Help with 7.5 gear and posi swap

[Bonnewagon]

Well I'm glad you are willing to learn because it's another lost skill. But I must reiterate reading up on it. You must know about pre-load, backlash, contact pattern, and the theory behind how hypoid gears operate. The first time I tried it I thought the factory manual was just a bunch of baloney and I didn't need to listen to it. So I swapped gears around just like you are considering. I buttoned it all up and thought what a genius I am. I got about 15 feet before the gears crunched themselves to bits destroying a rare 3:55 gear set. I them forced myself to learn the right way. To be fair, I once successfully set up a rear with nothing more than a set of feeler gauges. BUT, I knew what I was doing by then. As I acquired the right tools, my skills got better and every rear I've ever done since then is still working smoothly and quietly.

 

[Longroof79]

Mark,
What tools are required to do a successful gear change? I'm guessing a dial indicator is an essential tool on the list.

 

[Rick_B_86]

I'd suggest replacing the bearings and seals while you're in there.

Essential tools: Dial indicator, a good dial or vernier caliper. Bearing pullers. Torque wrench. Patience.

Non-essential but very handy to have: a pinion depth tool (they're pretty cheap off summit but it makes life a bit easier.) Dry ice and a deep fryer, or you can put the bearings in the oven and carrier/pinion in the freezer, works the same. NO HOTTER THAN 200F. Dummy bearings.

I spent 2 days getting my mesh pattern just right, gear lash set and bearing preload just right. Can't comment on if it works good or not, it's tough to drive a car when the body isn't on it yet.

 

[pontiacgp]

good idea on the seals and bearings, easiest method I know of the get the axles seals out is to use the tip of the axle to pry the seal out

 

[Turbo Zach]

I say go for it! It is a little tricky, but not rocket science. All you really need is a dial indicator. Torque rench for the carrier bearings would be a good idea. Most differentials set up all the same. If you can find a buddy that has done it a time or two, it will be no big deal. Most differentials are, 10 thousands back lash, 0 end play, slight drag on pinion, and proper gear mesh. I have done a lot of them in different things from payloaders to cars. If you were close I would help you. Should only take a few hours. Wost thing is, if you mess up and ruin your rearend. You are using used parts and a 7.5. Easy to get more. Best of luck.

 

[Bonnewagon]

It makes a ton of sense to replace all bearings and seals just because you are in there anyway. But most guys are trying to do the job on the cheap and won't spring for all those expensive parts. Jack, the Kent-Moore tool kit I have comes with a pinion depth tool (and various adapters for all the GM rears), and a dial indicator with mount. As mentioned, inexpensive depth tools are available. Any dial indicator will work but I like the kind with a big magnetic base. I have a 12 ton press and bearing clamp to change the bearings. You need an inch pound torque wrench for the pinion nut pre-load, and a foot pound torque wrench for everything else. A cut down bearing makes a great tester for the pinion shims as you can just slip it on and off. How come no one mentioned a shim kit? You know that you can't use the cast carrier shims that came from the factory because they are only correct for that carrier, with those gears, in that housing. Just like the factory, you need a shim kit to position the carrier so the ring gear is in the correct plane relative to the pinion gear. Once you do, and the backlash is right, and the carrier bearing clearance is .000", you add .008" pre-load by inserting a .004" shim on both sides. ( or adding .004" to the shims already there) That way the backlash doesn't change. Once the correct shim thickness is found the factory grabbed the same exact thickness cast shim off a parts rack and pressed it in place with an installer tool. Even if you are using the stock shims because nothing is being changed, banging it in with a hammer will break it. So you need to spend at least $50 on a shim kit, so might as well get the whole deal with shims, bearings, races, crush collar, ring gear bolts, seals, cover gasket, and dye for checking tooth contact pattern. I used to get those kits with USA bearings, for around $100-150. FWIW, if using GM gears and a GM carrier, in a GM housing, I have found that the pinion depth is usually within one or two thousandths of the original factory pinion gear shim. But aftermarket gears are all over the place and a depth tool is really needed. And carrier bearing shims are all different- so you need a shim kit. Again- read up on this.

 

[Longroof79]

Mark,
Thanks for all the good information and recommendations. Much appreciated.

 

[Bonnewagon]

Sure thing Jack. My goal is to keep guys from tearing into a rear without knowing what all is involved. It took me many years to learn the right procedure. As mentioned, it's not rocket science, but you have to read up on it and do it right. That's why I say to get a factory manual, any make, any year that uses the rears like in our cars, and read. I promise you doing your taxes is less mind boggling. If I can do it (and I'm an idiot) anyone can. But I would rather see a guy find a complete rear with the gears he wants, than break his and now he REALLY needs to find a rear.

 

[Texas82GP]

I attmpted a ring and pinion swap on my 2008 Sierra. I read up on it, bought a press, bought a dial indicator, etc. and still failed. At first it was great, then it started making noise, then it started leaking. I had a pro re-do it at that point as it is my daily and it had to go back into service. I am a detail oriented guy and I read and follow all directions. I ended up not crushing the crush sleeve enough and the pinion ended up loose. If I had time and did it again I think I could get it right. For the money though I think I will stick with paying a pro to do it. Especally if the housing is staying in the car. Thats a bit of a bear of a job to do on jack stands in the driveway.

 

[Bonnewagon]

It's a tough job even on a lift. Best way is out of the car, and at that point, you ask yourself "why am I not just swapping in a whole rear?". Jared, you were almost there. The concept is that the gears are held in suspension by the pre-load on the bearings. The .008" on the carrier bearings and the inch pounds of pinion pre-load all serve to hold the gears in position and hold steady under the gobs of torque that the driveshaft delivers to the gear set. Too much, and the gears and bearings make noise or get wiped out. Too little, and the gears flex out of position and make noise or get wiped out. What you were supposed to do was hold the companion flange steady with a bar then slowly tighten up on the pinion nut and check with a torque wrench how many inch pounds of torque it took to turn the gears and axles (NO BRAKES!). There is a spec for new gears and used gears and it must be right on or you get what you described. A new crush collar is not easy to compress and you will never know what's enough without checking the pre-load with the inch pounds wrench. As a bonus, if you crush it too much, you get to go back to the beginning with another new collar and start all over again. The final check is to mix up some dye in gear oil, apply it to the gears, and rotate the pinion while applying pressure to the ring gear as if under driving load. The gears will wipe the dye off each other and you can read the pattern which corresponds to the gear contact. Any factory book has a set of patterns to compare them with. That will indicate if the gears are meshing properly, or need to be adjusted again. Are we having fun yet?