High speed Deceleration clunking/hard rattle 60-70mph

[Jmoyet]

I couldnt find anything that described what I have just the usual clunk at changing gears typical of a bad mount. 

I have an early 1971 Datsun 240z with a 4 speed manual built in 1/71. All original to my knowledge. Besides the clunk at accel and decel (front diff mount that is no longer in production) I also have a loud clunking/rattle when i let go of the gas over 60mph then goes away under 60mph. Sounds like somebody with 2 hammers hitting the bottom of the car rapidly. When I have it up on jack stands it does the same on 2nd and 4th gear at idle. its louder on the drivers side wheel for some reason.. Quieter without the wheels. when i hold the drivers side wheel and let the passenger side spin away (yes its an open diff) it still makes the same loud clank on the drivers same if i hold the passenger side wheel. still louder on the drivers side.. and also jerks at the wheel when holding it.. Bad Diff? 

[siteunseen]

Could it be the tranny insulator's rubber is worn or gone completely?  When accelerating or decelerating the transmission could be banging around.  Just a thought.  

I bought a new one from Nissan the other day, $65 plus tax.  P/N # 11320-N3000

[Zed Head]

2 hours ago, Jmoyet said:

When I have it up on jack stands it does the same on 2nd and 4th gear at idle. its louder on the drivers side wheel for some reason..

When the wheels are hanging the u-joints of the driveshafts/halfshafts have too much angle and will bind.  Clang, clang, clang as they pop around.  If you want to test it on jacks spread the load with some wood under the control arms, to load the suspension and get the drive shafts in a normal position.

If you want to just check u-joints, it's best to use a screwdriver to pry the joint around with a bright light so you can see any movement.  Also look for rust dust around the bearing cup seals.

 

Edited January 12, 2016 by Zed Head

[Jmoyet]

I have a transmission mount ready for install. I ordered a solid mount for the differential. Waiting for it to arrive now to see if it fits the early differential mount without swapping everything out of the 72 model into my early 71. But yeah, I get the normal clunk when changing gear, Accel, deceleration typical of bad mounts. But the heavy pounding rattle at above 60mph only at dice is strange. When I press on the gas again it goes away and let go of the gas and rattles like hell. I won't be able to work on it till next week on my days off.. Yeah it's a daily driver.

[Mark Maras]

 IMO, Check the drive shaft u-joints first. I've experienced the rattle induced by deceleration in an RX7. It was caused by a front u-joint that had a TINY bit of end play in it. The clunks? A really bad u-joint will do it but trans. and or rear end mounts are probably the culprit. I think if the clunk was coming from the ring and pinion, you'd have worse symptoms than a clunk.

[John Coffey]

Look at the e-brake linkage above the driveshaft.  If you see shiny spots then the front diff mount and diff strap are gone and the driveshaft is hitting the e-brake parts and the tunnel.

[jalexquijano]

I have the same problem on my 1972 240z. Same clunk noise after releasing the gas pedal. There is even a back and forth shaking if you accelerate hard and suddenly drop the gas pedal. Do you recommend buying a OEM Nissan Differential mount? What else should be swapped?

[Chickenman]

On 1/13/2016 at 9:27 PM, Jmoyet said:

I have a transmission mount ready for install. I ordered a solid mount for the differential. Waiting for it to arrive now to see if it fits the early differential mount without swapping everything out of the 72 model into my early 71.

Personally, I wouldn't go with a solid mount. They will transmit a lot more Noise, Vibration and Harmonics than a rubber mount, especially on a DD . Just my .02c

[jalexquijano]

who sells this solid mounts for a 1972 240z?

[jalexquijano]

Just now, jalexquijano said:

who sells this solid mounts for a 1972 240z?

Do they need to be OEM Nissan or are they aftermarket?

[Zed Head]

There are eight points of rubber contact holding the engine, transmission, and differential in place.  The two motor mounts, the central transmission mount (called rear engine mount by Nissan), the two ends of the transmission mount crossmember (not readily noticed), the front differential mount, and the two mustache bar ends.  They can all add some looseness to the whole system.  The tail of the transmission is really not very tightly held.  That's a spot for rotational vibration that might be hard to diagnose.  Lots more mass also, small movement = big effect.

[jalexquijano]

Tail of the transmission?

Symptom: clunk noise after releasing the gas pedal. There is even a back and forth shaking if you accelerate hard and suddenly drop the gas pedal. Any clue???