I would like to build a posi 7.5 rearend for my cutlass. i found a donor on craigslist, the rearend looks a little rusty from setting outside. just wondering how much rust is too much to mess with. plans are to have the tube welded to the center section hoping it will help strengthen it up a little. I'm using a donor so the car wouldn't have to set while it's getting rebuilt, or would it be best just to use the original rearend.
posi rear end upgrade
- Thread starter scarborough
- Start date
Pictures of what you're talking about would help more than you could ever imagine.. just sayin...
weld the axle tubes? check
put a cast differential cover on?
change out axle shafts?
replace with new differential with which gears?
don't forget to install new dog ear bushings, pinion gear crush bearings, and convert the drums to rear disc?
I wouldn't pay more than $100 for a used axle you're going to strip down and do this to
put a cast differential cover on?
change out axle shafts?
replace with new differential with which gears?
don't forget to install new dog ear bushings, pinion gear crush bearings, and convert the drums to rear disc?
I wouldn't pay more than $100 for a used axle you're going to strip down and do this to
While a 7.5 posi rear end will handle a few hundred horsepower comfortably as long as you aren't planning on using sticky tires, (I have a 7.5 factory posi with 2.29 rear gears currently in my Malibu) you may want to add up parts costs and compare to a Quick Performance rear end. Unless you are doing ALL the work yourself, I'll bet you will be pretty close to the cost of a new stronger rear end than building a 7.5. Once mine breaks (383/TH400) I'll move forward with QP rear end instead of building my original.
Just something for you to consider.
Just something for you to consider.
My original plan for my Regal was to take it to a Ring and Pinion shop and have them upgrade my 7.5 to a 28 spline carrier with Moser axles and new bearings and seals, since I am currently only running a 355/200R4.
But since it was only a few hundred dollars more to get the QP 9" and build it here I went that route and the price difference more than covers the fact that I now have 31 spline axles in a brand new housing (instead of my rusty 7.5" housing) and new 11" brakes versus my factory 9" used brakes.
Plus I tend to upgrade engines so it is best to have a foundation I can build on if that happens.
Another thing to consider with the 9" swap is you may need a different length drive shaft, I ended up getting one from Denny's and it was very nice.
But since it was only a few hundred dollars more to get the QP 9" and build it here I went that route and the price difference more than covers the fact that I now have 31 spline axles in a brand new housing (instead of my rusty 7.5" housing) and new 11" brakes versus my factory 9" used brakes.
Plus I tend to upgrade engines so it is best to have a foundation I can build on if that happens.
Another thing to consider with the 9" swap is you may need a different length drive shaft, I ended up getting one from Denny's and it was very nice.
Unless you pick up used gears and something like the Torsen worm gear posi from a F body, it isn't worth the expense. It adds up quickly to a much stronger 9" housing. Even with the used 4 gen F body gears and posi, you still need either used GN or new Moser 28 spline axles. Yes the reinforcement is worthwhile. Part of the problem with F bodies were thinner housings than even the G bodies and with the T-56 would twist axle tubes. Not sure why GM thought it was a good idea to use a 7 5/8" rear in the F bodies when power was climbing every few years and started at 275 HP and TQ to match!
Without seeing it, I couldn't say how much rust is too much. I can appreciate the idea of building a 2nd on the garage floor to minimize downtime though.
I went the super budget route with mine. Went to the local Pick N Pull yard on a half-off day and pulled 3:42 gears with a Gov-lock from an S-10 and an Auburn limited slip from an '87(?) Camaro. That was probably 15 years ago and I don't remember what it cost now, but it wasn't much more than $100. I cleaned up the Gov-lock and sold it on Craigslist for $50 or $75. Since the gears were 3-series and the Auburn carrier was 2-series, I needed a spacer ring. I got the spacer and a set of bearings from Speedway. I got a no-name cast rear cover with cap preload screws off Ebay. Everything else was reused. I never could get a perfect pattern with the used gears and at certain speeds it howls a bit under coast. This combination will probably hold up indefinitely behind my stock 305. I'll be putting a QP 9" in before the BBO goes in. If I'd had the money 15 years ago, I'd have done the QP.
I agree with the guys who say to add up the cost of upgrading the 7.5 rearend. You'll probably find that a 9" isn't much more than upgrading a 7.5 with good parts. Buy once, cry once.
I went the super budget route with mine. Went to the local Pick N Pull yard on a half-off day and pulled 3:42 gears with a Gov-lock from an S-10 and an Auburn limited slip from an '87(?) Camaro. That was probably 15 years ago and I don't remember what it cost now, but it wasn't much more than $100. I cleaned up the Gov-lock and sold it on Craigslist for $50 or $75. Since the gears were 3-series and the Auburn carrier was 2-series, I needed a spacer ring. I got the spacer and a set of bearings from Speedway. I got a no-name cast rear cover with cap preload screws off Ebay. Everything else was reused. I never could get a perfect pattern with the used gears and at certain speeds it howls a bit under coast. This combination will probably hold up indefinitely behind my stock 305. I'll be putting a QP 9" in before the BBO goes in. If I'd had the money 15 years ago, I'd have done the QP.
I agree with the guys who say to add up the cost of upgrading the 7.5 rearend. You'll probably find that a 9" isn't much more than upgrading a 7.5 with good parts. Buy once, cry once.
see if the person selling the diff will remove the cover to see if the rust/moisture has gotten inside and run it's course.that's an unfortunate suprise after shelling out your hard earned money.if the seller won't open it then...
I was doing a tune on a 2002 Camaro SS, 6spd car. Literally a stock car even with the A.I.R. stuff still on it, turned up the timing and fuel and blew up the 7.5 rear doing a couple pulls.
If I had a 7.5 in my SS right now I wouldn't make it out of the driveway. 🤣
If I had a 7.5 in my SS right now I wouldn't make it out of the driveway. 🤣
I ordered the 7.5 posi 3.42 kit with the pinion shims instead of crush sleeve from quick performance. Did work myself and put a summit aluminum diff cover. Im in it for 650.00 ish . The 9 inch is over 2000.00.
I couldn't justify the cost difference for cruising around town and occasionally getting on it with a mild 350.
Look at what your use is. Hot engine/drag racing get the 9". Street cruiser build the 7.5.
I dont regret building the 7.5 and my next project will keep the 7.5 also.
I couldn't justify the cost difference for cruising around town and occasionally getting on it with a mild 350.
Look at what your use is. Hot engine/drag racing get the 9". Street cruiser build the 7.5.
I dont regret building the 7.5 and my next project will keep the 7.5 also.
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