Not A Good Idea.
KH: I have a 12 bolt rearend out of a 1968 Impala wagon. There are no numbers on the housing to check. Both axles rotate in the same direction when turned. By turning the pinion and counting the axle rotation it is close to 3 turns.I have never heard of a 2.73 Positraction, but if it is, can I use the carrier and put in a 3.50 or 3.73 gear? This is going into my 12 bolt non-Positraction '71 El Camino with 2.73 gears.
W_ : Pull off the cover plate and count the teeth on the ring gear and the pinion gear. Divide the ring gear by the pinion to verify the ratio. You can also verify that it is a Positraction axle.
There are three different carriers used in GM rear ends; the 2 series, the 3 series, and the 4 series. The 2 series would be for ratios in the range that you are quoting (a 4 series would be for 4:11). As I understand it, you can use spacers to install 3 series gears on a 2 series carrier. You can't go in the other direction, i.e., you can't put 3:50 gears in a 4 series carrier.
O_: I also have a 2.73 Positraction out of a '71 Monte Carlo and was planning to put 3.73-4.11 gears into it. After trying five different shops in this area, I got basically the same answer: it can't be done.
The 12 bolt Positraction carrier goes: series 2 = up to 2.90; 3 = 3.08-3.90; and 4 = 4.11 up (or so I was told by an "expert"). This is not necessarily true for a 10 bolt from my understanding. What I found is that there is a spacer which allows you to go up to the next series gear (not recommended with Positraction). Richmond makes a more expensive gear that is extra thick that basically does the same thing.
After hours spent at the parts counters and long distance bills contacting "experts" at driveline shops across the country, the only answer I came up with was that the 2 series Positraction carrier couldn't use a spacer or thick gear because of lack of clearance for the cross pin. Further, it probably wasn't worth my time to research it anymore because the stock Positraction wasn't that great for a serious street/strip car anyway (90% said it couldn't be done; 10% said it "might" be possible).
One thing I didn't find out is if the pin clearance problem could be solved by adding a c- clip eliminator kit to the housing so you could possibly use a thick gear without the pin? I got sick of the hassle and ended up sticking a 9" (out of a '71 Torino). I put a poor man's Positraction (mini-spool) and 4.11's under it for less than it would of cost me for a good aftermarket Positraction. I had it locally modified for my 4 link and had him build the upper mounts a little long to do the same thing as no-hop bars on a GM axle. It works great.
I recently bought a 1968 SS (with 12 bolt and a spare 12 bolt) and plan to make one of those work with a good aftermarket Positraction and no-hop bars. I need the 9" for my 1950 Sport Coupe and the mini-spool is a little radical for the street (so far no problems and it launches real straight). My suggestion finally is to spend the money for a good Positraction that is the right series for the gear you plan to run. You'll be happier in the long run.
Archivist: Tom Wilson
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