wp-content/uploads/2010/11/diffmountinstalled.jpg

 

^That should more or less be how the stock setup is. The mount should be fairly low. There is an early and a late cross bar, but if it came off your car I would imagine it should be the correct one.

 

I think on my car with the RT mount I just have about a 1/2 inch puck from ES that pushes up the nose of the diff from the bottom on top of the cross bar, it is mostly captured from up top by the RT mount.

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I have a couple of comments:

 

First, Have you considered lowering the differential by flipping the poly bushing in the mustache bar or by adding a spacer between the mustache bar and the frame.  I have the differential in my car lowered by about 1/2" using washers above the mustache bar.

 

Second,  how do you like the TTT Drop Mounts?  Are they aluminum?  I am contemplating getting either the TTT or the ArizonaZCar drop mounts.

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So I've taken out the sti diff and have it on my shop floor with the ttt mustachbar attached, sitting right next to the factory diff with the factory mustache bar. All the dimensions, height location of bushings are the same. However the factory bar has twist in it. With the ttt bar installed the front of the diff is way too low, if I force it up into position it tries to twist the aluminum bar,

post-53929-0-43436200-1482894183_thumb.jpg

post-53929-0-48800600-1482894208_thumb.jpg

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Way too low for what?  It's still not clear what you're trying to mount the nose of the diff to.  You said it was some sort of custom front diff mount.  Could be that the flexibility of the stock bar just allowed you to use a "wrong" diff mount in the front.

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It appears the the factory bar is either twisted from the factory or from wear. And the new ttt one doesn't allow for the nose of the diff to bolt to the body, I have an aftermarket upper front mount to replace the strap (picture attached). Like I said I can force the nose up into place but it puts a lot of stress on the ttt aluminum bar.

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It's still not clear what you're trying to mount the nose of the diff to.

 

Sounds like you're trying to force together two aftermarket parts that aren't compatible.  The aluminum bar was probably deigned for the factory front mount.  What you're describing suggests that your aftermarket upper front mount is incorrect for the application.

 

Is it so ugly that you can't show a picture?  Might explain the situation.

Edited by NewZed

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The front upper fits fine, and the diff site fine between the upper and lower front mounts. I've attached a picture of the front upper installed, and a picture of how far I have to push the nose of the diff up to bolt up the lower to the body when the ttt aluminum mustache bar is installed. With the stock steel bar the diff is angled up in the front and fits fine. At this point I would go back to the steel bar but it won't do around the new control arm drop mounts.

post-53929-0-63223600-1482898438_thumb.jpg

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I only see one shadowy picture of things that are hard to define.  Bad picture and only one.  I can tell that you don't have the crossmember installed though.  That would be a good clue of how far off the front mount is.  I attached a picture of a diff nose sitting in its stock mount (borrowed it from classiczcars.com).  Notice how close it is to the crossmember.

 

The rear bar and the front mount have to work together.  Obviously one of your parts has to go.  Pick one.  You seem too attached to that front mount, like you don't want to give it up.  Did you make it yourself?  If the T3 bar is right then the front mount isn't.  Pretty simple.  Good luck.

 

 

post-8864-0-94179800-1482903135_thumb.png

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Could also be that the car has been in an accident.  If you give the name of the company that made your front mount people could tell you if it should work with the T3 bar.

Edited by NewZed

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Wow, that's some serious modification. The height of the bars and holes are the same, the ttt bar just points the diff down further. I would prefer to not have to modify the lower Cross member like that. My front upper is a ron Tyler style mount from technoversions, the lower Cross member and lower mount are stock. The rubber bumper is trimmed to fit with the stock lower mount and crossmember.

Edited by Robfixhondas

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Don't overlook that one very important part of mounting the diff is that the angle of the diff's pinion shaft has to match the angle of the transmission's output shaft.  So that the u-joints of the propeller shaft will be in-phase, to avoid vibration.  You shouldn't be modifying anything until you know the angles are right.

 

The T3 bar shouldn't change the angle of the diff if it's designed to work with the stock mount.  And the Technoversion piece is well-known and seems to work well.  And, since you're using the Technoversion piece with a stock mount, you shouldn't really be worrying about it.  It's not really a mount the way that you're using it, it's just a snubber for the nose of the diff.  You could remove it entirely while you figure out how to make the stock factory mount work.  Then install the snubber.

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I understand how the front upper and lowers work and that the upper is just a bumper. And I understand pinion angle, that's why I thought at first that the ttt bar was too low in the rear until I compared them side by side and found the twist. The front bolts up fine, but the twist in the stock rear mustache bar apparently should not exist. I'm going to check the pins on the frame for the mustache bar, I'm assuming they should be square to the frame/ straight up and down? If they are bent or not at the proper angle that could explain why the stock bar is twisted and the new one is at the wrong angle.

Edited by Robfixhondas

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The stock factory mount on the crossmember is most likely right.  The factory mustache bar is flexible and it's mounted with very flexible rubber bushings.  You're replacing a bunch of sloppy flexible stuff with rigid parts.

 

You could just slot those urethane bushings to make those parts work together.  If the distances are correct, up and down, take a rat tail file and slot the holes to give the bolts room to let the nose move up.

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Mustache bar mount studs are supposed to be perpendicular to the frame.

 

The modification didn't make any sense to me either, as it should located it in the same place, looks and actually are can be deceiving. That is why one of the things this forum likes are numbers and dimensions. If anything the measurements will tell you how off you are, the stock bar can flex quite a bit under the weight and such, wanted the control arms at a JY and had a diff touching the ground while the car was jacked up only being held in by the mustache bar.

 

Clear pictures with a light source to illuminate the parts may show something that we are missing. If the car was running really loose or broken bushings for a while or like new zed says an accident could have twerked the mustache bar structure.

 

Did you round out the mount for the Technoversion kit? I know mine has about a 1/4-1/2 inch groove so that the ES mount from up top captures the curvature of the diff.

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The factory mustache bar is supposed to have that twist in it. Are you sure you have the correct T3 bar because they make 2 'stock' versions, one for the r180 and one for the r200? Also, have you tried bolting the original diff in with the T3 mustache bar, that will tell you right there if the bar is the problem.

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I've contacted t3 and they said the bar should have no twist to it. I do have the bar for an r180 and that's what I removed. I'm installing an r160 from a Subaru which has the same external measurements as the the 180. As far as I'm aware t3 only makes 2 different bars, an r180 or r200 style

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You say that the two diffs have the same external measurements. What really matters in this case is: What is the relationship between the location of the rear mounting studs compared to the location of the front mounting points, in an up and down dimension? My guess is that the two diffs are different in this respect and that's what's causing your twisting issues. If the dimensions on the Subie diff were the same as the Datsun diff, then it should bolt right in.

 

Mike Mileski

Tucson, AZ

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