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Old 08-21-2012, 09:58 AM   #1
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88 Formula firebird rear end or tires or something else help!!!

I have a 88 formula firebird with a 305 h/o tbi and a 5 speed tranny with a limited slip posi rear end. Please read entire post before replying. Here are my problem's

2 different problems

1. when I got the car I noticed that when I get above 72 mph there is a loud vibration coming from wither the rear end or drive shaft as it feels in the center of the car but I can only hear it at high speeds.


2. I switched to a bigger set of tires on the rear end and decided that those tires were affecting my mpg so bought 2 brand new 215/65r15's for the rear. my car has aluminum american racing rims on it and after i put these 2 new tires on when I am going down the road I keep hearing a loud flat tire sound. I took the tires to two different shops and they both tell my that there is nothing wrong. However when the car was jacked up they could pull the passanger side tire in about out about an inch and said I had a bad axle bearing.

I then took the car to a rear end ship and they took the rear end apart and said the pinion gear and spider gears looked okay and the axle bearings looked okay but I did need carrier bearings and a pinion bearing so i replaced them.

the problem is still there so here are some of my questions.

is it normal to be able to pull the tire in and out a little bit ?

what would be causing a helicopter noise/flat tire sound noise if not the tires?

The only other thing I can mention is that the casing over the spider gear/ring gear has teeth marks on it as noted by attached pictures.

would it be a bent axle? there is nothing on the driveshaft. please someone help.
Attached Images
File Type: jpg pn2.JPG (33.1 KB, 31 views)
File Type: jpg pinion1.JPG (92.4 KB, 28 views)
File Type: jpg pinion3.JPG (95.3 KB, 31 views)
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Old 08-21-2012, 11:55 AM   #2
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Re: 88 Formula firebird rear end or tires or something else help!!!

I had that EXACT problem, and those EXACT markings on my diff, just like yours.

Out in California, it was an 87 Formula with a VIN F engine and a 5 speed stick. The first problem was the trans mount and torque arm bushing at the trans. They were not ripped through, but they allowed a lot of movement. I replaced both with poly mounts.

The next problem was the rear end. You have two problems here and I am surprised that the rear end shop didn't catch them. The first problem is those marks. The match the end of the pinion gear and they happen when your pinion starts getting pushed into the rear end by the driveshaft. The problem is the factory crush sleeve on the pinion. You have to completely disassemble the axle, remove the crush sleeve, and replace it with a solid pinion spacer for a 7.625/7.5 10 bolt GM rear axle. I got mine for 11 bucks. Call Summit if you don't want to do a lot of searching for the cheapest one. They have them.

Your problem of the wheel moving is because the axle shaft is allowed to "walk" back and forth in the differential, not usually at the wheel bearing outer. The culprit is usually the c-clip is worn and then it pushes into the center pin and grinds it down. You replace the pin and the clip and you are back on the road.

Print this response out and take it with you to the rear end shop that screwed you. It may have been unintentional, but whatever the case the problems should have been corrected when they rebuilt your axle assembly.

Good luck!
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Old 08-21-2012, 11:59 AM   #3
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Re: 88 Formula firebird rear end or tires or something else help!!!

Do check your axle bearing when you have the shaft out, you may have to buy the one called a "bearing buddy" or something like that. Its kind of oversize and has a thing like a "speedy sleeve" that provides a new surface that your bearing can ride on. If it its walking back and forth that much, your bearing is eating the surface of the axle shaft too.
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Old 08-21-2012, 12:57 PM   #4
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Re: 88 Formula firebird rear end or tires or something else help!!!

when we put it back together with the new carrier bearings and new pinion bearings there is absolutely no slack.
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Old 08-21-2012, 02:31 PM   #5
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Re: 88 Formula firebird rear end or tires or something else help!!!

you may not feel it by hand. It really only happens under load when the pinion has input pressure.
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Old 08-21-2012, 02:34 PM   #6
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Re: 88 Formula firebird rear end or tires or something else help!!!

spoke with mechanic a couple of hours ago and he said that the crush bearing was fine.
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Old 08-21-2012, 02:45 PM   #7
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Re: 88 Formula firebird rear end or tires or something else help!!!

