10 bolt limited slip or upgrade to a 12b?

[Old Painless]

I've heard factory 10 bolts (8.5" ring gear) get a pretty bad rap as far as torque handling goes. I've got a one legged 3.42 behind 6 liters of ls, and i'm wondering if it's near the threshold. Engine is probably putting max 400 ft/lb on a good day (LQ9 with headers, 3" dual exhaust and a bit of tuning).

My question is will I be pushing that 8.5 if i throw a limited slip in it? I know the c-clips are the achilles heel of those things, but i'm willing to take a chance if it's got a fair shot at holding up. I'm guessing after I do that the 4l60e will grenade, but that's the point of a project, right? It's got p295/50/r15 M/T Sportsman S/T tires on it, so I think it'll hook up enough to cause concern.

I'd love to get my hands on a 5 lug 12 bolt or those freak 5 lug 14 bolts, but craigslist has been pretty dry out here.

Here's the deal, my brother and I are in a bit of a shootout between a project we've been working on and this truck. The project is our '65 Pontiac Tempest with a pretty hot 355 that we just built pushing a th350 + tci 2200 stall (not that big of a deal i know) and factory 10 bolt posi - if you're curious I can post cam specs and other engine build info.

The reason Tempest>1500 right now is posi. I know what you're about to say... well the tempest has an 8.2 which is weaker yadda yadda, either way the thing has held up and launches pretty well with the new engine and converter. While we haven't done an actual drag yet (thanks California), I'm pretty sure the tempest will whomp on my truck off the line at least - the LQ9 might have it beat in the end, but I'd bet the 355 is almost tied with the 6.0 output-wise. The bottom line is my truck needs limited slip of some sort.

I think with limited slip the thing has a shot of staying together and really whipping the '65. I considered a locker but have a feeling that would be it's demise.

 

[Rusty Nail]

I dunno.....I really like to praise the 8.5 inch 10 bolt! I beat the tar out of one for better than a decade. Long story short : the car didn't outlast it.
Running ET/street on 28x10 street slicks, I spun the ujoint off of it and had to upgrade the pinion, as the straps eventually gave up. Lol.
I also spun the hubs off of the DRIVER SIDE axle, twice, before I bought STRANGE axles.
It was fine until daddy got on slicks...

It ran 12.7s, chirps the fronts at 40 mph, and still has c-clips in it to this day WITH the factory posi.

From what I gather a TRUCK 12 bolt is NOT equal to a CAR 12 bolt, but i'm bettin you knew that.

I'm with you. That's what makes a project (like that) fun. If you don't break, it's not going fast enough. Amirite?

Swapping out to the 8.5 will prove to be loads of fun. Then when you get done fixing it, and it won't break anymore, you'll be singin the same song.

[yt]E38-VDzvIZg[/yt]

 

[gmachinz]

I'll be overhauling my 8.5 with a Posi carrier and gears along with a disc brake conversion while I'm at it.

 

[Old Painless]

Nice, I love hearing success stories like this! I figured I'm being too cautious with it - while it is a daily, I have several (including the Tempest!) in case I lunch the thing, so not much risk there. I'm guessing at 12.7 you were throwing a fair amount more power at yours than I will be on mine.

Yes early 70's 12b's were 5 on 5 in blazers and some other trucks I think? I can't remember exactly but I know the option is out there.

Part 2 of the reason i want to keep the 10b is due to bolt pattern. The truck came with wheels that I'd have a hard time letting go of.. So bolt pattern would be nice. Everything is new up front so I don't want to deal with converting the front to 6 lug if I had to go modern 12b. Ideally I'd have 5x4.75 so I can swap wheels/tires with the tempest!
Probably vice versa would make more sense huh...

You're definitely right! I finished this thing in december and I haven't even had to pop the hood on it - way too casual for my appetite! I started looking into cams and heads before I came to my senses and figured I needed to make use of the power I have before investing in further one-tire boiling.

 

[74propu]

I am running a 10 bolt posi in my 74 with a 5.3 twin turbo running 560whp at 6 lb of boost turning 31x19.5 15 mickey thompsons . The rear end has has been in it 3 years and out lasted 3 transmissions 2 of them from monster transmission . So go for it and keep the 8.5 .

 

[Old Painless]

Nice, now I have 2 people to blame when mine detonates! Thanks for the input - looks like the summit fairy will be showing up soon.

 

[Honky Kong jr]

If you are buying a new carrier get a 30 spline and use 30 spline axles. Minor machining needed to use 12 bolt axles in an 8.5 or use 89 up burban axles. I upgraded my front rear to 30s and have a set of rear 30s in waiting. 90 burban stuff.

 

[Old Painless]

If you are buying a new carrier get a 30 spline and use 30 spline axles. Minor machining needed to use 12 bolt axles in an 8.5 or use 89 up burban axles. I upgraded my front rear to 30s and have a set of rear 30s in waiting. 90 burban stuff.

Sounds like a plan, I'll definitely beef it up where I can. I've heard the pinion can be a weakness as well in these, but if straps and u-joints are failing before that, I have a hard time believing such tales.

 

[Honky Kong jr]

Sounds like a plan, I'll definitely beef it up where I can. I've heard the pinion can be a weakness as well in these, but if straps and u-joints are failing before that, I have a hard time believing such tales.

The pinion shaft of an 8.5 is marginally larger then that of the 12 bolt. The car 12 bolt is stronger then the truck in the carrier but uses the same axle dimensions. A car carrier can be swapped into a truck housing tho. If you want strong find a rear out of a 454SS truck 14 bolt with smaller then 3/4 ton drums. The extra weight on the tires wont hurt either.

 

[Keith Seymore]

I've been running the original 8.5" ring gear 10 bolt in my Chevelle for almost 40 years. Welded tubes, Mark Williams spool and axles with C clip eliminators.

Best ET is 9.85 at 136 mph at 3950 lbs (just under 1000 HP).

I did break the pinion once but I am ok with rebuilding the rear axle every 25 years or so...

K

.

 

[bucket]

The 8.5 10 bolt is fine for 2wd applications. The stock carrier is the weak link, so an aftermarket posi or spool takes car of that. It's the 4x4 applications and heavy ass Suburbans that mainly kill the 8.5 and give it a bad name.

 

[Old Painless]

Sounds good to me! Thanks for the input everyone - now if I could only get my fordophile co-worker to shut his yapper about doing a 9" swap.. Not gonna happen. Ever.

 

[Honky Kong jr]

Sounds good to me! Thanks for the input everyone - now if I could only get my fordophile co-worker to shut his yapper about doing a 9" swap.. Not gonna happen. Ever.

I have a friend that's Fordtarded and knows I have a posi 9". He keeps telling me to put it in and use a ford 44 up front. My 8.5 is pretty much built to 44 specs only difference at this point is the 44 has a little heavier tube wall thickness. Why would you want to put a 9" in anyway they eat more hp to turn then an 8.5 or 12 bolt,yes they have extra pinion support but like many 14 bolts I've pulled apart the support boss is broken. 454SS rear is the rwd indestructible answer and they are 5 lug too.

 

[Old Painless]

454SS rear is the rwd indestructible answer and they are 5 lug too.

I've been looking, trust me. My brother found a guy parting one out but of course that thing went in a split second.

 

[Honky Kong jr]

Best thing you can do to the 8.5 is disc brake swap it that way if an axle breaks it stays put without shredding your bed. There are threads on here to do it on the cheap. I made my own for my V10 nice big vented rotors too.