Loud, sharp thud after differential install

[pridemore54]

Hey guys I just got my diff in and the car back on the road.

Something new that wasn't there before is that driveline rear end thud sound after getting on the gas and taking up what must be some new slop. It does it both getting on and off the gas.

The only thing that should be new is the FRPP diff cover. Any idea what I did to create this? It's pretty bad. Makes me afraid to drive it.

I'm certain I put the driveshaft back the same. Torqued it to spec too.

 

[pridemore54]

Just got back from the initial drive. Rear end thud is so loud and sharp, I don't even want to drive it. It also does the thud both getting on and off the gas.

Something is wrong. Just trying to think of what it would be.

 

[PC03GT]

Just got back from the initial drive. Rear end thud is so loud and sharp, I don't even want to drive it. It also does the thud both getting on and off the gas.

Something is wrong. Just trying to think of what it would be.

Probably a loose bolt, I had the same thing on mine. The bolt on that hockey puck on the differential was just a hair loose

 

[pridemore54]

Probably a loose bolt, I had the same thing on mine. The bolt on that hockey puck on the differential was just a hair loose

What hockey puck? If you're referring to the dampner, mine doesn't have it. Didn't have it prior to this project

 

[PC03GT]

What hockey puck? If you're referring to the dampner, mine doesn't have it. Didn't have it prior to this project

Yes. Well I couldn't tell you then

 

[MG0h3]

Get back under there and check your work.

If you just changed the cover, it’s not like the backlash in the ring and pinion changed.

Did you get the FRPP case with the little brace studs on it? Just curious but that wouldn’t even matter.

Crossmember bolts loose? Diff mount loose?


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[98 svt]

You refilled it right?

 

[pridemore54]

Get back under there and check your work.

If you just changed the cover, it’s not like the backlash in the ring and pinion changed.

Did you get the FRPP case with the little brace studs on it? Just curious but that wouldn’t even matter.

Crossmember bolts loose? Diff mount loose?


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Yeah I obviously need to check all of my fastners, altho I was quite deliberate about going back and making sure everything was tight and/or torqued to spec.

Trying to think of what you mean by brace studs.

 

[pridemore54]

You refilled it right?

Fair point haha. Yeah it's filled properly. To the fill level plug in the middle of the diff cover

 

[Dave.O]

Recheck the front diff mounting bolts
One of my track cars did this when it spit one of the mm slotted spacers out
The resulting slop sounded like suspension bottoming out when getting on and off throttle
Next I would also the rear sway bar is firmly secured, had a bracket fail. Sounded similar


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[MG0h3]

Yeah I obviously need to check all of my fastners, altho I was quite deliberate about going back and making sure everything was tight and/or torqued to spec.

Trying to think of what you mean by brace studs.

The FRPP diff cover has a couple studs that go through the case and apply pressure on the bearing caps.


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[pridemore54]

Recheck the front diff mounting bolts
One of my track cars did this when it spit one of the mm slotted spacers out
The resulting slop sounded like suspension bottoming out when getting on and off throttle
Next I would also the rear sway bar is firmly secured, had a bracket fail. Sounded similar


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"Suspension bottoming out" actually is a lot like what it sounds like.

So I guess I'll start with those front diff mounting bolts. I made sure the spacers were there when the bolts went through the brace and to the top of the mounts

 

[pridemore54]

The FRPP diff cover has a couple studs that go through the case and apply pressure on the bearing caps.


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Oh those main hub load bolts? Yeah I torqued those to spec, then torqued the jam nuts to spec

 

[fitforspeed]

Hey guys I just got my diff in and the car back on the road.

Something new that wasn't there before is that driveline rear end thud sound after getting on the gas and taking up what must be some new slop. It does it both getting on and off the gas.

The only thing that should be new is the FRPP diff cover. Any idea what I did to create this? It's pretty bad. Makes me afraid to drive it.

I'm certain I put the driveshaft back the same. Torqued it to spec too.

Check the two large bolts that go through the rear of the cover. I think they attach the cover to the cradle iirc. It’s been quite a while back when I did mine but I remember having to go back and really crank those bolts down. I think the factory bolts may have also been to short and we used some larger/longer heavier duty ones. Once we torqued them super tight fixed the issue that sounds like what you are having. It’s pretty tight for room in there also but can be done.


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[fitforspeed]

Check the two large bolts that go through the rear of the cover. I think they attach the cover to the cradle iirc. It’s been quite a while back when I did mine but I remember having to go back and really crank those bolts down. I think the factory bolts may have also been to short and we used some larger/longer heavier duty ones. Once we torqued them super tight fixed the issue that sounds like what you are having. It’s pretty tight for room in there also but can be done.


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Just went and looked at the FTBR site and those two bolts I was referring to are the cover to diff mount. Says they are 60 mm long and use 80ft lbs. The rear diff to subframe bolt is 80 mm long and gets 90 ft lbs. I think the two bolts I used came from my previous Steeda diff brace I had on the car. I do not know the specific size though Steeda probably has that info if needed. It was like 10 years ago when I did mine so best I can remember. Hope it helps.


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[MG0h3]

Been a while for me too since I did the FTBR job.

Wonder if you can interchange those bolts accidentally.


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[pridemore54]

Just went and looked at the FTBR site and those two bolts I was referring to are the cover to diff mount. Says they are 60 mm long and use 80ft lbs. The rear diff to subframe bolt is 80 mm long and gets 90 ft lbs. I think the two bolts I used came from my previous Steeda diff brace I had on the car. I do not know the specific size though Steeda probably has that info if needed. It was like 10 years ago when I did mine so best I can remember. Hope it helps.


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Where does the rear diff to subframe bolt go in relation to the diff? I don't recall that one. I've got the two in the back, and the two in the front for sure

 

[pridemore54]

Check the two large bolts that go through the rear of the cover. I think they attach the cover to the cradle iirc. It’s been quite a while back when I did mine but I remember having to go back and really crank those bolts down. I think the factory bolts may have also been to short and we used some larger/longer heavier duty ones. Once we torqued them super tight fixed the issue that sounds like what you are having. It’s pretty tight for room in there also but can be done.


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I'm actually using the FTBR bolt kit cause my diff had a brace when I took it out.

Just found the last FTBR bolt hidden in some other stuff, which is that 80mm one you mentioned I believe. Please tell me I don't gotta take it back apart..

 

[fitforspeed]

I'm actually using the FTBR bolt kit cause my diff had a brace when I took it out.

Just found the last FTBR bolt hidden in some other stuff, which is that 80mm one you mentioned I believe. Please tell me I don't gotta take it back apart..

Sounds like you may have found your issue. The two 60mm bolts attach the rear cover to the diff mount. The 80 mm bolt attaches the diff mount to the subframe assembly. I think you know what you have to do from here.


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[pridemore54]

Update:

Got under the car to check the 4(?) diff mounting bolts. I was easily able to get another half turn or more on all 4.

My guess is after hooking everything up to the diff again, it took out some binding I didn't realize was there.

I'd like to test drive and see if the slop/thud is gone, but it's been raining here all day unfortunately.

Side note, I noticed my exhaust tips point downwards at an angle. Not sure how I managed to do that. Gonna see what I can do to correct that if anyone has advice.