To start, i just recently replaced my open 2.56 rear end with a 3.91 posi SL stamped unit from a 69 hurst car. I am by no means a rear end tech and this is my first posi (how sweet huh). I swapped bearings and seals, cleaned out all the old fluid and filled it up with 80-90w with a bottle of posi additivel. Coming up on 300 miles now and I notice an intense "shudder" and/or vibration if I start out with the wheels are turned. It happens left and right, forward and reverse but really only at very slow tight turns. I did get on it around a corner and i heard a strange screetchy, howling noise but only for a second. I am not sure if i am a worry wart.........or something is about to grenade. Any input would be of great value. I was going to just PM 507olds but figured this way it may help other enthusiasts as well. Thanks all........Cheers.

 

my Merc with 4.10s does that all the time at low speeds. The 3.42s and 3.23s in the 442s seldom. Maybe its because of your steep 3.91 (and my 4.10s). I'd be curious to hear as well

 

Did you add enough of the posi fluid?? Sometimes it takes just a bit more.

 

I have not yet, i only put 1 full tube in at first. I dont know if you can over do it with the additive (again first timer assumption)

 

I had the same problem in my 68 442. I just added some more posi additive. Make sure when you add it to take the car into a parking lot or something and do some slow circles left and right to allow the additive to get into the gears.

 

Well if it wont hurt to add more in, i suppose i know what ill do after work now! Anyone want to go drive circles with me?

 

My '68 4-4-2 did that for years - from the factory in 1968 to 1981 (my parents bought the car when it was new). It has just a 3.08 posi but the shudder was very noticeable at low speed tight turns to the right or left. Starting in mid-1981, the car then sat dormant for 26 years. We took the differential apart in 2007, noted no extraordinary or undue wear, cleaned it up and put it back together again with the recommended amount of posi additive. It has never shuddered in tight turns from the time the restoration was complete in the summer of 2007. I don't know what the difference was between the early years and now, but the shuddering is gone.

Randy C.

 

I have a 12 bolt Chevy rear with 3.90 gears in my 70 Cutlass and my car hops during corner turns. The reason why is because one side wheel tries to catch up to the speed of the other wheel. Being its a posi rear is like the rear axles are locked together. So when you make a corner turn, one wheel is slower because of the sharp turn while the other wheel tries to catch up to the speed of the slower wheel. Its not that you didnt add enough posi additive. One bottle is enough. Its the way how the posi rear works.

 

I agree with ELY442. My 68 Cutlass built with a Chev type posi with 4.10's hops, shudders or chirps the tires if you have the steering wheel turned hard right or left while moving slowly. It did that when I bought it in 1998 and still does after changing the fuid last summer. I was always told that it was normal. Like ELY442 said it happens because both wheels are locked together and when turning they travel different distances. One tire will have to slip on the pavement.

Adam, not an expert, but wish I was. :-)

 

Sounds like its pretty common then? If thats the case I feel alot more at ease. I have the basic understanding of different posi units and such, i just didnt know how it would act. I will just keep driving her the way i do and let you know when i blow it up Just kidding....or so i hope.

 

Since GM took the whale oil out of the additive in the mid-70's, seems like they all develope this problem!
Best stuff - bar none!
The reason why I always stop at garage sales that have automotive parts - you never know what you'll find!

 

If it gets worse do remove the unit and go through the clutches there could be issues. Being that it sounds like the posi unit installed was not driven for a while the clutches could have gotten some rust area on the clutch pack making it grip more than normal. Change the fluid and readd fresh with additive and see if that does not calm it down some.


In a way you should be happen for some chatter as that means the posi is actually trying to stay locked up.

Another thing is some could have re shimmed the unit a bit on the tight side for more performance you can check this again with a disassemble of the unit and measuring the pre-load shim plate thickness pack. more than .100 will be pretty tight. .075-.095 is pretty normal for new clutch set-up

Good luck on the noise. Take care, Jim

JD