I saw a used posi close to me on craigslist and the price is good so I wanted to know how to identify it easily. He is selling both a gm 28 spline and a "chevy"30 spline with the 3.42 gears that i want. First question, can i take the ring and pinion from his 30 spline unit and put it on his 28 spline unit. I dont want to change axles. Second question, how to identify these units?, I know what they look like and it does look like a posi gm rear end. Third Question, Except for looking for wear on the teeth on the ring and pinion, is there another wear point to look for? I'm going to attach these pics so you guys can take a look. GM 8.5 28 spline posi GM 8.5 30 spline posi unit with 3.42 gears
I wouldn't mess with that,...would suck to do all that work, buy it etc etc and it be burnt up and be no better than what you have
Those S clip posi's just aren't very good for one and 2 they didn't think much of it based on its appearance. I'd get with Brian or Jim and get a new Yukon and have peace of mind
I think the S-spring style posi units are one of the best units ever made. They can take plenty of abuse. I would just make sure the pin bore is not elongated or sloppy. There are new clutches available to rebuild it,but the originals might be fine. Even the original clutches in that are better than any clutch that comes in the new units. I just tune those units by shimming the clutches tighter,and removing the S-spring. It will make a nice street-friendly unit that will likely outlast all of us. There are two different series of those posi units. The 2-series,for 2:42/2:56,is casting#410409N,and usually have large 9’s cast into it. The 3-series,for 2:73-up,is casting #410408N,and usually have large 8’s cast into it. If you can get those gears at a decent price,they might be worth getting,but you do NOT want that other posi unit. It looks like a Gov-lock,which is not a good unit.
Ok -Get the casting number on the S spring unit as Brian said. If the unit is a 2 series You just talking more headache than it is worth. Good and bad. The other unit is a GM auto locker AKA GoV Lock unit- Gears will fit and appear to be decent from the pictures. Gov Bomb can be bench check with a 30 spline axle in a vice and a quick spin left hopefully a click and lock... then a spin right and spin quick right ... another hopely click and lock.... (PLEASE WEAR GLOVES when doing this ...you can cut your hands if the unit locks correctly on the sharp edges of the casting. ) Down side of the 'S' spring unit is there are NO Aftermarket parts to fix the unit... YES we have clutches BUT who knows how long that will continue ... I personally have 9 sets on the shelf. 200.00 a set. Spider gears NONE... you will always have to scavenge from another used unit. ( Might be a good reason to get this one even if just for parts IF THE PRICE is right) I know every one likes the 'S' spring unit and wants to believe they are superior because they are factory unit... That is fine and for the street and some stip they are good. BUT in all actuality the 2 big huge open windows are the units GREAT weakness. The flex in that case under hard drag racing load will usually make the spider gears pop and top each other . You have to remember when this unit was designed...When torque was starting to go away and emissions were coming in. The preload on the spring at best is 200# AGAIN I am talking for high performance applications.... I am sure for the street this unit is GREAT and do last a long time from the steel clutches that were used. flip side the steel clutches create lots of heat and will chatter more than aftermarket carbon fiber and composite too. This will require more frequent oil changes with ADDITIVE . Eaton / Yukon etc cases are clam shell and case is much thicker providing more stability to the spider gear mesh under load ... Spider gears are hardened - clutches come in a variety of steel, carbon fiber composite for tailoring the unit to your needs now and down the road...with that you can even tailor the preload spring pressure from 200 to 300 to 400 to 800 or NONE ... Personally my bet is always on NEW parts if you can afford them. BUT that is me. There there is another point of view on the posi stuff..... Not trying to make anyone mad... just give my points on the posi tractions and after seeing failure in different type units. Ultimately you have to decide what your car is going to do for you... pick the correct unit and price range you want to be in. See what the price is on the units get a close up of the spider gears on the s spring, and get that casting number 410409 or 410408 or 725 or 726 (bolt in axle units only side gears) Jim J D
