Front Differential Mount Replacement

[JoelH]

Ok, my new Z runs great! It's a 72 and I just drove it 1500 miles home and it runs amazing but I did notice a few cosmetic items and one big "clunk" when shifting gears. It appears from searching the posts here this would most likely be the front diff mount needing replacement.

I haven't checked it under the car yet but I have reviewed the manuals and I can't find anything in detail about replacing this part.

Is it as easy as removing the crossmember below it and unbolting it from the front of the diff? I would assume I could suuport the diff with a floor jack but does anything else need to be removed? Is this pretty staright forward?

Thanks!

[Jeff G 78]

I just did mine on my '78 Friday night. Jack up the front and back of the car to give yourself plenty of working height. Support the rear of the car at the body just before the wheel opening (make sure the car body is not rotten!). Support the diff with a floor jack. Remove the NVH weight if equipped and then remove the nut and washers from the center of the diff mount. Unbolt and remove the crossmember. At this point, you only have the two vertical mount bolts to remove. They have a washer and nut above the diff that you have to hold. The way the mount is designed, you can't get to the bolts with a socket to remove them. Since the mount was bad anyway, I tore the two halves of the mount apart for easy access to the bolts, then removed them with an impact on the bolts and wrench on the nuts. That's it. Installing the new mount is a bit tougher since you have to use an open end wrench to hold the bolts while you tighten the nuts with a ratchet on top of the diff. I didn't try it, but an air ratchet might make this easier. There is very little room to work. I let the diff down as far as it would go (hits the rear sway bar in a 280Z) to give more clearance at the top. Once the mount is tight, jack the diff up and install the crossmember, center nut, and NVH weight.

The job took me a few hours total including several trips to the house for eye cleanings. Even with safety glasses, I got grit in the eyes.

The clunk is gone and the car accelerates smoother with the new mount.

[sblake01]

Might want to replace the 'belt-arrester differential torque' as Nissan calls it, while you're in there. They're usually broken or at least stretched to the point where they really don't do what they were designed to do.

Jeff: kudos to you for knowing the name of that item (NVH weight), few people do. Although it would be more a NVH 'damper' than a weight. For those of you that don't get my rambling, NVH damper=Noise/Vibration/Harshness damper. That's the metal thing that bolts to bottom of the differential crossmember. Most S30s I've seen don't even have them either because they've been lost in the shuffle over the years or never there to begin with.

[Jeff G 78]

Jeff: kudos to you for knowing the name of that item (NVH weight), few people do. Although it would be more a NVH 'damper' than a weight.

Hee, I had better know what it is as I am an automotive engineer . I was going to use 'NVH damper', but I wasn't sure he would know what I was talking about. I figured 'weight' was more obvious.

It's been a while since I had a 240, so I couldn't remember if all S30's have NVH dampers or not.

[sblake01]

Well that explains it because, IIRC that part doesn't show up in the microfiche and is only vaguely mentioned in FSM as a 'mounting damper' with no description. Years ago, my cousin, who is an airline mechanic, saw the one from my car while it was sitting on the bench and said "What's that, some kind of an NVH device"? Then he explained to me what that meant. I hadn't heard the term since then until you said it today.

[JoelH]

Might want to replace the 'belt-arrester differential torque' as Nissan calls it, while you're in there. They're usually broken or at least stretched to the point where they really don't do what they were designed to do.

I don't see that part on my drawings. Is that the piece that is above the differential?

[Jeff G 78]

Yep. It looks like an old leather belt.

[Arne]

I've found that removing the nuts from the long bolts at the nose of the diff can often be very hard with the diff in place. For me, it seems much simpler to drop the whole diff instead. Support the car as noted above. Then unbolt the half-shafts from the diff flanges, as well as the driveshaft at the pinion. Don't remove the driveshaft from the rear of the tranny, the gear oil will all drain out through the rear. Just support the driveshaft in place but out of the way. Then remove the two diff to mustache bar nuts, support the diff with a floor jack from the rear of the car and unbolt the cross-member from the body. Slowly let the jack down and forward.

Doing it this way allows everything to be torqued properly, and also is necessary if you plan to replace the upper strap.

[JoelH]

I ended up taking my car to a Z mechanic that just opened shop here a few months ago and he seems to be very good. I took my car there to be inspected the other day and we got it up on the lift and looked her over really well and she looks good!

I had him replace the front diff mount this morning and it still clunks. He took off the drive axles and they look good and the clunk seems to be coming from the rear center.

He thinks there seems to be some play in the diff. I'm looking at buying a used diff and letting him put it in. The car drives fine but I want it to be perfect I guess. I even read somewhere today that early Z's sometimes had diffs with play and they clunked a little when new. This was noticed in some cars. Don't know if that's true but I can't believe someone would accept a clunk like this for 30+ years!

Anyway I'll probably replace the diff seeing how the insulator is new, the strap looks good, drive axles look good and rear mounts look good. He did have a bulletin for later Z's that said the stub axles could be a source of clunk but he thought the play in the diff was the more likely culprit.

Sound plausible?

[sblake01]

If your Z mechanic is 'very good' as you say, have him rebuild your differential. You have no way of knowing if a used one would be any better than the one you have now until you install it. When I had the noise in mine, it was more of a 'thud' than a 'clunk'. While up on jackstands, I could push the differential up off the broken insulator and stretched strap. Replacing the mount and the strap solved it on my car.

[JoelH]

He doesn't rebuild differentials. He said he would send that out to a shop that will rebuild it for $800. I know a used diff may not be better but I can probably pick a used one up for 25% of that price.

I examined the strap and it seemed very solid and in place and the insulator was replaced. With a pry bar there was little to no movement anywhere in the rear end.

[d240zx2]

I had a "clunk" in mine early on during my restification process. Found it the diff-to-moustache bar mounting bolts had loosened. Tightened them up properly and no "clunk" since. Maybe a cheap way to end your problem.

Frank