it`s time to tinker with the rear end of my 89 305 tbi firebird
i have a 2.73 posi in now and want to change to 3.73
ive been shoping for gears to fit the 2 series carrier and found a set of motive new from one of our suppliers fairly cheap butthere listing the gears as fiting 2 &3 series
i looked on motives web sight and see they make a spacer is that how they do it or is it just a mistake in there catalouge?
any help would be great
thanks

 

One set of gears cannot fit both carriers.

Look at the pic below, and you'll see why, and what the spacer would need to look like. They are 2 carriers of the same model: the stock Auburn unit for these cars, the one you would have if you had posi which you almost certainly don't unless you put it there.

THink about gears for a minute. We always talk about the ratio being the tooth count. But it's also something else: it's the ratio of the diameter of the ring, to the diameter of the pinion. Which means, recalling that the ring is always the same size in a given model of rear, that as the ratio gets higher, the pinion gets smaller. In other words, the ring has to get thicker. So, rather than produce massively thick gears, the factories opt to produce gears in a series of ratios up to some convenient break point; then move the ring gear flange on the carrier over toward the pinion, and produce another series of ratios with relatively thin ring gears; and perhaps to do it again as the ratios get even higher and the ring starts to get too thick again.

In this pic, the 2 series is on the left. Notice how the ring flange is almost flat, where in the one on the right, it's cone-shaped.

 

rb83l69 thanks for the reply i had seen that info from you in my search
but then what is the pinion/ ring spacer sold on there web sight (www.motivegear.com sorry i dont know how to link)

"7.5" GM car and truck- allows you to use a 3.08 and down case with a 3.23 and numerically higher gear ratio. Part Number 075050

8.5" & 8.6" GM car and truck- allows you to use a 2.73 and down case with a 3.08 and numerically higher gear ratio. Part Number 085050

8.78" GM car - allows you to use a 3.73 and down case with a 3.90 and numerically higher gear set. Part Number 088050

8.78" GM truck- allows you to use a 2.73 and down case with a 2.76- 3.42 gear set, or a 3.42 -2.76 case with a 3.73 and numerically higher gear set. Part Number 088050"



as for the rear axel it is a posi but you coud be right i was told by my salvage supplier it was 2.73 but didnt check for myself i will do that this weekend

one more question is there problems with running 3.xx gears made for the 2series all the searches said just get a new carrier yet most companys sell gears to do just that
sorry about all the questions just trying to understand thanks again

wayne

 

The spacer goes between the ring gear and that flange on the 2 series carrier, which moves the gear over toward the smaller 3-series sized pinion by an amount roughly equal to the distance that the carrier is different by.

I wouldn't run the thicker "special" gears; not for any technical reason, but more because if you do that, you can't ever change to the right carrier for the gears, even if you want to. But with the spacer, all you have to do is to remove it, if you ever get a 3 series carrier. The cost is about the same. The "special" gears are about $30 more than regular gears, or the spacer is about $50-60. In the grand scheme of a gear & carrier swap, it's nothing.

 

RB83L69 thanks for the help so all i need to do is check on price of spacer, check my gear ratio , set aside the time to do it, ask 6 or 8 more dumb questions and im all ready to bust a knuckel and get gear lube in my eye

much thanks
wayne