Don't come back crying when your rear diff fails. What was HIS explanation for the gear tooth marks through the center of the diff?

I'm speaking from experience with identical symptoms. Advice is free, so you can take it or leave it and be no worse off. Just trying to help.
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Old 08-21-2012, 02:48 PM   #8
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Re: 88 Formula firebird rear end or tires or something else help!!!

I hear what you are saying, but he said the teeth marks were because of the pinion bearing having way to much slack in it and the wrong shims.
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Old 08-21-2012, 03:05 PM   #9
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Re: 88 Formula firebird rear end or tires or something else help!!!

The reason it had the wrong shims in it is because they used to be the right shims along time ago. That spacer sleeve squishes down when you torque it down. Over time it actually collapses on its own, just a thousandth at a time, but eventually it gets enough slack that it gets loose and then gets beat back and forth against the differential.

Most GM rear ends have this problem, but our 7.5s from 3rd gens, 4th gens, and S-10s are the most common that fail this way. If you start doing a little research you'll see that its common. By your guy adding the shims its now back to OEM standards, FOR NOW. It won't take too long to come back out of spec though. Hopefully you don't experience a bad failure.

Just a word to the wise; always replace the crush sleeve with a solid pinion spacer on a GM rear axle. It just eliminates the headaches later on.

Good luck!
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Old 08-21-2012, 03:12 PM   #10
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Re: 88 Formula firebird rear end or tires or something else help!!!

I hear what you are saying and he could be pulling my leg not wanting to take it all back apart. all I know is that I have a noise of a helicopter sound/flat tire going the road at around 55. and if I get over 70 a totally different noise happens "audible vibration" extremly loud at 80.
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Old 08-21-2012, 03:20 PM   #11
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Re: 88 Formula firebird rear end or tires or something else help!!!

I am willing to do what needs to be done but I know very little about firebirds.And I have but a few tools and I am not very mechanically inclined
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Old 08-21-2012, 03:38 PM   #12
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Re: 88 Formula firebird rear end or tires or something else help!!!

The axle moves in and out because the Auburn differential that you have is worn out. It has nothing to do with the c clips or axle bearings. The axle gears move farther away from the center of the differnential as the cones wear. This creates more space between the end of the axle and the cross shaft in the differential, which allows the axle to move in and out. There is no way to rebuild the Auburn, so the only option is to replace it. I recommend the Eaton Posi or Yukon Dura Grip. These are both rebuildable and much better quality than the Auburn. When you do this you will notice that the axles will move very little.

The marks on the case are from the pinion bearings having wear and allowing the pinion to move when decellerating, or the pinion nut came loose at some time. This also has nothing to do with the crush spacer. The spacer does not hold the bearings in place. The spacer only allows you to preload the pinion nut. If there was no spacer in there then the pinion nut would have very little torque on it and it would constantly be coming loose. The only way that the bearings can move is if the nut comes loose or the bearings wear out.

The vibration that you have at 72 MPH could have been from the bearings that you replaced, but it is more likely to be from the drive shaft. A drive shaft is made to run below a certain RPM. Most stock drive shafts are made from thin tubing and 3 inch diameter or less. When they are ballanced they do this at a RPM that is below the RPM that the drive shaft distorts. When you run up to 72 MPH your drive shaft is above the RPM that it was designed to run. When your car was built the national speed limit was 55 MPH. The drive shaft worked fine at these speeds, but when you turn it fast enough it begins to bow in the middle and causes the vibration that you are feeling. A larger diameter shaft or thicker tubing shoudl cure the problem.

Last edited by big gear head; 08-21-2012 at 03:45 PM.
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Old 08-21-2012, 03:59 PM   #13
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Re: 88 Formula firebird rear end or tires or something else help!!!

