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JMortensen

The dreaded "diff" CLUNK

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Z cars are prone to clunking noises when they transition on and off power. There are many causes for the clunk, but the most common suspect that is almost never guilty is the differential itself. Time and time again we read about someone who "checks the backlash" in the differential by rocking the driveshaft back and forth, or jacks up one wheel and rocks it back and forth and thinks that they've found the source of the noise. Occasionally one of these people will take the diff out and wonder why they can't reproduce the clunk with the diff out of the car, or how it is possible that the backlash is in spec. Some more unlucky people go to the trouble of replacing the whole differential with a known good unit or worse, replacing all of the bearings in the differential, which is an expensive job, only to find out that the clunk is still there when they're all done.

 

Checking backlash with the diff installed is very very difficult; I would say impossible. If you grab the driveshaft and turn it back and forth, you're probably not going to be able to isolate the feel the backlash because of everything else moving, and you probably can't move the pinion enough to isolate the backlash without everything else also moving. First you have to overcome the resistance in the transmission, and by the time you do that and the shaft starts to move, you've already put too much force on the pinion to just move the pinion alone without also moving the ring gear. So when you move the driveshaft, what you're likely doing is moving the transmission gears until they lock solid, the driveshaft splines and u-joints, the pinion flange and pinion splines, the ring and pinion, the pinion gears and the cross pin shaft, the diff output stubs and the side gear splines, the CV joints or u-joints on the halfshafts, the stub axle and companion flange splines, wheel bearings, and any brake slack that might get taken up. It's not a good test of backlash, and is probably only good for finding a very big problem somewhere such as a u-joint that is about to fall out.

 

Likewise on the jacking one side of the car up and twisting the raised wheel back and forth. You're going to feel any and all slack from the wheel on one side to the wheel on the other. Slack could be coming from stub axle/companion flange splines for both sides, wheel bearings for both sides, CV or u-joints for both sides, output stub to side gear splines for both sides, and spider gears (and there is more slack in the spiders if it's an LSD). Some of this slack or gear lash is absolutely necessary. The transmission gears, ring and pinion, and spider gears in the diff MUST have lash, otherwise they'll burn up.

 

Even on a bench with the cover off and rocking the ring gear to check backlash you need to be a little careful. The 87 FSM says backlash on the R200 is .0051" to .0071", and getting the carrier to rock back and forth such a small amount without turning the pinion requires a bit of attention.

 

There are cases where diff backlash is a problem. There are many more cases (common enough drag racing issue that it shows up on the forums from time to time) where the cross pin shaft on the carrier wallows out its hole, and this leads to a larger amount of pinion rotation before the outputs begin to move.

 

More likely though, the ring and pinion is not the cause of your clunking. So what is? Well, it could be any number of things, but we can make a list of the most common issues pretty easily.

 

1. Diff strap/front diff mount. It’s hard to imagine that Nissan sold a million Z cars with this kluged design, but they did. The diff mount is below the front of the differential housing and the nose of the diff wants to lift when you apply power to it. The mount is rubber and tears pretty easily, so Nissan added a strap over the top of the diff to limit the movement. This is a less than satisfactory fix in practice, and over time or with added power the straps break and the mounts tear, and the nose of the diff can lift surprisingly far up. It can raise up so high that the driveshaft hits the transmission tunnel. Solutions: Ron Tyler’s diff mount (which he designed but other people are now producing), clamshell design diff mount addition where a bottom half is added to the diff mount which curves under the diff crossmember and prevents the mount from moving up, solid diff mounts also work but are generally not recommended because they put a lot of noise into the cabin and can cause stress at the crossmember causing it to break, and beefier replacement straps (usually metal or chain).

 

2. Mustache bar bushings. The diff hangs in the rear on the mustache bar. The mustache bar is insulated from the frame by rubber bushings, and they can wear out. These are typically replaced with polyurethane, which might solve a clunk but also transfers more diff noise into the car.

 

3. U-Joints. Worn driveshaft or halfshaft u-joints will definitely clunk on accel/decel. Replace as necessary. Just as checking backlash is difficult with the diff installed, I find checking u-joints on installed shafts damn near impossible. I have a friend who swears he can do it, but I always take the shaft out.

