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What is the deal with solid diff mounts?

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so i just ordered a new steering rack from msa and i was going to order a solid diff mount BUT the sales guy told me that it would transmit ALOT of harshness back into the car, and that the solid mount was mainly for race cars... now i want to know what personal experiences are with going with a solid mount.. i'm thinking about going with the arizona z car mount or the msa mount some advice would be appreciated

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The noise will vary depending upon the condtion of your diff. Some have had worse experiences than others

 

The bigger problem is they will cause your front crossmember to crack and ultimately destroy itself.

 

http://forums.hybridz.org/showthread.php?t=108276

 

There are other threads where people have posted pictures of the destruction caused to their rear suspensions when the cracked cross member fails.

 

Not pretty.

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This is what you need to go along with the solid front mount. A solid rear mount.

billetrearmount.jpg

 

 

The solid front mount was designed to be used with a solid rear mount. The factory mustach bar is a spring and it bends into an S-shape. This will fatigue the crossmember and the lower chassis mounting points.

 

 

This is not my picture but one for the record.

torncrossmemberresults.jpg

 

lateraltearing2.jpg

 

lateraltearing1.jpg

 

The crossmember is not only the next weakest link but it will fatigue to the point that it will literally fall apart on it's own.

 

 

 

Here are some pics of a cracked but still holding together crossmember. This one was never subjected to more than stock power. The damn thing made all sorts of creaking and clanging noises before I gave everything a good look. This is what I found, cracks everwhere.

crackedcrossmembertop.jpg

 

crackedcrossmemberdetail1.jpg

 

crackedcrossmemberdetail2.jpg

 

crackedcrossmemberdetail3.jpg

 

crackedcrossmembertopcenter.jpg

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Another approach to this problem is the Ron Tyler mount.

 

Differentialmockupfront.jpg

 

ron_tyler_diff_mount_diag.gif

 

 

The R.T. mount was designed to lower the nose of the differential for the JTR V-8 swaps. You will need to adjust the dimensions to suit your engine tranny combo.

 

This picture shows that it lowers the differential right to the crossmember's top surface.

crossmembermockuprear.jpg

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Wow. VERY nice photos demonstrating the problem. I know I switched BACK to urethane from a solid mount (did not drive with it, only mounted it), after doing hours and hours of reading. I agree with Pop.

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Try a little searching. There is a sticky post in the drivetrain forum that has info on several different front diff mounts. You won't find a poly mount, and the stock system with the strap and mount underneath is REALLY crappy, so you might look hard at the alternatives instead, including the one that bjhines showed above.

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I just replaced my worn out stock diff. mount with...

 

A brand new stock diff mount.

 

I had ordered a solid steel mount but cancelled it when after doing

research and ascertaining that a solid steel one might be too noisy and as I have already lost the upper registers of my hearing range extra noise is

something I would like to avoid.

 

The weak point in the mount is the rubber which when it is new is not a problem, it is only after a long time of degradation that it fails, and really

after normal driving how often do these things fail due to that?

 

Every decade or so, if that?

 

I guess it depends what kind of driving one is doing, 'cause like they say;

 

"If you are not breaking anything you are not trying hard enough..."

 

______________________________________________________________

 

'76 280 Z-daily driver "Sometimes stock is good enough"

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You can take a stock one and make it solid with a couple small welds. You weld 1/4" stock between the outer and inner shell on the bottom by the crossmember.

 

The stock mount has a much larger surface area and wraps around the crossmember. This helps to disperse the load. Those solid metal pieces make for such a pinpointed load they tear up the crossmember.

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old school answer is to buy 8 feet of 1/8 or so plastic wrapped cable from local hardware store, and 2 clamps.

 

Wrap the cables under the front cross member, then up and over the top of the differential. Pull it tight with a claw hammer, and tighten it up.

 

You can use a shot front diff mount with this solution.

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What are the symptoms of a broken or worn Diff mount? I ask because I recently discovered a movement in the rear of the car that occurs when I accelerate or decelerate. It is more noticable on deceleration but is there on acceleration as well. When I let off the gas the car seems to slide out in the rear to the right. I have a Quafie rearend and Wolf racing cv joints. I recently ran sticky tires on the car as well.

