Tracking Down The Clunk - Worn Lower Diff Mount?

[Hardway]

I am currently in the middle of trying to pinpoint the rear clunk I am getting from my '71 240z.  I recently installed a GM transmission mount that replaced the trimmed cone bump stop in my RT diff mount bracket.  I did not re-install the lower diff mount as many owners have claimed not to need it once they moved to this setup.  Additionally, there does not appear to be enough room for the old mount.  Unfortunately the clunk is still there but I wanted to get opinions on the state of my old lower diff mount as I do not think I will be keeping the GM trans. mount due to the head of upper bolt greatly interfering with the parking brake cable mechanism.  Looking at the condition on the surface the mount does not appear to have very many miles on it.  It is a true Nissan part but I am curious, should the lower plate and upper plate of the mount be that close together or should there be more space between the two?  Using my digital calipers in the middle of the mount, the distance from the outer edge of each plate is 25.15mm.  Also, the rubber you see between the two plates in the middle is just flashing when the other rubber blocks were were molded in to the mount.  If they come in contact with each other you should definitely hear metal on metal. Does anyone have a new lower diff mount sitting around they can provide some photos and measurements of?  Everything I see online does not have a good side shot of the mount.  Thanks!

[Zed Head]

There's a measurement shown n the Rear Axle chapter.  9 mm and 2 mm.  The instructions are kind of odd.  I think that they're trying to describe a range.

Once you get past the diff mount and the u-joints then every other rubber part is suspect, along with bearings and nuts and bolts.  Somebody recently described how their sway bar bushings were causing a clunk.  Loose strut inserts are fairly common.  Maybe your diff is worn out, they're durable but it happens.  Someone described the small bushing in the hub casting being completely disintegrated, causing a clunk.  Many possibilities.

[Patcon]

Have you tried just jacking the car up and rotating a rear wheel to look for where the slack is?

[madkaw]

I did that set up and thought it worked out pretty well, but I had driveline vibration that I couldn't figure out and thought it was the cause since it lowers the pinion .

i went back to the mount sandwich - Techno mount snubber and stock mount.

make sure no twisting motion allows those bolt heads to contact the crossmember since they are so close .

I now have just learned to live with the seldom clunk I get. It seems the harder I drive the car the less clunk I get. 

[Hardway]

Thanks for the info guys.  I know my strut inserts are good as I did them not that along ago.  I double checked to ensure they were seated properly and allowed no movement with the gland nut installed.  I have a lift but I cannot put enough rotational torque on the driveshaft to simulate the load needed to move whatever is being moved.  With the previous "sandwich" setup I tried moving everything I could using my adjustable safety stand.  Everything seems rock solid.  I am debating on removing the webbed bushing from the mustache bar and install some poly bushings.  I have some extra Prothane snubber bushings so when I go back to the sandwich setup, I may cut less off so the diff is tighter in the sandwich.  Wife is out of town this weekend so the Z will get plenty of attention.

[madkaw]

Know anyone with a 4 post lift. Drive it up on there and chock everything and put a load on the driveline, or just rock it in gear with someone watching

[Zed Head]

I thought that you had a picture of your mustache bar end bushings somewhere, but I can't find it (edit - found it, #10 here http://www.classiczcars.com/topic/58053-worn-transmission-mount-best-replacement-video-inside/ )  They looked kind of loose.  They can move many different ways.  (Had a different description here but the forum ate it).  Buy you can't swap n the urethane without destroying the rubber.  So that's a big decision.  I tested mine by getting some big washers at the hardware store and inserting them between the rubber stopper and the bar, to clamp the bar ends down.  It stops the up and down but the sideways motion is still possible.  But it did reduce the thump.

U-joints are really hard to diagnose.  You need to stick a large screwdriver in to the joint and reef it around.  Moving a wheel by hand or twisting the propeller shaft and looking for movement at the u-joint doesn't really cut it.  The joints are under very high loads in-use so you're not going to break them with a screwdriver.  Look for gaps at the seal, red rust dust, or just wiggliness.

 

Edited July 14, 2017 by Zed Head

[Namerow]

Thinking out loud here...

Has anyone ever checked to see whether any of the other, still-available Nissan steel-sleeved isolator bushings could be adapted to the moustache bar?  The make-or-break dimension would be the outside diameter of the external sleeve, which would need to be pretty much spot-on.  Length could be trimmed to suit (or two under-length bushes could be stacked and then cut to length).  Inside diameter of the inner sleeve would only need to be within 10%, I would think (could be sleeved up, but not the opposite). 

The S30 bushes that come to mind are the spindle pin bushes, the front control arm inboard pivot, and the transmission  mount.  I have all three on hand and can provide their measurements.  What I don't have are the the three key dimensions for the moustache bar bushings:

Length
Outside dia. of the outer sleeve
Inside dia. of the inner sleeve

Anybody?

Even if none of the other S30 bushes will work, there are hundreds of steel-sleeved bushings out there on the market.  Captain Obvious has shown an appetite for looking through online parts catalogs, so with the right dimensions in hand, maybe we (I mean, Captain Obvious ) can find a suitable replacement for the NLA Nissan parts.

As for the wavy-shaped rubber end washers, I think suitable facsimiles could be cobbled up pretty easily from thick rubber washers, using a small-diameter sanding drum in a Dremel.  Fiddly work, but would probably only take 30 minutes to make four.  An alternative fab strategy would be to drill a set of radial holes through a double-thickness rubber washer and then slice it in half, creating a pair of the desired washers in the process.  The biggest challenge here would be finding the right rubber washer to start with.

[Captain Obvious]

Haha!! You know me well, my friend! As a back burner project under @Patcon's guidance, I've been looking into this off and on for a while.

The most promising bushings I've seen are things like engine torque strut mounts. The upper bar they strap on transverse mounted engines to keep the top of the engine from wretching around as the engine works. if you can't find something that has the exact same size as the original bushing, I muse that you could use something that is a little smaller than the stock bushing, and press it into a thin wall cylinder of appropriate ID and OD. Make the length of the cylinder long enough that there's extra material to swage over the mustache bar.

Here's some pics of common cheap bushings that are probably large enough to consider. I bet with a caliper at a junkyard you ought to be able to turn up something .100 or so smaller than the original. There's got to be something out there. Little smaller OD than stock. Same or larger bolt through the middle. Length shouldn't matter so much. Here's some possible candidates. Next time I'm headed to the yard, I'm going to take my caliper:

73-79 Civic

Camry:
 

Excel:

Paseo:

 

[Namerow]

Happy hunting, CO!

Still hoping that someone can post those three measurements for the Nissan OE bushing so that we've got a reference point to work from...

[Patcon]

@Captain Obvious

Can you post up the dims from the NOS replacements?

[Zed Head]

There was a thread on a different forum about using two part urethane to pour your own engine mounts.  I commented that sufficient adhesion might be difficult to achieve, considering the shearing forces at the interface of "rubber" and steel, but this application seems about right for it.  If the material fails everything is captured so that no parts come loose, unlike the motor mounts.  Remove the rubber from the bar end, build a mold, and pour.

The inside of a mustache bar end will give the dimensions needed to find a replacement from another car.  I have one with the old sleeve still inside.  There are really two dimensions that you could consider, old sleeve in, or bare bar.