 

4. Control arm bushings. If the control arm bushings are worn out, they can allow movement which can cause a clunk. They are a royal pain in the ass to replace. Search: “spindle pin†for more details.

 

5. Loose bolts. A lot of these clunks are caused by the two big nuts that hold the diff to the mustache bar coming loose, or loose driveshaft bolts. Also check diff cover bolts and suspension mounting bolts.

 

 

 

 

If you have a clunk and you’ve gotten to this point and haven’t found the source, you’re into the weird stuff. At this point you need to work your way through the whole driveline looking for problems. Look for loose splines on drive shafts and axles, bad driveline mounts, anything. Arne over at www.classiczcars.com even reported that his clunk turned out to be caused by a woodruff key inside the transmission. “Mine was in the transmission itself. The woodruff keys that locate the drive gear on the front of the countershaft were worn, allowing the the gear to move on the countershaft. Clunk once under forward torque, and thud back under the reverse. Sounded for all the world like the clunk was coming from much farther back.â€

 

Dealing with a clunk is like pulling the spindle pins on the rear control arms. It's a not so glamourous rite of passage that most of us go through. I'm hoping others will add their oddball clunk causes as well.

Edited by JMortensen

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Good info, Jon.

 

I will say from experience that if you do have a clunk, you might want to take the diff out and check the front mount before trying to launch the car at an autox. Or you could be in for some undercarriage damage and further embarrassment. Or so a 'friend of mine' tells me. As a matter of fact, I would say that if you do not know that the diff mount was recently replaced it's probably a good idea to just go ahead and replace it. It's cheap and not too terribly difficult. Also gives you a chance to do a little further trouble shooting while you are at it.

 

 

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Dealing with a clunk is like pulling the spindle pins on the rear control arms. It's a not so glamourous rite of passage that most of us go through.

 

So very, very true. I remember the struggle I had two years ago with the spindle pins, it was a good learning experience.

 

Thanks for taking the time to do the write-up. All of this is very, very true. I just went ahead and did my own take on an RT style diff mount, brand new poly bushings everywhere and new u-joints. Tightened everything up nice and all the horrible noises went away.

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Jon,

I have been chasing clunk issues for a while and with some success. Recently I had found that vmy gland nuts were bottoming out on the threads before putting adequate pressure on the shocks. This was more of a clunk or rattle when the car was put in a twisting motion over bumps.

 

I am still fighting an issue and I think I know what it is--see if you would concur.

I made my own RT mount from drawings offered on this site. I think my RT mount has shoved the diff to far back and there is hardly any clearance around the rear control arm inner brackets that mount to the chassis. I have heard that the moustache bar actually twists a bit under torque-is this true? If so I can about guarantee this is my clunk issue.

Also, shouldn't the halfshafts be a true 90 degree angle from the diff or where? Putting a square against my rotor shows the diff being pushed back back a few degrees from 90.

 

What I have done:

checked tightness of bolts

new rubber mount bushings moustache bar

Rt mount

Made a strap for over the tranny mount. Old mount had a lot of slop.

Everything in the rear suspension has been replaced or rebuilt,ie; spindle bolts, urethane bushings

Edited by madkaw

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I don't know if the halfshafts ride at a true 90 degrees from the diff. It's pretty close, but I've never put a square on one like you have done.

 

As to the mustache bar, yes, it twists. In fact I've seen a few instances of people who have broken the front mount and strap and had the nose of an R180 diff lift all the way up until the U joint was hitting the bottom of the trans tunnel. The R200 mustache bar is quite a bit thicker, but I think there is a considerable amount of flexing going on there too.

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Good read thank you. Its making me think maybe my diff. on the 78 R200 isnt broken. I had a random clunk going on when then car was not under heart transplant and it was very random driving in a straight line on the highway steady rpm and speed then out of nowhere clunk sounds like a rock or a piece of wood hit the bottom rear of the car. I have all poly bushing back their and the strap was intact when I dissmenteled the car for sandblast nothing seemed broken. Still have to check back lash and the rest but it always seemed weird that it wasnt while under accel or decel.

