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Can't Find Source Of Clunking In The Rear- Driving Me Crazy

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symptom is - sudden throttle movements cause clunking from somewhere in the rear.

going over bumps DOES not cause clunking

gradual application of throttle is fine

1. all rubbers and mounts are brand new

2. u joins have been replaced and all free of slop

3. checked all the bolts

4. diff strap is in place

5. rotated the wheels with car on stands and a bit of slop in the differential, but nothing out of the ordinary

6. exhaust and exhaust mounts all new

what the hell could it be?

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Always hard to diagnose over a forum, but i'd be very confident its the diff.
What are the mustache bar mounts like? If they are worn or soft they could be allowing the mustache bar to move up or down on the big rubber chassis mounts under load.
You should be able to put a jack under the diff (while its on stands) and lift the weight up to see how the bushes move under chassis weight and free hanging, should give you an idea of how soft they are and roughly how far they'll compress under load (about half as much as when they droop)
you say the diff strap is in place, but is there room for the nose of the diff to move under sudden load? (quick acceleration)
Id imagine that when you give it a boot it's loading the diff nose upwards and stretching the strap, then when it unloads it comes down with a clunk.
Maybe you could temporarily secure it with a ratchet strap or something jammed betwwen the nose and mount to take up freeplay just to diagnose the noise.

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1. diff mount + moustache rubbers are all new

2.have jacked up car by the diff front nose and no movement (shdn;t really do that probably),

3. married to the girlfriend, so figured she's not under the spare tire

 i can try the strap trick just to be sure

other then mounting a go pro underneath ,

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1. diff mount + moustache rubbers are all new

2.have jacked up car by the diff front nose and no movement (shdn;t really do that probably),

3. married to the girlfriend, so figured she's not under the spare tire

 i can try the strap trick just to be sure

other then mounting a go pro underneath ,

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I have the same issue and my assumption is the front diff strap is cactus. I'm moving to an RT style top mounted bumpstop and also replacing the moustache bar bushes for good measure.

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Stock rear drums and no brake pulsing, I only get it when I nail the throttle, lift after heavy acceleration or over *some* bumps

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I got it too. Have spent hours diagnosing with no definitive answer.

Please let me know when you find it.

Go Pro is prolly my next step.

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I know my car's rear end is a bit non-standard and I have the Wilwood brakes, etc, but I chased a similar sound for months - even swapped the diff.

 

I simply never considered the brakes as it occurred after hard acceleration without touching the pedal. As Cozza knows, I now believe it to have been an issue with the brake calliper/pistons/pads - probably not bedded-in in some fashion. After removing the rear wheels and sitting the LCAs on Cossa's hoist, engaging second gear and gently applying the brakes for 10 minutes, the problem went away

 

Try hard acceleration while applying light pressure on the brake pedal and see what happens.

 

I know this sound completely absurd but I hate to see folks going through the mental anguish I went through, so just give it a try.  

Edited by PeterAllen

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my other theory is the exhaust pipe banging on the cross member where there's the circulat cutout cutout?

http://whiteheadperformance.com/wp-content/uploads/2013/01/1971-Datsun-240Z-Restomod-Front-Diff-Xmember.jpg( not my car)

how much clearance do people have between the pipe and the cross member?

is it meant be handing pretty low or right up against the

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Temporarily clamp/ziptie a bit of rubber sheet piece, to crossmember, to insulate.

Go for drive, if noise has changed,/gone, then yes its exhaust hitting crossmember,

 

Do one test at a time, or you wont know which it is.

 

Jack up rear of car, release handbrake, put car in gear.

Rotate one tyre, back/forth. Clunking? Excess CW&P backlash.

Remove diff, send it to a diff/box shop to have backlash adjusted out.

You still need some, to allow for growth under heat, talking a few thou's

 

No, you cant do that at home, you need dial guage, and possibly a new pinion crush tube. They may also report it needs re-lapping, if teeth wear is bad.

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...

Jack up rear of car, release handbrake, put car in gear.

Rotate one tyre, back/forth. Clunking? Excess CW&P backlash.

