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Andrew Bayley

R/T Mount: how much clearance do I need between diff and X-member?

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I just installed my home-made R/T diff mount this evening and it seems like I still need to do some tweaking. :(

 

When I was originally bolting everything together, I noticed some interference issues with the mounting hardware on the differential against the cross member. I switched the direction of the bolt (goes up now instead of down) and removed the flat washer. This gave me about 1/4 inch of clearance between the mounting bolt heads and the cross member.

 

On the test drive, I was getting a loud banging noise under mild torque loads. Since this is the first time I've gotten this noise and the mount was the only thing I change, I can only assume that one of the bolt heads is hitting the cross member.

 

So, all the people who are successfully running this mount, how much clearance is between YOUR cross member and mounting bolts?

 

I'm thinking I might need to grind a little more material away from the bottom of the urethane mount. All I did to prepare the mount was grind the bottom portion of the mount so it was flush with the gold metal tabs. I'm thinking that maybe I should have grinded a little more.

 

Oh well, at least the side pipes I installed last summer make differential swaps / work TONS easier. :)

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To get my u-joint angles equal I had to lower the diff nose untill it actually interfered with the crossmember. Since the crossmember no longer takes any direct torque forces from the drivetrain (with the RT mount) I had no concerns about grinding away at the top of it to make clearance for the diff nose. I made myself about 1/2" clearance with the angle grinder and I have no more interference issues. Albeit, if I jump on it in reverse I can bounce the diff nose off the crossmember but no contact under nornal driving.

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Read my post about R/T install? Because I initially hung my bolts down, I ground a bit to clear bolts, less than 1/8 inch. Until this is a proven design, I had the clearance, though compressed, to use the stock mount as well. In my case the diff hung down slightly less than 5mm lower than my MSA solid, resulting in about 1 degree more difference in driveline angles. I originally had left the upper washer out of my mustache bar bushings, urehtane, and have the trans crossmember lowered under the chassis about 1/4". This was perfect for my solid mount < 1 degree.. Adding the upper washer will add more clearance to the rear crossmember and probably get me back to where I was with the solid mount for driveline phasing. I've launched mine pretty darn hard and have no interference, amazingly quiet, but occasionally slight contact with E brake linkage. I also have all new urethane bushings throughout. Probably don't need to mention, but If you have bad vibes, phasing can be done with either trans cross member, trans mount, or diff depending on clearance.

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Thanks for the response(s) guys.

 

The problem I’m getting is definitely an interference hit. Under normal street driving, everything is nice, quiet and smooth. However, under heavy torque loads, something is banging around down there. First gear is obviously the worst (as the banging goes).

 

After I originally put the mount in, I could easily get the differential to “thump†in all three gears. After the disappointing initial drive, I shaved a v-shaped “notch†into the polyurethane GM Transmission mount. Before I did this, there was a good ¼ inch of space between the mounting tab(s) and the differential (where the bolts feed through). After I “notched†the mount, the mounting tabs sit flush against the diff mounting surface.

 

Another thing I noticed was that the stock differential mounting bolts appear to be too long. Rather than using the stock flat washers, I simply used a couple spare lug nuts as nuts for the mounting bolts. As I torque’d down the bolts, I noticed that I was bottoming out the threads of the bolt before the mount was tightly fastened against the differential. A washer was fitted onto ONE of the bolts, but this pushed the mount too far over to use a washer on the second nut. Since I was in a rush to finish up, the second bolt was installed WITHOUT a spacing washer.

 

If one of the mounting bolts isn’t pulling the mount firmly against the differential (even though the other one is) would this be a cause of the loud “thumping†I’m hearing?

 

I’ve checked all the u-joints and they are all perfect. Also, while time for replacement bushings, the moustache bar and bushings are all still intact. As it sits, I’ve got about ¼ of inch clearance between the bolt heads for the differential mounting bolts and the original cross member. Is this enough?

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