1968 Delta 88 Posi Upgrade
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As Seen on Roadkill
Joined: Oct 2022
Posts: 30
From: Highland, Michigan
1968 Delta 88 Posi Upgrade
Looking to swap out my 2.56 open to a posi unit. What rear end type is in this car? It’s a “12 bolt” but I’m wondering if it is an O type with the 8.5 ring gear or if it is a regular 12 bolt.
Registered User
Joined: May 2009
Posts: 2,273
From: Northern California
You’re on the right track to put the 2.56 away. It gets great mileage, but no one is keeping these cars for mileage these days. You very likely have type O or maybe other exotic (type P) rear end, it is very unlikely to be “regular” anything. Don’t even ask about swapping in anything Chevy. Just plain different.
Your safest swap is a complete donor rear end from another big Olds from 1965 to 1970. The idea there is to get a complete replacement and use the best of both to build something safe & reliable. If you’re in the Midwest, check out French Lake Auto parts in MN. They have more than a few big Olds from the mid-60s.
You might get really lucky and find an Olds “Anti-Spin” (aka posi) unit, but these were rare in the day. They also degrade with use and I don’t believe they’re rebuildable. I’ll be happy to be corrected if I’m wrong. I’m not a differential specialist. Just a hobbyist.
It took me many, many years to find 3.23’s for my big Oldsmobiles. And both of mine are open (aka not anti-spin/posi) rear ends. I believe the most common big Olds rear ratios were 3.08:1. If you chase performance models as donors (Starfires/ Police specials) you may find 3.23’s as well. Either of these will be a big performance improvement over 2.56:1.
My point here is, if you come across a 3.08 open rear end, that may be enough performance improvement to keep you happy for a time, perhaps a long time, until you find the ratio you’re looking for. Out at the performance edge Olds probably built a few big cars with 3.42’s, but I’ve never seen one.
When you get to redoing the rear end, keep your axles and if you’re swapping in different gears, that’s a good time to do rear bearings as well. And rear brakes for that matter.
If you want to go high buck exotic, I have the impression that Ford 9” custom with custom axles might be the ultimate, but pricing that would be something like a mortgage payment.
Cheers
Chris
Your safest swap is a complete donor rear end from another big Olds from 1965 to 1970. The idea there is to get a complete replacement and use the best of both to build something safe & reliable. If you’re in the Midwest, check out French Lake Auto parts in MN. They have more than a few big Olds from the mid-60s.
You might get really lucky and find an Olds “Anti-Spin” (aka posi) unit, but these were rare in the day. They also degrade with use and I don’t believe they’re rebuildable. I’ll be happy to be corrected if I’m wrong. I’m not a differential specialist. Just a hobbyist.
It took me many, many years to find 3.23’s for my big Oldsmobiles. And both of mine are open (aka not anti-spin/posi) rear ends. I believe the most common big Olds rear ratios were 3.08:1. If you chase performance models as donors (Starfires/ Police specials) you may find 3.23’s as well. Either of these will be a big performance improvement over 2.56:1.
My point here is, if you come across a 3.08 open rear end, that may be enough performance improvement to keep you happy for a time, perhaps a long time, until you find the ratio you’re looking for. Out at the performance edge Olds probably built a few big cars with 3.42’s, but I’ve never seen one.
When you get to redoing the rear end, keep your axles and if you’re swapping in different gears, that’s a good time to do rear bearings as well. And rear brakes for that matter.
If you want to go high buck exotic, I have the impression that Ford 9” custom with custom axles might be the ultimate, but pricing that would be something like a mortgage payment.
Cheers
Chris
Old(s) Fart
Joined: Mar 2007
Posts: 50,841
From: Northern VA
Your car uses the big car Type O axle, which has a 12 bolt cover and a 9.375" ring gear held to the carrier with 12 bolts. Aftermarket parts for these do not exist. Factory posi units with descent ratios are unicorns. Impala axles do not fit. Unless you can find a factory unit (and if you do, buy a lottery ticket), an aftermarket Ford 9" is about your only option.
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