Remove diff, send it to a diff/box shop to have backlash adjusted out.

You still need some, to allow for growth under heat, talking a few thou's

 

No, you cant do that at home, you need dial guage, and possibly a new pinion crush tube. They may also report it needs re-lapping, if teeth wear is bad.

 

Be careful here. You would be assessing 'play' from the clutch through to the wheel bearing using this technique.

 

To check the diff, grab the pinion input flange with one hand and output flange with your other hand, and rock back and forth. You need to check individual components not the system. 

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I'm on the diff backlash bandwagon. I have the same issue, and have new bushes throughout, new uni joints in the driveshafts and tailshaft, and the RT diff mount.

 

Noise became a fair bit worse with the increased NVH from the poly bushes, and I can see/feel play when I perform the same test described by dat2kman. If you move the wheel back and forth and watch what play you have before the input pinion moves it gives you a fair idea of what play there is in the diff.

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thank to everyone for the suggestions, hopefully its knowledge for all

1. followed the suggestion ( kato's?) to place a some rubber on the exhaust pipe where it meets the cross member. no difference, other than numbnuts did not remove at the end of the experiment and was then driving around with a burnt rubber smell. so dead end there 

2. so took the car for the RWC and the guy found it.

ready?

ok,

1. here's the diff mount i got from zcardepot

https://zcardepot.com/catalog/product/view/id/336/s/differential-mount-insulator-oem-240z-260z-280z-71-78.html/

standard nissan issue

2. its not damaged in the car, just like i said

3. however, instead of sitting symmetrical on the cross member like this

http://woodworkerb.com/wp-content/uploads/2014/11/Figure-RA-3.png

its stretched tight on the front and compressed at the back to the point that the mount lips hits the cross member when the diff moves

4. i have the straight rear cross member like this

https://c1.staticflickr.com/1/407/19369581915_445221a6b8_o.jpg

5. stewart wilkins ( god bless that guy) from sw motorsports tells me that i need an EARLIER version of the mount - that no one makes anymore - so that the diff sits more forward

6. i think this is right because lots of the suppliers list a 72-78 mount and not 70-71

55415-E4102 - this is the early mount

55415-N4300/4301 - this is generic 280z one that i have

7. from what i can see, the difference seems to around 5mm between the two in regard to the bolt centres

 

so ........ question is:

 

A. have i got the right end of the stick

B. surely someone has done this before - is it a matter of ovaling the holes on the mount or just turning it around?

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here it is

https://www.flickr.com/photos/62957688@N08/36574132803/in/dateposted-public/

skinny end first

https://www.flickr.com/photos/62957688@N08/37386129365/in/dateposted-public/

for the mount, the centre to centre distance for the holes is apporx 12 mm. i figure the ser 1 shd be 17 to move the diff forward

does anyone have a ser 1 to measure?

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ok, made some more progress

have concluded that the problem are the strap brackets - they're "skinny" in ser 1 cars and the startp is unable to move backwards to suit the move of the diff

so my choices are

1. i have to find a ser 1 mount or have it rebonded and the diff stays in its original position

2. i keep the existing mount, move the diff backwards, get the curved cross member AND mod the strap brackets

3. i hack up the later mount to move the holes - maybe weld on a 5mm plate

try my luck in the wanted section

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for #3, it just doesn;t look feasible - the centre to centre difference is a whopping 2.5 cm, and the stampted shape is very different

welding a brackets would just introduce massive stress points. looking at the chunky M12 bolts, looks like this area takes a beating

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Not sure if this is relevant however, maybe as the 240z cross member was not as "deep" (36mm vs 43.2mm) as the 260z model then the later ('75 onwards) bracket is hitting on the base of the crossmember?

 

From the Nissan Motor Co. 1973 Mods of Datsun 260z which outlines the changes from the 240z to the early 260z:

 

post-105219-0-17766200-1506233386_thumb.jpg

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that would be a factor for sure. as you can see from the photo, the edges of the mount are very close to the edge

what other info do you have in your magic book?

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