73-87chevytrucks.com

73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Rear Drivelines, Drive Axles and Gearing => Topic started by: JohnnyPopper on November 17, 2020, 02:04:46 PM

Title: Somewhat Urgent Question about Interchangability
Post by: JohnnyPopper on November 17, 2020, 02:04:46 PM: I'm having a 3.42 installed to replace a 3.73 diff. There is a stock posi-traction on the 3.73.

With help from this forum, I discovered it will drop my highway RPM from 3000 to 24-2500. That's where the torque peaks as well.

I don't off road, it's my daily.

Just got a call from the shop, they said the posi will not work now, and that they need to install a standard diff.

Yesterday when I was there, there was no mention of a problem.

Q: are these interchangeable? Or are they trying to sell my posi from under me? In any case, I told them to box up my posi in case I want to restore it.

Any help would be appreciated. :D
Title: Re: Somewhat Urgent Question about Interchangability
Post by: bd on November 17, 2020, 02:13:16 PM: Which year/model truck and model axle?

The differential deck height for an 8.5" 10-bolt allows 2.73:1 and greater gear ratios to employ the same case. So ask for a specific reason that your LSD won't work. Without more information, I agree. It sounds suspect.
Title: Re: Somewhat Urgent Question about Interchangability
Post by: JohnnyPopper on November 17, 2020, 02:30:00 PM: '78 12 bolt
Title: Re: Somewhat Urgent Question about Interchangability
Post by: bd on November 17, 2020, 02:43:17 PM: The truck 12-bolt uses three (3) different cases, dependent on gear ratio range: 2.73:1 down, 2.76:1 - 3.42:1, and 3.73:1 up. The deck heights between the cases for the gears you have are different. The 3.42:1 gear requires a differential case that moves the ring gear further from the pinion gear centerline.
Title: Re: Somewhat Urgent Question about Interchangability
Post by: JohnnyPopper on November 17, 2020, 03:20:18 PM: So it is legit. Thanks bd!
Title: Re: Somewhat Urgent Question about Interchangability
Post by: VileZambonie on November 17, 2020, 05:11:15 PM: You are changing the rear ratio to 3.42:1 - from 3.73:1 posi? What about the front? Curious why you downgraded yourself?
Title: Re: Somewhat Urgent Question about Interchangability
Post by: JohnnyPopper on November 17, 2020, 07:56:31 PM: Originally the shop didn't indicate that I would lose the posi... not real happy about that.

I don't off road this truck, daily driver. This will put highway speed right at 2500 RPM.

Front is disconnected for now.
Title: Re: Somewhat Urgent Question about Interchangability
Post by: VileZambonie on November 18, 2020, 04:50:45 PM: :o
Title: Re: Somewhat Urgent Question about Interchangability
Post by: JohnnyPopper on November 19, 2020, 07:52:28 PM: I'm keeping the stock parts, will probably have the front converted or restore the rear end at some point in time.

Ultimately OD will be the solution.
Title: Re: Somewhat Urgent Question about Interchangability
Post by: ehjorten on November 20, 2020, 01:56:44 PM: If you had been going from 3.42 to 3.73, you could have kept your Posi and used a "thick" 3.73 ring gear set, but you're doing the opposite.