12 Bolt Gear Selection Q's

[GeoffP]

My 68 Camaro has an original 67 12 bolt in it that has an Eaton 3 series carrier with 4.10 (or 4.11 not sure which) gears installed. They're noisy enough that I want to replace them. I have been told that the gear is fine - it was setup wrong in the previous housing it was installed in and was bought used to go in the housing I now own. The posi was rebuilt at the time of that install, so it works well enough for now.

What I want to know is what is the difference between 4 series gears designed for a 3 series carrier and 4 series gears for a 4 series carrier. I know it's thickness and pinion size, but I'm not sure if it's thicker or narrower. I would love to be enlightened!

I am considering changing to a 3.90 or 3.73 depending on what I learn from the answer to the above.

Thanks!!

[chicane67]

The differences in 12bolt "series" are from a measured distance (height) at the end of the bearing journel to the ring gear flange. There are three different heights.

To make a 4 series gear fit a 3 series case... the ring gear is manufactured thicker.

A 4 series gear will naturally fit a four series case, as it was manufactured to do so.

And... to make a 3 series gear fit a four series case... the ring gear is manufactured thinner.

Pinion size has really nothing to do with it. The pinion gear head will obviously become larger due to a higher tooth count... and it is that tooth count that makes the requirement for different flange heights... and to keep the proper helical mesh, as well as, so that the ring gear will maintain enough thickness for strength.

If the gear set is making noise... and its gear set related... they will always make noise, as there is no real mechanical way to make them quiet post break-in.

If the differential is making noise due to another issue... like that of a bearing noise... a simple bearing overhaul should take care of it.

You raito selection sould be in consideration of your tire diameter, type of transmission as well as its gear ratios... and some of your engine parameters. Then you should select a gear ratio that suits your your driving requirements... like do you spend a lot of time on the freeway, is this a weekend car, what MPH do you regularly drive at... and this is all key when figuring OD ratios as well.

So basically... we would need to know more about your combination.

[GeoffP]

Thanks for the input. I think this info is what you're looking for: The engine is an LT1 running Comp Cams cc305, Comp Pro Magnum 1.5 RR's, stock injectors, 94 PCM. My research shows that it "should" produce about 330 HP on average based on what others have gotten from the same combination of parts with a good tune. The transmission is a 93 T-56 (lower first gear than 94+). With the 4.10/1, first gear is nearly useless. It will break the tires loose at any time in 1st with a simple tap on the throttle. Wheels/tires are 17x9's with 275/40-17's. My intent for the car is to drive it about 50% of the time with my DD (87 IROC) back and forth to work (~6 mi a day round trip) and occasional Auto-X, possibly some drag racing. The engine runs about 2200 RPM at 60 in 6th gear with the existing rear gear.

I think this is enough to help you to help me...just let me know if you need other info.

[GeoffP]

Found some info on the 93 F-body T56:

Year 1993 (M28) 1993 (M29) 1994+ (MN6)

1st: 3.36 2.97 2.66
2nd: 2.07 2.07 1.78
3rd: 1.35 1.43 1.30
4th: 1.00 1.00 1.00
5th: 0.80 0.80 0.74
6th: 0.62 0.62 0.50

I'm 99% sure mine is an M28

[GeoffP]

bump! Any more love on this?