Experts in rear axles, I acquired a 3.31 geared open Chevrolet 12 bolt and everything seems in decent shape. I'd like to install a posi unit, however I don't want to spend alot on it. What units are decent quality for a decent price? I figure stock ones are hard to find, Jegs has one for $330 and they go up from there. Not for racing, just street use and probably never over 400-425 horsepower. I'd appreciate some input. Thanks Brent
dO NOT GO CHEAP ON SOMETHING THAT MOVES YOUR CAR....!!! lol. If your going to keep the car spend some money 400.00 and up Jim JD Race we have Yukons and I try to give the best deals I can for these to the registered forum goers
I would either find a good used OEM unit,or get a TrunTrac unit. ALL of the clutch type units on the market have junk clutches in them. Doesn’t matter if it’s Eaton,Yukon,sister Yukon,fake sister Yukon,Monkey box,or whatever. By the time you put the good clutches in the unit to make it nice,you will be at or above Tru Trac money. The OEM units have the best clutches ever made for them. I would also suggest getting a new pair of c-clip shafts. The 50+ year old shafts are probably worn on the bearing surface and not really made to handle the modern power we are making today. It’s just good insurance. What car is this rearend going into and what will the purpose be with the car?
My preference would be to find a used OE unit, so it's not made off shore. I've the used trutrac units before and they seem to work well, but they are made in Taiwan. I've also installed a few of the AAM gear units in 30 spline apps and they work well also. I pulled the axles and they look pretty good, you can see where the bearings have been running, but no pitting or wear. This unit will actually just go on the shelf for a future build, probably a mild big block in a street car. I built 1 12 bolt in college because I listened to everyone else and believed they were the best. Once I was properly educated I have built 8.5s ever since, lol. This one just fell in my lap and I'm running out of 8.5s. Does anyone have a used one they would sell? Thanks Brent
The original posi units are floating around. You want the #140 casting for the 3:31 gears. Yes,the Tru Trac unit IS made in Taiwan,BUT it IS a very nice unit. I have plenty of them in rears that are holding up under 9-sec cars amd they are your typical full-weight or close,not a gutted potato chip. The next option between the Tru Trac and a spool is the Detroit Locker,which I run in one of my custom 12-bolts. But with your plans and intentions with the rear,you don’t need anything like that. The Tru Trac is nice because it doesn’t have clutches or cones to wear out and it corners smoother than a typical posi unit. No additive or special oil needed.
yOU WANT A REAL NOISE MAKER? BUY THE ORIGINAL GM EATON UNITS. yOU BETTER BE READY TO INSTALL PLENTY OF ADDITIVE...LOL. Steel clutches are just inherent noise makers...The spider gears start to get smooth and round off clutches get glazed all this dooms these units to NOISE. So now if you think your going to rebuild the posi unit for cheaper than a NEW unit...Think again. spiders 200.00 and clutches 150.00 your already at 350.00 in parts. preload spring kit 50 -75...??? Labor if you do not do you own mechanics... You have to understand, Americans put up with these imperfect cars back in the day...ALL the old cars were rattle traps, windows leaked, main seals leaked etc. We fell in love with cars cause it was CHEAP freedom form the daily grind. Go take a drive to the country, tear up the road, go to the drive in, eat at a drive thru... Listen to music! WE did everything in our cars!!! How could you not have memories... We just do not remember all the negatives about old cars because that was all there was.... NOW we all are modernizing old cars the best we can to make them more reliable drive-able in all ranges LS motors fuel injection A/C custom interior better suspension parts etc. WHY because technology has improved the cars and were getting ready to CHANGE again! ELECTRIC SKYLARK ANYONE!.....lol...IT WILL HAPPEN if not already! Jim JD Race
Jim, When I build or rebuild the Eaton units with the steel clutches,I shim them properly and leave the spring pack out. Quieter around the corners and more life out of the clutches because they aren’t constantly engaged. Everything still works as it should.
I know Tom taught me how... TOM is the guy who let me in the business first....lol that was a while ago. Tom can get away with that he was the posi Eaton GURU. Most people want to see the spring pack in that rear or the feel they are not getting what is paid for. I do not have the time to explain that trick (NO PUN intended...lol) to everyone I sell Eaton posi unit to. For street strip application drivers, most can use the Carbon fiber and composite clutches with out any issues especially corner chatter. Also Tom said you have to keep up with the clutches for proper engagement... It is NOT a shim it and forget it deal. There are many ways to skin the cat for sure. If it works for you run with it. Remember to the NEW cases Yukon and most aftermarket Eaton are now thicker cases where the spider gears live. THAT is important again to keep the deflection of those spiders in check....WHICH cases most failures to posi units. That is another reason the true trac worm gear units are better than the factory GM units the true trac bodys are hardened steel the GM units were cast iron again and with worm gears sliding back and forth through those worm holes you know the hole will wallow out after time with out a harder material. (7.6 and 8.6 factory units) Jim
Another thing with the Tru Tracs is that the load or force is spread out to more parts versus a traditional posi unit that has contact between the spider and side gears. The Tru Trac has more contact points or more surface,however you want to describe it. They really are a good unit. I don’t do too many tuned posi units anymore because by the time I get the unit,get the good clutches,and make it all good,you can be into Tru Trac for less money and yet still have a superior unit. The only ones I still do are the rears that don’t have a Tru Trac available,like the BOP 8.2”. Yea,I know,trying to convince the customer that the unit will work fine without the spring pack can be a challenge as well. The stiffer 800lb springs are just a band-aid for a bad clutch system. They wanted a quieter clutch,which is less-aggressive,but still needs some bite. The other problem is the newer clutches are thicker,so there are less clutches in the pack to make friction,so another reason for a stiffer spring. The original 18-plate set is still the best. The thinner 22-plate are too thin and I have seen them rip the tabs off,but they also have a stiff spring in them.
So Brian and I can sit here all day long and get down right over technical about everything and anything because we are always looking for the tiny advantages we can get...THAT is how it gets when you do this stuff day in and day out. BUT for everyone else...I really think we agree If you buy NEW you usually can not go wrong for trouble free operations... and most warranty for a year from most any defects. Buy from a reputable person and I think your going to be good. I personally will try to go over what you the customer want from your car and what you want to do with your car/ truck / vehicle...THEN we make a good educated choice. Used posi units are a pig in a poke. Sometimes you get a winner sometimes not so good.... I have trained myself to look close for tell tale signs on used unit in the yards and swap meets. AS stated before ....do not go too cheap on the drive line parts or you will be stuck on the side of the road at the mercy of a tow truck drive having a BAD DAY! Good luck. Jim JD Race
I am restoring a very rare rearend right now,and it is nothing but one problem after another,but I will get it. I will run out of parts to replace.
That happens when your posi is not tight enough and the spider gears spin like an open differential in dirty oil. ALL the floating silt draws to the side gear that is spinning fast and making a magnetic current fusing the material and making that mess. Jim
It’s your typical factory assembled unit. They were done very vaguely back then,and this can be a common result,especially if it was driven hard or raced. This particular one came out of a rare Oldsmobile W27 with the aluminum center. Square-tab,S-spring posi. You know the deal.
This is a prime example of why you don’t take a used or junkyard posi unit or rearend and “just throw it in there”. Service it,rebuild it,whatever. Same goes for the cone units. Jim does an excellent job of re-machining them to tighten them up. Good preventative maintenance.
Yah if it was tracked ...one tire in the water one not is a torture to the posi clutches as one wheel will spin the other will hook...OUCH
