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Discussion Starter #1
I just got a set of 3.73 gears for my 78! The posi unit is from a 78 trans am with 2.41 gears. So if I understand correctly, I can use the 3.73s on that carrier with the use of a spacer? Thanks

Gary
 

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yep ya can.. be sure to use the spacer bolt set.. the bolts are shouldered for the spacer
 

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and lots of green loctite

Miles
 

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Discussion Starter #5
I know I have a lot of research to do yet, but Im still trying to figure out why the spacer is needed. The 3.73 ring gear is much thicker than the 2.41 gear. The carrier is also thicker, so why would is the spacer needed?
 

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I think the 2.41 and 3.73 both have 41 teeth* on the ring gear, but the 2.41 Pinion is huge (17 teeth?) so the carrier flange is offset more to the left to compensate. The 3.08 up pinions are smaller so if put the 3.73 (11T) on a 2:41 carrier it won't mesh. Are you sure you have a 2.41 posi? It's very rare since the performance gear was usually ordered with the posi option on Trans-Am's. I worked in a Pontiac dealership in the Smokey and the Bandit era. Worked on lot's and lot's of black T/A's.

Here's more on Carrier ring gear placement and ratio/breaks at Randy's Ring and Pinion

*
41/11=3.73
41/17=2.41
 

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If they make one I would use the thicker gear designed for the 2 series carrier. They make a thick ring gear to use in place of the spacer. It's supposed to be stronger than the ring gear spacer also.
My parents had a 79Trans Am with a 2.41 gear also. I don't know how fast it would go but the needle on the speedometer would hang straight down on the interstate. I'd guess around 130-135 Mph.
 

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The carrier break size on the 10.5 10 bolt rear is 2.56 & down //2.76 & up

Some company's will make a higher ratio ring & pinion to fit the lower ratio carrier.

If installing a 3.73 gear to a 2.56 or lower ratio gear you will need a spacer.

Mr. Gasket 908A



1978 Trans Am rear codes
PS - 2.41 Rear Gear (Posi)
PT - 2.56 Rear Gear (Posi)
PW - 3.08 Rear Gear (Posi)
PX - 3.23 Rear Gear (Posi)
PP - 3.23 Rear Gear (Posi)
 

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Discussion Starter #9
The ring gear is stamped with 41 and 17. I divided and got 2.41. The 3.73 ring gear out of the blazer is really thick. I wonder if its a gear that Novajay is talking about. Ill get some pictures loaded
 

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If the 41 tooth 3.73 ring gear is thicker than the 41 tooth 2.41 ring gear on the posi, you may have lucked out. You'll still need the matching 11 tooth pinion from the Blazer. Mock it up and try it out. I use a honed out bearing test set for trial fits so I don't have to press the bearings on and off. Since you have two setups, (and you should buy new bearings, crush sleeve anyway), you can use one of the old sets as a mock up set. Save all the old shims even if you don't use them.
 

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They don't make "thick" gears for an 8.5 so you will need the spacer or a 3 series carrier.

Miles
 

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If the 41 tooth 3.73 ring gear is thicker than the 41 tooth 2.41 ring gear on the posi, you may have lucked out. You'll still need the matching 11 tooth pinion from the Blazer. Mock it up and try it out. I use a honed out bearing test set for trial fits so I don't have to press the bearings on and off. Since you have two setups, (and you should buy new bearings, crush sleeve anyway), you can use one of the old sets as a mock up set. Save all the old shims even if you don't use them.
I agree- Having a honed pinion bearing to setup the pinion depth saves a lot of messing around.
You might be able to mount those gears on the carrier and get a good gear mesh but often times the carrier will be offset so far that the axle c-clip won't go on. There is a way to make the spacer work ok and a friend of mine has done it many times. There is a register on that carrier that centers the ring gear tightly and when you use a spacer you lose that register as it isn't long enough to seat the gear properly. To do it right you have to make some welds (8 beads) spaced equally on the carrier extending out from the register then mount the carrier in a lathe and cut the welds down to be flush with the register-this way when you mount the spacer and ring gear the gear will have a surface to center on. Use locktight on the ring gear bolts. Last year I helped another friend build a 403 79 trans-am and his had the 2.41 posi also-I think it was very common, he will doing the gear upgrade soon but currently getting 25mph with 430hp
 

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Discussion Starter #13 (Edited)
the carrier on the left has the 2.41 gear. the ring gear on the right is the 3.73. I bought the ring and pinion for $50





I didnt know I could use the old sets for mock up, thanks! I was going to order extra bearing. I read that also on a write up.
 
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