OK so I bought a 9-bolt rear from Hawk's Thirdgen (great transaction BTW, I highly recommend him for parts) and it has 3.70 gears in it. When I found that out I assumed the gears were aftermarket, because I was under the impression that 3.45 was the steepest that GM ever used. However, the factory ID tag on the diff cover says "ratio-3.70" as well as stating the part and serial numbers. Were 3.70s actually used stock? And if so, what car is this rear from? Thanks in advance...

 

Its my understanding they were used in 85-86 Trans Ams with the L69 engine and a T5 transmission. 87 and up have the 3.45 as the steepest ratio.

 

I also bought a 9-bolt with stock 3.70's, rare but they are out there.

 

As a side note my '84 Z-28 HO 5.0 came with one of those from the factory... man was I mad when I grenaded that thing

 

Looks like the 3:70 9-bolt was offered in only 85 on the Camaros and may be on the Firebirds. Not a very common axle ratio, with a limited productin lifetime. The other was a 10-bolt 3:73.

 

No, it was offered in 87-89 IROC's, and 85-89 Formulas, Trans Ams and GTA's.

 

What info do you have that the 3:70 came STOCK
ON 87-89 IROC & FIREBIRDS 85-89 FORMULA/TA????
What was the RPO Code for this STOCK 9-bolt 3:70 axle ratio on those cars.
Also the STOCK engine and transmission combo with the 9-bolt
3:70????

 

A few of the very first 9 bolts had 3.70 ratios. Very rare. Ratios to 4.10 are available at www.9bolt.com

Miles

 

Miles
I know that the 3:70 was on early model 9-bolts and that you sell replacement parts for these ratios. But I do not belive they were STOCK on the above models. What is your take on this??

 

Those cars listed are they cars that got 9 bolts, but not all of those years got 3.70's.

Ben

 

As best as I've been able to determine, there was a few factory equiped 3.70/3.73 ratios in 85. That's all I know about the factory options

Miles

 

3.73 10-botls were available from 1982 through teh 1985 model year. they are most commonly found in teh 4cyl cars actually, but they are best known for their place in teh 305HO motored cars, it was an option in both cars. the RPO code for the 10bolt, 3.73 is GT4.
as for the 9-bolt 3.70 ratios, they are very very rare. they were only supposed to be in a handful of early 4wheels disc brake cars, but i have seen them with drums on them before. you definately got your hands on an uncommon piece.

later
tim

 

So in other words this rig is WELL worth the $400 I paid for it. I knew it was a rarity (since I had never even heard of them being offered) but I didn't know it was THAT rare. Hell I thought a 3.45 was my white elephant to search for....guess I stumbled across a neon green one instead.....

BTW it is a disc brake rear.

 

i woudl say it is worth a lot mroe than teh $400 you paid. to a person doing a resto who has lost that rear and needs it it woudl prolly be worth 1200-1500 if it is in good working order.
congrats on your find man!!!!!!

later
tim

 

It's in excellent working order. The pads and rotors don't even need replaced. The diff cover is bent up a bit where the last owner apparently jacked the car up under it a few times, but other than that it's perfect. Excellent wear pattern, tight posi.....the works!! Hell the rubber plug isn't even dried out!