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Installed Poly Urethane Mustache Bar bushings on the car and I do hear a little more noise coming from the differential as said in the number 2 point. So all cars that install Poly Urethane bushing on Mustache bar will hear a little more noise? I don't hear a lot more than before, but I can tell there has been a little more differential noise.  

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I'm finding my welded R180 will rock back and forth at the two studs that secure it to the mustache bar, even on jackstands at an idle. It seems the holes in the mustache bar are pounded out, or the studs should have a wider collar on them to prevent this. Tightened them up as snug as I could last night, will see what happens over time. If this is just a fiction fit sort of design, I think I'll make a modification or two. I still have relatively large amounts of play with the on/off throttle. Suspension is new, bushings are new, tranni mount is new, diff mount is a new RT style sandwiched against a prothane bushing. Wheels, tires, brakes, all new. Diff does not move at all if raised with a jack, and I can't physically move it side to side or otherwise by hand. 

 

Driveshaft front ujoint has significant play though...

 

Is it normal to have lateral play in the mustache bar?

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. Tightened them up as snug as I could last night, 

 

Is it normal to have lateral play in the mustache bar?

The torque spec. is 54 to 69 ft-lbs.  Not enormous but more than snug.  

 

The only play should be what the bushings allow.

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You were clear.  It's a self locking nut on a stud with either a washer under it or a flanged clamping area.  Designed for zero movement once it's torqued to the proper spec., locking the bar to the diff cover.  The size of the hole is not a big factor unless it's so wallowed out that the nut and washer is slipping inside.

 

Take it apart and remove any burrs on the clamping area, clean all grease/dirt/oil from those areas, including the flat area on the diff cover.  Put it all back together and torque it down.  A clean, dry surface with good surface contact and it shouldn't move at all.  Burrs, dirt, defects, with lubrication, will allow it to move and the nut loosen.

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Anyone know why my diff may be torquing down and striking the diff mount...? It has chewed out a good piece of the diff mount.

 

This happened after I launched the car with new Bridgestone Potenza RE-11A's. Since I have put new half shafts on and a new drive shaft and a new diff mount.

Here is a thread I started many months ago and myself, many forum users, and mechanics can't figure it out. 

 

 

 

http://www.zcar.com/forum/10-70-83-tech-discussion-forum/357305-death-clunk-knocking-4.html

Edited by JM280z

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Anyone know why my diff may be torquing down and striking the diff mount...

 

Since I have put new half shafts on and a new drive shaft and a new diff mount.

What goes up usually comes down.  New diff mounts actually have a little bump of rubber in between the metal pieces to absorb the shock,

 

The stuff after "Since" doesn't quite make sense.

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You were clear.  It's a self locking nut on a stud with either a washer under it or a flanged clamping area.  Designed for zero movement once it's torqued to the proper spec., locking the bar to the diff cover.  The size of the hole is not a big factor unless it's so wallowed out that the nut and washer is slipping inside.

 

Take it apart and remove any burrs on the clamping area, clean all grease/dirt/oil from those areas, including the flat area on the diff cover.  Put it all back together and torque it down.  A clean, dry surface with good surface contact and it shouldn't move at all.  Burrs, dirt, defects, with lubrication, will allow it to move and the nut loosen.

 

A proper designed bolt/nut clamped at the required torque (i.e. use a torque wrench if possible) should be designed to have enough clamping force to hold the mustache bar to the diff tight. The shape/size of the hole doesn't matter as long as it does not interfere with the clamping surface (flange of the nut in our case)

The friction between the mustache bar, the diff and the nut matters. I second your advice about making sure there's no grease, dirt or defects in between parts.

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What goes up usually comes down.  New diff mounts actually have a little bump of rubber in between the metal pieces to absorb the shock,

 

The stuff after "Since" doesn't quite make sense.

"Since" meaning,  once the problem started I have put on new diff mount, half shafts, and new drive shaft. 

 

A substantial piece of the diff mount has been grinded out by the flanges of the half shaft. During downshifts and reverse only. I can floor the car with no problem. 

 

All bolts are solid, so I think. What could be allowing this...? Could the diff itself be screwed up...? 

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