I currently have a 455 with 96 heads and a 223 comp cam with a M20 and a 10 bolt 3.55 rear end. I want to install a 3.23 or 3.08 posi. Should I replace the the 10 bolt with a 12 bolt? I don't want to buy a posi and new gears for 10 bolt and then have it break. I have not dyno'd the engine but my friend Paul helped me rebuild it. He is an ASE master mechanic and estimated about 400-420 hp.
Depends on how you plan to drive the car. If you're going to "play nice" then the 10-bolt will last a long time. However, if you see drag race style launches in your future, then you'll want something stronger. I put a Moser 9" in mine It was the one 'aftermarket' piece that truly was a bolt-in and didn't require me to modify everything around it to get it to fit.
Be advised that originally, 455 powered A bodies came with a 12 bolt rear end. There was a good reason for this. With your level of power, I would spend the $$$ on upgrading a 12 bolt rather than a 10 bolt. Do it once, and do it right, and you won't regret it. I have personally run nothing but 10 bolts in my 389-400 powered GTO's over the years. Still am. I have blown a couple up, though. The one in my '65 4 speed car was rebuilt after I blew it up 20+ years ago. I re-geared it to 3.36, and it's been fine since 1990. My slightly modified 389 probably makes about 400 HP.....
There's a whole lot to be said for 'do it once, do it right'. I really tried to do that with my car and even so, I'm -still- working though little bugaboos on it at least every month it seems. I don't think they're ever truly "done".
If you do go with a 12-bolt, keep a couple things in mind. If you go 'used', there are folks out there who will sell you a "Type O" rear-end out of an Oldsmobile and claim it's a 12 bolt because it has 12 bolts holding on the rear cover. They aren't. Inside they still only have 10 bolts on the ring gear and aren't any stronger. Sometimes they truly just don't know, sometimes they're being dishonest.
On a 12-bolt, I'd recommend making sure you use a c-clip eliminator kit, and make sure it's one that's approved/recommended for street use (not all of them are). If you ever DO go to the track, most sanctioning bodies require them (and besides, they're just safer).
That's one of the reasons I utimately decided to go with a 9" - that, plus the ability to change ratios by swapping pumpkins and they're stronger. (but they do have slightly more parasitic drag)
Strange Engineering makes 12 bolts, 9 inch Ford and Dana 60 (the ULTIMATE) rears for A-body GM cars. All top notch stuff and they offer brakes and all accesories for rear axles too. I got all my 12 bolt stuff (aside from the housing) from them and everyone there was very helpful. I highly reccomend them .
I believe it was Summit who sold a 9" that would bolt right up to an A body. A Moser rear if I'm not mistaken.
I bought mine through Spohn Engineering - the same place I bought nearly all of my suspension parts. Spohn Engineering
Quality parts for sure. My Moser 9" was a brand new housing with an added drain plug and fill bung. I ordered it with Wilwood rear disc brakes (4-piston Dynalite Pro), nodular iron center section, Wavetrac posi with 3.50 gears, heavy yoke with 1350 series U-joints. I ordered it in bare steel and painted it myself. It bolted right up and was a perfect fit. The only "extra" I didn't order with it was the housing brace.
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