I have the opportunity to swap out the stock 10 bolt open rear end currently in my 1970 442 for a 12 bolt open rear end out of 1970 Cutlass. Are there any real benefits to swapping out the rear end?

Knowns:
12-bolt rear end is in a running 1970 Cutlass
It is original to the car
It is an open / peg leg (no posi)

Unknowns:
Gear ratio, I haven't checked the ratio yet
I don't know if the 10-bolt is original to my 442. The engine and transmission are long gone, so the original rearend could have been replace as well.

Any input would be greatly appreciated

 

A 10 bolt isn't original to a 1970 car to my knowledge. It was a 71-72 thing

The 12 bolt o type rear would be original equipment, ......but in my opinion. The 10 bolt 8.5 is a better rear end, both because of aftermarket support, and axle strength.

I personally would keep the 10 bolt if it's an 8.5. If your 10 bolt is an 8.2, I'd get rid of it.

 

If your not sure which rear end is under your car, could you snap some pictures of the inspection cover? That may help us key it out for you.

John

 

If the O-Type 12 bolt has a better gear ratio it may be beneficial to do the swap, assuming you want different gears and don't want to pay for new gears and having them installed. I did this a few times back in the 80s on my '70 Supreme and really enjoyed ditching the original 2.56 gears for 3.08s and then later 3.55s.

If they're the same or similar ratios, I would keep the 10 bolt (assuming it is an 8.5" 10 bolt and not an 8.2"). The 10 bolt will be easier to find gears and posi units, again assuming you are going to want those things either now or in the future.

If you are wanting to keep the car more original, then get the O-Type as it will be correct for your year car. You can always change gears and get a posi for it; that will just cost more than the same parts for a 10 bolt.

 

Thanks Fun71, I was thinking the same thing. I will be swaping the smaller rearend in my convertible for the twelve bolt rearend in the 1970 parts car. It's free, basically. I have the rearend in a 1970 Cutlass I just bought. I am also pulling other parts off of it in an attempt to make the 442 as original as possible. Thanks again for the advice

 

Why are you referring to the 10 bolt as SMALLER when they both use 8.5" ring gears? What is smaller about it?

What is the axle ratio on the Type O that you are swapping in?

 

I am confused. No 3.55 gears exist for the Type O 12 bolt so how could you have swapped them in?. What am I not understanding?

 

I meant swapping rears to get better gear ratios instead of changing ring and pinion in the existing housing. I swapped the original 2.56 O-Type for a 10 bolt 3.08 rear, then later swapped the 10 bolt rear for a Chevy 12 bolt rear with 3.55s.

Each swap was a huge improvement in performance.

 

I plan on pulling the cover off in order to get the gear ratio to each rearend prior to the swap. But I am leaning towards the 12 bolt O-type and getting the gear ratio I want later. I would like the car to look as original as possible. Thanks for all the help.

 

There should be a two letter code on the right axle tube on the top or the rear about midway out. Also, you can jack and stand the whole rear up and turn the prop shaft however many times it takes to get one full wheel revolution (a piece of tape sticking down from the tire to a line on the floor helps you guesstimate). Unless you WANT to smell the gear oil, hah.

 

Thanks Koda,

I want to check the teeth as well. And I do love the smell of gear oil. LOL

 

When you pull the cover, turn the ring gear until you see the stamped in alphanumeric characters on the edge of the ring gear. This will be the number of teeth on the ring gear and the number of teeth on the pinion gear, along with some other info. My 2.56 rear had 41A16 which is 41 ring gear teeth and 16 pinion gear teeth. 41 divided by 16 = 2.56.

The stamped characters look something like this:

 

Thanks! That helps out a lot. And you just save me some time. I usually have to refer to youtube or return to this forum for information such as this . Thanks again.