All of that is very informative.

However, after suffering the same gear marks, and talking to a lot of racer's mechanics, I found out about the crush sleeve. When it collapses, this allows the torque to come off of the nut because its not held tight anymore against the bearings. The nut will make its way loose a bit more maybe, but mine was a lock nut and had loctite on it when I took it apart. It did not appear to have backed off, but the entire pinion was loose. After a retorque with the back cover off on a lift, the measurements came out that the pinion was now too deep in the hole. Only more shims would bring it back out to spec. That seemed weird to me and prompted my investigation.

I'm just telling you what I found, and what fixed it. I didn't do anything to the driveshaft but I did replace the torque arm mount and trans mount because they were starting to deteriorate anyway and seemed loose to me when I was pulling stuff on the lift. My grinding noise went away after the solid pinion spacer was installed. I will run one every time I tear down a 7.5 from now on. But maybe that's just me.
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Old 08-21-2012, 05:58 PM   #14
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Re: 88 Formula firebird rear end or tires or something else help!!!

I doubt that your problem was the spacer. It was probably the pinion nut. Think about this for a minute. When you crush the spacer it takes about 250 to 300 foot pounds of torque on the nut to do the job. If the spacer colapses any more after the rear end is in use then that 250 to 300 foot pounds of torque is now being applied to the pinion bearings instead of the spacer. They are not going to get looser, they are going to get much tighter. It is much more comon for a pinion nut to back off a little than it is for the spacer to cause any problems. I have been building rear ends for 32 years and I owned a rear end shop for 15 of those years. I have seen MANY pinion nuts back off, but I have never had a rear end come into my shop that failed because of the spacer. All pinion nuts are lock nuts, and all of the rear ends that I build have red Locktite on the threads when they leave my shop. I have had 3 pinion nuts back off on rear ends that I built, but I have never seen a spacer cause a failure. A spacer failure is going to cause the pinion bearings to have way too much preload on them, causing them to overheat and wear rapidly. If the pinion nut backs off it will make the pinion bearings (and pinion gear) loose.

I'm not saying that a solid spacer is not a good idea, but I haven't had any problems with the crush spacers. In some very fast drag cars a solid spacer might be needed, but I seriously doubt that a street driven 3rd gen Camaro is going to apply enough power to the rear end to cause a problem with the spacer.

Last edited by big gear head; 08-21-2012 at 06:04 PM.
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Old 08-22-2012, 07:45 AM   #15
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Re: 88 Formula firebird rear end or tires or something else help!!!

Update!! I went by yet another mechanic yesterday and they shook the passenger tire and you could hear the brake drum clanking and he told me that that is way too much in and out slack in the axle and I need either a new rear end or new axle.
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Old 08-22-2012, 07:52 AM   #16
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Re: 88 Formula firebird rear end or tires or something else help!!!

btw none of problems have been resolved despite replacing carrier bearings and pinion bearing. I will tell you that I have the vibration at 73 mph long before this other problem started and another mechanic had said it could possibly be an unbalanced drive shaft.

the helicopter sound has turned into a sound similar to that is a tire with a know on it or a bent rim but I took it to another mechanic yesterday and they went through the brand new tires again and checked the rims so that is not it. I found a used rear end for 150.00 but it is not a posi rear end and since I do not know how to begin I am stalled. Could it be a bent axle?


Do you guys know any test I can perform to eliminate what it may or may not be and I will video it and put the video up.
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Old 08-22-2012, 08:21 AM   #17
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Re: 88 Formula firebird rear end or tires or something else help!!!

Quote:
Originally Posted by KrisW View Post
All of that is very informative.

