change gears

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Well-known member
Sep 9, 2011
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chicago ridge,il
My Car
1972 mustang convertible
2008 ford edge
2006 cadillac cts
hi need help do i need to buy new seals,bearing and the rings, plust the new gears or can i just buy 3.50 gears and just change that,i have 2.75 gears can any one give me some advise thanks Ron

I am getting ready to put in 3.89 gears and also upgrade to a limited slip carrier. I also currently have a 2.75 rear end.

I have some noise coming from my rear end... I suspect it is a pinion bearing. I plan to replace all seals and bearings. They are 40 years old, and I definitely have a bad bearing in there somewhere. If I was you, I would replace everything.

Jegs has a complete kit that has all the seals and bearings and other things you will need.

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hi need help do i need to buy new seals,bearing and the rings, plust the new gears or can i just buy 3.50 gears and just change that,i have 2.75 gears can any one give me some advise thanks Ron
Gear choice is determined by what the goal is for the car, trans type, tire height, HP of the engine. What is the end goal for the car? weekend cruiser? dragstrip? frequent or long trips on the highway?

3:50 is a good general purpose gear but you might find that 3.25 or 3.89 or 4.11 might better suit your end goal for the car.

I replaced mine last year from stock 2.75 to posi 3.50 and last week I noticed some vibration coming as if the drive shaft was loose or something. I have new u-joints, and had the rear gears checked at the machine shop. Can anyone tell me if they have experienced anything like it, or know where to start? Vibration comes and goes at about 65-75 mph or more. Wheels and tires are new and balance from factory.


possible issue with bearing preload causing vibration. when the gears were changed was the case spread set correctly, and was the crush sleeve replaced and the preload on the pinion set correctly?

a lot of people just swap out gears and tighten things down till they think it is tight enough. there is more to it then that.

A lot of transmission shops that work on these don't even know what they are doing. most are familiar with setting up a chevy rear end which is very different from a ford setup.

The biggest mistake most transmission shops make is they reuse the crush sleeve on the pinion. a crush sleeve can only be used once, if you overshoot the torque spec on a crush sleeve, you have to take it all apart throw it out, replace it with another new one and start again. usually they will reuse a crush sleeve, torque it lower then original spec, and call it a day. 50 miles later the pinion shaft starts dancing around and you start to get all vibration noises at speeds over 35mph. the dancing around then basically ruins the gear set and the bearings.

additionally aftermarket gear sets (replacement gears) have completely different backspacing then OEM gears from 40 years ago. a lot of shops just assume the aftermarket gears for a ford 9" are manufactured to OEM spec. they are not, not even close usually. so when they do a gear swap, they reuse the same spacer on the pumpkin and then loosly setup the gear mesh that they can never get right because the spacer is now incorrect, they just get it close and call it a day. on a chevy the spacer is set on the pinion shaft, on a ford there is a spacer gasket that must be changed between the nose of the pumpkin and the body of the pumpkin. it can get involved because you need a spacer kit that is a special order and then unless you have a calibrated spacer tool(OTC) which is VERY rare and impossible to buy new today you basically have to setup the gears, see how they mesh with the grease and take the entire thing apart replace the spacer, reset the backspacing on the ring gear, reset the case spread and start again.

this eats up HOURS AND HOURS of labor for a transmission shop so they basically screw the customer and throw the chunk together and hope the customer doesn't notice the new noises from the rearend.

this is why there is a big market for Used OEM pinion and ring gear sets. ford setup all the OEM gears with the same spacing, so this allows a much faster swap where you can reuse the same spacers and come inside spec. the problem is a good used set in a specific ratio might be impossible to find. after 40 years not many matched pinion and ring gears set survive. many ford gears are also timed or lapped matched so you cannot mix and match from 2 sets of gears they have paint markings showing the proper keying of the teeth.

All that said there could also be issues with the U joints on the driveshaft. but if you have no issues before the gear change and now you do the problem is in the pumpkin.

Hey Hcode,

I killed the crush sleeve by overtightening it, which is why I took it to a well known speed shop and he did everyhing you mentioned above and said it should be good to go. First 400 miles never felt anything and now I feel the vibration. I'll check again.


couldn't hurt, to loosen the rear Ubolts on the driveshaft. take the drive shaft down DO NOT pull out the front yoke from the trans, it will spill the trans fluid out

then with the car rear lifted up, turn the yoke on the diff and try to wiggle it around and see if you have any play in it. pull it in an out and side to side, any knocking sounds and it is loose.

then maybe inspect the U joints, hold them together and twist the caps roll them around and see if they are tight. just to make sure.

then maybe rotate the drive shaft and inspect,, maybe a balancing weight came off it, if it had one there would be a missing spot on the drive shaft.

now on the diff if you spin the yoke and the rear keeps going for a bit that isn't good that means the pinion preload has gone, it should have some resistance to turning so if you spin the tires it stops almost instantly.

you wouldn't know but it is possible when he changed the crush sleeve he didn't change the nut also. the nut has some crimps in it to keep it from backing out but if you overshot the crush sleeve torque it may of damaged the nut slightly. over time it loosened and the preload went away. usually you get a crush sleeve and the main nut for the yoke in the same baggy.

you could also check the preload with a small dial torque wrench on the car turn the yoke and see if the preload is still in spec.

it is possible that the yoke was catching on a burr during original install and it was torqued to spec, then during a heat cycle when driving the burr collapsed and the pinion loosened up again.

you should also check the transmission yoke with the drive shaft in place try to wiggle the transmission yoke and see if you have a lot of play in it.

and double check the U joints make sure none of the clips popped out of the driveshaft.

Thanks for the info HCODE72. I'll let you know if your strategy works.

you know we still didn't rule out a wheel bearing going on an axle shaft too.

I guess if you notice the sound is louder on one side of the car verse the other.

maybe have somebody in the back seat listen to the sound and try to pin point it.