 

Any help would be great

Jeff

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The picture below is an example of a bad diff mount. You can see that the rubber is seperating from the metal. The first time you really GET ON IT (floor it or drop the clutch) the rubber will completely seperate and the diff will smack the upper diff strap (located above the diff to "Arrest" the diff under diff mount breaking conditions.

 

You can tell if it's wearing out by dropping the four bolts that hold the crossmember and letting the diff hang by the mustache bar. While it's hanging, apply a trwisting motion to the crossmember while watching the rubber and metal area or the mount. (as shown in the picture)

 

Dave

DSCN0565_thumb.JPG

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OK, so here's my "Question of the Day" for any of you, Brad, BJHines, etc, etc. In the pictures on post #5, It shows 2 bolts holding the GM mount to the R/T mount. So why did my R/T mount only come with 1 hole? And which of the 3 holes on the GM poly mount should I use? Should I drill another hole? Wouldn't 2 bolts be better than one?

 

Some one? Anyone?

Dave

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You're mistaken. Two bolts hold the GM mount to the diff housing and one bolt attaches to the RT mount. You don't need to drill another hole and two bolts won't make anything better. The mount is in compression under load.

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I'm not mistaken, I'm misinformed and confused. There are 3 holes on top of the GM mount. Should I just use the middle one? I know the mount is in compression when the car is under load (forward motion) but what about if the car is in reverse and the only thing holding the diff up is 1 bolt?

 

Just trying to cover all the bases. For safety, clarity and peace of mind.

Dave

 

31951350230_medium.jpg

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I'm not mistaken, I'm misinformed and confused. There are 3 holes on top of the GM mount. Should I just use the middle one? I know the mount is in compression when the car is under load (forward motion) but what about if the car is in reverse and the only thing holding the diff up is 1 bolt?

You could drill another hole in the RT mount and use two holes if it makes you more comfortable, but here are a couple questions for you that may ease your mind. How much torque do you put to the pinion in reverse vs driving forward? Enough to cause that bolt to fail under tension? How many bolts hold the stock diff mount to the crossmember that's taking 100% of the torque under acceleration? What size is that bolt compared to the one on the GM mount?

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All good questions and I considered that after I hit the "Submit" button. I just like that "Bullet-Proof" idea before the wheels even hit the ground. I won't have that much HP to the wheels but it's always nice to know that it'll never fail under "Dumb ass" conditions.

 

Thanks again Jon,

Dave

 

P.S. That 79' 280ZX at the Arlington Pull-A-Part has a complete rear in it. (Diff, Mustache Bar, extra diff in the hatch area, etc, etc.) Just FYI. It's in Isle I-2 in the middle and the 78' 280Z is in Isle G-1 toward the back fence.

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FYI... driving in reverse isn't the highest torque load you can put on the diff mount. A bad downshift that locks the rear wheels puts a pretty good shock load on the mount. I've done that and not had any problems. Even if the mount fails the crossmember is still in place to keep anything bad from happening.

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I know, I just keep thinking of the stock mount and keep forgeting about the interlocking (failsafe) feature of the GM mount.

 

Thanks guys, I can never be too informed. The more answers I get, the more informed I am, the more info I can give when someone asks me. You get the idea.

 

Dave

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So here's my Progress so far. This is all on an early 260Z (2/74) I got the mount and POR15'd it after media blasting it to help the POR stick better. I went to go mount it and found that the 260Z's exhaust hanger is in the way. 4 spot welds, an air hammer and large mallet later, the hanger is gone and I'm ready to slip the RT in place.

 

Of coarse, the fuel lines and the rubber bracket are in the way. So I move the bracket forward and gently push the brake and fuel lines upward to clear the R/T. After that, the instal was smooth. Couple smacks with the plastic mallet to align the bolt holes and we're good to go. Had to lift the diff into place and ballance it on my shoulder while I was bolting it to the Mustache bar, then lifted the front to bolt up the poly to the R/T.

 

In the 4th pic, you can see that I tightened the Poly so much (to make sure it was tight) that the inner plate started to twist. I fixed it shortly after the picture was taken.

 

Here's the pics....

 

Dave.

DSCN0725_thumb.JPG

DSCN0728_thumb.JPG

DSCN0729_thumb.JPG

DSCN0731_thumb.JPG

DSCN0732_thumb.JPG

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