However, after suffering the same gear marks, and talking to a lot of racer's mechanics, I found out about the crush sleeve. When it collapses, this allows the torque to come off of the nut because its not held tight anymore against the bearings. The nut will make its way loose a bit more maybe, but mine was a lock nut and had loctite on it when I took it apart. It did not appear to have backed off, but the entire pinion was loose. After a retorque with the back cover off on a lift, the measurements came out that the pinion was now too deep in the hole. Only more shims would bring it back out to spec. That seemed weird to me and prompted my investigation..

Ok, the pinion shim is to set the depth of the pinion gear on the ring gear for proper pattern. On these rearends the shim is between the pinion head and the "big" pinion bearing. It really has nothing to do with the crushsleeve and bearing preload.
When the pinion nut is torqued it puts the proper "preload" on the bearings thu the crush sleeve. A crush sleeve failure is fairly rare. (altho I do recomend a solid sleeve on everything, It takes longet to setup but its worth it)
Its normally a install,backed off pinion nut after over torquing or a bearing related failure. Even with locktite and locking nut the P-nut can come loose.
When the Pinion nut backs off the pinion can move freelly back and forth which will cause it to hit the carrier.
I have built close to 300 rearends.

The OP sounds like you need a new posi unit, not a complete rearend. A inch of play is a bit much 4 sure.
It also sounds like your DS needs rebalanced but B4 you do that, put in new Ujoints. Depending on your gears, I recommend a aluminum DS, esp if you have 3.42-3.73-4.11 gears. The critical speed of the steel shaft is exceeded very quickly and starts to look like a jump rope at elevated driveline rpm. That and aluminum helps absorb driveline harmonics..

this is a dana 44 but its a similar setup other than the pressed on axle bearings and position of pinion shims.
Click the image to open in full size.
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Old 08-22-2012, 09:03 AM   #18
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Re: 88 Formula firebird rear end or tires or something else help!!!

Which would be better or cheaper a used rear end or new posi unit?
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Old 08-22-2012, 09:34 AM   #19
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Re: 88 Formula firebird rear end or tires or something else help!!!

Cheapest is a used rear axle that you take to your house and rebuild, then swap it into your car.

I have not built or setup 300 rear ends, more like 20, the majority of them being GM stuff. I have never had an after build failure (that I know of.) You can do this yourself, its not difficult. It is time consuming. You will need a good tech manual and some tools like dial indicators and such, but they can all be had over at harbor freight for not too much. The money you spend on manuals and tools you will save versus an hourly shop rate of labor that can really add up. You will always have the books and tools.

Lots of guys here, like the guys above who have built a gazillion of these already, can easily help out if you get stuck or don't understand something.
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Old 08-22-2012, 11:02 AM   #20
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Re: 88 Formula firebird rear end or tires or something else help!!!

http://www.nastyz28.com/forum/showthread.php?t=19773
Read this and see if you think you can do it. Setting up the 8.5 and 7.5 is the same, so this will work on either one. If you can do this then get a new Yukon Dura Grip, a set of bearings and shims and get started. If not then look around for another rear end. Finding one that is in better condition than the one you have might be a challenge.
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Old 08-22-2012, 11:15 AM   #21
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Re: 88 Formula firebird rear end or tires or something else help!!!

I would just replace the diff. by the time you find a replacement rear, pay for shipping or pickup,fix anything that may be wrong, you'll prolly be close to just buying the new diff.
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Old 08-22-2012, 01:10 PM   #22
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Re: 88 Formula firebird rear end or tires or something else help!!!

Question what was the factory stand gear ratio for this car was it 3.73
it has a limited slip posi with a 5 speed tranny. I might of found a used rear end out of a camaro but want to make sure.
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Old 08-22-2012, 02:14 PM   #23
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Re: 88 Formula firebird rear end or tires or something else help!!!

If you find a used rear end with the Auburn differential it's going to be worn out too. You wouldn't be any better off. The Auburn was used in almost all of the 3rd gen cars with the limited slip option. A few got the Governor Lock, which was worse than the Auburn. You would be better off just replacing yours with a new Dura Grip or Eaton Posi.