I never had any luck with them,..I've had a few sold them to local fellas from under parts cars and would get a call saying they wouldn't lay rubber or one side would be light. Maybe just a coincidence idk. I'd buy a new one regardless
Jim, I have found by tuning the unit,shimming the clutches tighter,to eliminate the excess slop between the side gear and spider gear,and doing away with the spring(s) completely. The side and spider gears are making better contact when engaged deeper,and push out against the clutches,much like the springs do. Now the the springs are gone,they can relax or disengage while cornering,which will also lessen or eliminate the clutch chatter,and also decrease clutch wear. By having the springs in there,the clutches have pressure on them all the time. I do this to the S-spring units and the Eaton units. It makes a good,quiet,trouble-free unit for street driving. I have this same type of unit in my street/strip car,and it hasn’t given me any issues at the track. If it is more of a streetable race car,that sees the track a lot,but does not want a spool,then I will tune it,and install the clutches for extra insurance,but they have been known to chirp tires going around corners. Not what you want in your family cruiser. This is also the same reason you see the newer units with stiffer springs. They make the newer poly-whatever clutches that are not as aggressive,to favor the quiet cornering of street driving,but need more pressure for good bite.
No I am talking about the 8.5 posi case is just NO MEAT ...those 2 big windows are the problem. The flex in the case is why they are inferior...Nothing to do with the tightness of the spiders... If the car never hooks this is all a moot point. WERE talking raw stuck to the ground HP. As far as the street, anything will be fine as long as your not trying to do a burn out with a car that does not have enough power to get the job done or a slow progressive tire spin burn out which is torture to the spiders & clutches etc. Jim
I personally have not seen one of those S-spring carriers break,but I am sure it is possible. I have seen the original Eaton carrier break in half,as well as the modern copies. They all happened at the drag strip. You can’t really add any additional material to them because of of the inner diameter of the ring gear. That is your limiting factor. You can only fit so much into a confined space. Otherwise,you could have bigger spider gears,thicker cases,etc. With the double-window S-spring carriers,I see axle shaft failure before carrier failure,and at the point the carrier is removed because the rear is upgraded with 30-spline shafts and a matching carrier.
This is some great information and some good opinions. I think the price is right and I'm gonna get this s spring unit. I'll have to message you Brian and get pick your brain how to make these as strong as possible. This is a street car for daily driving, not much track use.
Well I always heard that the 71 and up has the 8.5 rear end. My 71 looks like it has a 8.2 it doesn't have the points on the button of the pumpkin ughhhh!!!!
NO Please provide more info. THAT is NOT a goot enough picture. Picture of the yoke ans back side of the backing plates too pleaSE.... aLL CAN BE SEEN FROM UNDER THE CAR AND NOTHING REMOVED FROM THE VEHICLE. hold tight BEFORE JUMPING OFF A CLIFF. lol. Oh man sorry about the caps not yelling.
Heres the info, Four bolts go through the backing plate into the axle housing, when i take the drum off, it doesnt have the 3 holes for the bolt on axles, as shown in the picture it doesnt have the right angle casting on the bottom of the pumpkin. It raining right now but tomorrow i can take some pictures, im not sure on the yoke so well see. Im just upset I planned everything out and forgot to verify the rear end.
You can’t go by the bottom of the pumpkin. There were two different castings. One had the square lugs,and one had the fangs. This is the style that came under all of the 71-72 Skylark/GS’s,and all of the 71-72 Cutlass/442’s that were NOT built in Lansing Michigan.
The housing around the pinion bearing and the tubes are a little bigger. On 8.5 housing. But if your not used to looking at them this wont help u much. The yolk will be bigger in the middle. Usually a bigger nut too. You can also tell when u pick one up. The 8.5 are noticably heavier
Cant see your tubes . youl know rite away. Its like looking at a fosters and a bud in can side by side
