which posi unit to get?
 

I'm looking for a posi unit for my 1971 swb. I found this on ebay.

what are your thoughts?

http://cgi.ebay.com/ebaymotors/gm-ch...QQcmdZViewItem

The seller is mrboss420r so you can look it up if the link doesn't work.

I hope that is the correct way to post the web address.

Trying to do this swap correctly and on a budget.

My other thoughts are a new eaton or detriot locker.

Re: which posi unit to get?
 

I bought a brand new eaton posi off there a while back for $180.00 If you are going to put any real horsepower to the rear end I wouldn't mess with a stock gm posi because I have seen quite a few fail. Just my 2 cents.

Re: which posi unit to get?
 

I might be a little careful with that. Auction says that will fit car or truck. As far as I know, the car & truck 12 bolt posi units are different. Could be a car one you would be getting. Someone else may chime in on this......

Re: which posi unit to get?
 

I believe that the trucks are a series 4 unit.

Re: which posi unit to get?
 

That is a govenor lock. You don't want it.

Re: which posi unit to get?
 

Truck & car units are not the same. The bearing carriers are different. The unit that is on ebay I would not touch. Truck units came in 3 sizes, 2 series, 3 series and 4 series. 2 series handle 2.73 gears, 3 series 3.08 - 3.55, 4 series 3.73 - 5.13. Any truck 12 bolt carrier will work in your housing, it depends on what gears you have to match the center section/posi unit. If you buy a second hand unit series 3 center section you can get extra thick gears on the 3.73 and up gears, this would allow you to run a 3 series center section with the taller gears. As far as the GM units not being any good. GM used alot of makes over the years, Dana was used in the 4x4's, Detroit was used in the 14 bolt rears in the 90's but Eaton has produced the largest number of units for GM. I have spent alot of time the last 3 months researching center sections and the common theme that came from 99% of the folks that build rear ends or sell rearend products is you can not beat the Eaton units. FYI the Moroso center section is a heavy duty Eaton unit. Not reccomeded for street use.
When you go thru your rearend make sure and buy 2 bottles of Posi lube not 1. Your center section will thank you for it.

Re: which posi unit to get?
 

I'm not sure if you consider 435 at the motor serious horsepower or not!?!?! I want to move to a 308 gear from the 411s in the truck. Not much RPM and burned dino for me with the 3 speed automatic. I've heard nothing but good things about the Eaton.

Anyone chime in on the auburn posi unit?

what about a detriot locker?

Re: which posi unit to get?
 

The auburn units are now considered throw away units. They have gone to a cone style clutch pack, when they fail, which seems to be sooner rather than later you throw them away, more $ to rebuild than replace.
The detroit unit that I had, 454SS, was a good unit, however they have a small spring in them that seems to break and cause' the unit to not work properly. FYI - I see on ebay tonight another 12 posi for sale - this one is out of Canada and should work in your truck.
Why do you want to move away from the 4.11s?

Re: which posi unit to get?
 

The 411s seem to make alot of RPM at 65-70mph. I don't have a tach, but with the 3 speed automatic it seems like I'm up there in the RPMs.

I don't have a posi right now, so I thought while I have it apart-just put in a better highway gear. Should still have a good punch off the line.

What do you think?

Re: which posi unit to get?
 

I have a 308 in my blazer and hate it. Of course I am running a 29.5" rear tire also but I would at least run a 3:42 or a 3:73. A 3:42 should be a real good gear if your not running that tall a tire. Just my 2 cents and I am no expert. My brother just put 4:56 in his 99 silverado, now what was he thinking? But he is also putting in a t-56 so they won't be that bad.

Re: which posi unit to get?
 

If I had the money, I'd just go with a gear venders unit. Best of both worlds then. That's some crazy money there.

I even thought about moving to an overdrive tranny, but the transmission I have has been seriously built up by the prior owner. He was saying titanium and kelvar internals.

Re: which posi unit to get?
 

Well put...Most of the true limited slip carriers in these trucks were eatons...The one in my 71 was. I am gonna rebuild it and use it in a different project and buy a new 4-series Eaton with carbon fiber clutches for my 71...