The factory gear ratio will be listed on the RPO decal on the bottom side of the console lid or on the inside of the spare tire cover. It will be a code that starts with a G. Code GU6 is a 3.42, GT4 is a 3.73 and all of the others are listed in the sticky.
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Old 08-22-2012, 02:17 PM   #24
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Re: 88 Formula firebird rear end or tires or something else help!!!

I really do not care if it is a posi rear end or not as long as it keeps me rolling
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Old 08-22-2012, 02:22 PM   #25
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Re: 88 Formula firebird rear end or tires or something else help!!!

If you just want to keep rolling, then get the Camaro rear and throw it in. You can change the speedo output stuff to bring the speedo back to correct.

Then you can tear yours apart in the garage and fix it the right way, taking your time. Put it back when its done.
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Old 08-22-2012, 02:23 PM   #26
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Re: 88 Formula firebird rear end or tires or something else help!!!

speedo output ? how do I do that?
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Old 08-22-2012, 02:31 PM   #27
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Re: 88 Formula firebird rear end or tires or something else help!!!

You remove the tailshaft housing of the transmission and change some little drive and driven gears to match the rear axle diff ratio to your speedometer. You might want to get those when you get the rear axle from the Camaro.

Most Camaros came with a cable drive speedometer, but Firebirds switched to electric in 86 I think. So the likelihood of you getting exactly what you need is iffy, but I would get them anyway just in case they will work.
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Old 08-22-2012, 02:33 PM   #28
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Re: 88 Formula firebird rear end or tires or something else help!!!

I think my speedo is cable because when I first take off in the morning it sticks and I hit the dash it starts working again.
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Old 08-22-2012, 02:37 PM   #29
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Re: 88 Formula firebird rear end or tires or something else help!!!

An 88 Firebird has an electronic speedo. I am SURE about your car, its the Camaro that is in question. I think they didn't switch to electric until 89 or 90, not sure.

Electric ones do get sticky, trip odometer stops sometimes, and all that jazz.
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Old 08-22-2012, 02:43 PM   #30
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Re: 88 Formula firebird rear end or tires or something else help!!!

gotcha, so If i go to a salvage yard to get a rear end do I need to get into a camaro tranny as wekk to get the gears i need ?
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Old 08-22-2012, 02:47 PM   #31
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Re: 88 Formula firebird rear end or tires or something else help!!!

Ideally, the speedo drive gears come from the SAME car you get the axle from.
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Old 08-22-2012, 02:50 PM   #32
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Re: 88 Formula firebird rear end or tires or something else help!!!

but I would need to get into the tranny to get them. correct?
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Old 08-22-2012, 02:59 PM   #33
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Re: 88 Formula firebird rear end or tires or something else help!!!

If this rear end is going to be a temporary swap until you get the other rear end back together then just leave the speedometer alone. You can use a GPS to find out how far off the speedometer is and just adjust your speed. Then get the other rear end fixed and back in the car.
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Old 08-22-2012, 03:02 PM   #34
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Re: 88 Formula firebird rear end or tires or something else help!!!

no if I do it then I am only doing it once
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Old 08-22-2012, 03:37 PM   #35
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Re: 88 Formula firebird rear end or tires or something else help!!!

Quote:
Originally Posted by Jeff f View Post
no if I do it then I am only doing it once
Do this then

http://www.summitracing.com/search/P...nd-Assemblies/
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Old 08-23-2012, 07:26 AM   #36
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Re: 88 Formula firebird rear end or tires or something else help!!!

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If I could afford a new rear end that would be an option.


Update while driving the car this morning I noticed that I was all over the road kinda like a busted belt in the tire.
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Old 11-11-2012, 02:58 PM   #37
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Re: 88 Formula firebird rear end or tires or something else help!!!

Jeff,

What did you end up doing with your differential and your vibration issues? I am having the same issue.
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Old 11-11-2012, 02:58 PM
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