Re: which posi unit to get?
 

I bought an Eaton unit for my '71 bigblock build from Summit. I have $1200.00 just in parts for the rearend build. If you are going to put some serious power to it , don't settle for cheaper parts. It will cost you in the rearend.

Re: which posi unit to get?
 

The problem with what you are saying is that Eaton used to provide a clutch-type limited slip to GM WAY back in the day, then at some point when they developed their Gov-Loc unit GM started using those instead. Most of what you will find in the used market are Gov-Locs and not the Eaton Posi (which is their trademarked name for their clutch-type units). The eBay diff in question here is a Gov-Lock, not a "posi" as most people so wrongly use the term.

The Gov-Loc has gathered more than a few nicknames over the years, one of which being the Gov-Bomb due to it's tendency to fail at the worst time. It is not a clutch-type unit, instead it uses a centrifugal weight system to lock only when a significant amount of wheelspin occurs on one side or the other. However, above a certain RPM of the carrier assembly, the unit will unlock altogether. You can visit traction.eaton.com to see how it works. It's great for the boatramp but not for someone wanting to harness some horsepower. An Eaton Posi would be a much better choice, as the clutches in those set a pre-load and you have to exceed a breakaway torque (difference between left and right wheel traction) for it to slip.

Someone asked for an opinion on the Detroit. For a street truck, I wouldn't do it. Drag racing or off-roading would be fine ( I have a Detroit in my Nissan and a Lock-Right in my Blazer) but the tradeoffs in road manners for DD use on a 2wd would be pretty bothersome to most.

Re: which posi unit to get?
 

Looks like the best bet is to purchase a new Eaton with the 308 gear

Re: which posi unit to get?
 

I was faced with a similar choice 2 years ago with my 69 C-10. I chose to go with a new Eaton unit purchased through an Ebay store. I think it and the new ring and pinion plus a bearing/shim kit and shipping was about $450. I paid about $250-300 for the Eaton posi part. I now punish it regularly with my 454 and 3.73 gearing and it's never complained. I'm not only happy with it, but confident in it's long term durability.

Re: which posi unit to get?
 

Has anyone out there had any experience with a Detroit Tru-Trac helical geared differential? Supposed to be pretty tough and doesn't have clutches to wear out.

Re: which posi unit to get?
 

True Tracs use the same type of design as Torsen differentials, which were/are OEM on H1 and military Hummers. Helical limited slips are also prevalent in many Japanese high-performance and exotic sports cars. These types of units are "torque biasing", which means that ultimately if one wheel has absolutely no traction you still aren't going anywhere. For street use they are sweet and as you mentioned do not really wear out over time.

Re: which posi unit to get?
 

When I rebuilt my Eaton the differntial shop talked me out of the carbon fiber clutches saying the steel clutches last longer (they would have made more money had they sold me carbon fiber). What is the real story on carbon fiber versus steel clutches?

Re: which posi unit to get?
 

This may confuse you even more but....I installed an Auburn posi in a 72 12 bolt with 3.73's about 8 years ago. Promptly punished it repeatedly...I mean punished it...our dragstrip has Friday night fun drags (which I drove it to from the house 70 mile round trip). I passed the truck off to all my buddies several weekends in a row so they could share the fun... and literally never turned the ignition off while at the strip. They will let you come right back to the staging lanes from the return road. We played with the timing and carb all night and basically hott lapped the track. The auburn has never popped, cracked, or left anything other than two 10 inch wide equal length black stripes behind it (from hoosier q/t dot slicks). I think it's great (at close to 400hp anyway)..just my .02 worth.

Re: which posi unit to get?
 

Yeah thats a gov bomb, no good, I use a detroit locker, you'll hear it when it engages.........

Re: which posi unit to get?
 

I just found my invoice. I paid $350 for the Eaton Unit, and $110 for a new Motive Ring and Pinion via Ebay. It was invoiced by Drivetrain Specialists. As I recall, for 12 bolt truck rear ends, 3.73 and higher gearing takes series 4 carrier. 3.42 takes a 3 series. You can run a 4 series ring and pinion on a 3 series carrier if you use a ring gear spacer, but not the other way around.