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Hell Camino's Rear Axle Upgrade

March 2009
Upgrading the rear axle to Posi was so involved I decided to make it a page by itself. This was my first time attempting it, so take my advice in stride. There may be shortcuts that I didnít know of, or didnít use, but I think the information here will get you started. I also have no idea how these instructions will translate to any other type of rear axle. I didn't find much information about the Chevy 10 bolt upgrade on-line, but Chevy High Performance Magazine had one of the best articles I could find so I referred to it often.

This project helped me address the last major hindrance to my goal of a 13 second quarter mile ET. My car came stock with a peg leg (non posi) 7.5 inch Chevy 10 bolt rear axle with a 2.73:1 gear ratio (good for highway driving, not at all good for racing). I had been searching the Internet for months trying to decide the best way to resolve this problem. I narrowed it down to 3 options; I could upgrade the stock axle with a posi carrier and maybe a few other parts to help beef up the less-than-desirable stock axle (not exactly known for their durability), or I could try to find a good used axle out of a Monte Carlo SS or Buick Grand National (more durable but very hard to find), or spend on a bolt-in unit from Currie Enterprises or Moser Engineering (nuff said).

Each option has its proís and conís, and I flipped back and forth for months until I made up my mind.  I wish I could have afforded the bolt-in Ford 9 inch axle, but I couldnít justify the cost. They offer these with a variety of options including rear disc brakes, the beefiest axle shafts available, your choice of gear ratios mounted in the center section, and factory mounting points pre-attached which makes these truly a bolt-in assembly. Currie and Moser have these available for about $2000 to $3000.

The Monte Carlo and Buick axles were heavier duty than mine, but they are very hard to find, and the people selling them know this. I have had conversations with people who made it sound like you could find these on any street corner, but trust me that isnít the case. I know swap meets are a good place to look, but I have been working night shift for the last year (with Sundays and Mondays off) and canít always make it to these events. When you can find them, they cost $400 to $800 or more.

My final decision was to upgrade my stock axle. The 7.5 inch ring gear had a 26 spline axle shaft and was used until 1988. Then Chevy upgraded to a 7.625 inch gear with 28 spline shafts. The more splines on the axle shaft, the better they will resist braking. I ordered a 7.625 inch, 28 spline Eaton Posi unit and a Liberty Gears 3.73:1 ring and pinion set from Power Train Parts online. The pinion gear and ring gear are a matched set; you should never replace one without the other. I also got an installation kit that included new bearings, seals and shims for the carrier, and outer axle bearings and seals from Summit Racing. Then I ordered new 28 spline axle shafts from Moser Engineering with the correct 5 x 4ĺ wheel bolt pattern. All together I figure I spent $1400, but I have an axle that should be more than capable of handling my 350 HP engine, an occasional run down the drag strip and a burnout or two on the weekends. It took a week for the parts to arrive, and another week or so for the weather to warm up enough for me to want to lie on the cold concrete to start pulling everything apart.

I know there are people who do this while the axle is still on the car, but honestly it isnít that difficult to pull it out and it makes the job much easier, especially for the first timer. I also spent an extra $30 on a spare rear pinion bearing (the larger of the two pinion bearings). This has to be pressed on, but you have to know what size shims to put behind it. There is a trial and error element here that will drive you nuts if you donít have the spare bearing. The chances of damaging the bearing while trying to remove it if you donít get the shims correct is great, not to mention you need a special tool. Youíll also need to have the bearings pressed on to the carrier (posi) unit, but that does not require shims and can be done right away. To get all the bearings pressed on cost me $10. I found that asking an auto shop that is capable of rebuilding a rear axle will charge $10 per bearing, probably because they lose money every time we DIYers attempt these projects on our own. The local auto parts store is a bit more reasonable. Unless you have a press available, donít try this yourself. Buying the spare pinion bearing allowed me to hone out the inside of the second one so it would slide on and off with little effort (and no tools). Youíll thank me for this tip later. Be careful not to hone it out too much, there is only about .004 inch difference between the size of the pinion shaft and the inside of the bearing. You want it snug, but not tight. I used a die grinder with a small 80 grit flap wheel.

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Removing the Axle Rear Axle Removed Hone the inside of the Spare Pinion Bearing

Itís very important to keep your parts straight as you remove them. The carrier bearing end caps MUST go back on the correct side. Also keep your shims separated too, this will come in handy later. Like I said before, fitting the correct shims is a process of trial and error, expect to install and remove the gears as many times as it takes to get it right. You donít want to spend this much time and money only to have it fail because you were in a hurry.

First youíll want to remove the old bearing races and install the new ones. These are very tight and will need to be knocked out using a good hammer and brass drift. Be sure to use a BRASS drift (a brass rod about ĺĒ thick x 8Ē long) so you donít break the seat on the housing, youíll use it again later when you are setting the pinion depth. Brass is softer than the other metals and damage the your new parts. Clean the area where the races sit to make sure there is nothing that will keep them from sitting flat, then drive the races on using the brass drift. You donít want to mar the bearing surface so be careful.

 After thoroughly cleaning the inside of the axle youíre ready to set the pinion depth. I was told a good starting point is add .030 inch shims under the rear pinion bearing, then either reduce or add shims as needed to get the correct depth. This is usually done with another expensive tool called a pinion depth gauge. You can ask a local shop to do this, but expect to pay for it. Here is the method I used; There should be a number either stamped or etched onto the flat surface of the pinion gear, this number (mine was CD 2.314) is the Check Depth, or pinion depth. This is the measurement from the flat surface of the pinion gear to the centerline of the ring gear with everything torqued down correctly. You can get this measurement by placing a straight edge across the surfaces the bearing race rests and measuring down to the top of the pinion gear. Then measure the diameter of the race outer surface and divide by 2. Now add the distance to the (half) diameter. If it doesnít equal the number on your pinion gear, you need to add or subtract shims to get as close as you can. These were the best instructions I found to explain this process.

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Cleaning out the Axle Tubes Brake Cleaner works good

Install the correct shims and pinion shaft using your spare rear bearing, do not install the crush sleeve until your final assembly, and then install the front bearing, yoke, washer and pinion nut. Youíll notice the front bearing will also not slide onto the shaft. This one is OK to leave alone. As you tighten down the pinion nut, it will drive the bearing down. When you remove the pinion shaft youíll have to use the brass drift to drive it off the front bearing. You can do it without harming the front bearing or the pinion shaft if you use a little common sense. You can either use an impact wrench to tighten the pinion nut or try using a torque wrench, I found the impact wrench worked better. Tighten the nut until the bearings seat on the races (the yoke has no more free play when you try to pull it out) then slowly tighten until the yoke is tight enough that it turns with little resistance, but will not spin freely if you try to spin it like a top.

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Moser Axle Shafts = Much Stronger Eaton Limited Slip Posi New Pinion Gear on Left

After installing the pinion the first time youíre ready to install the new carrier (posi unit). I started with the old carrier shims, which I set aside earlier, to get a baseline on how many shims to use. Thankfully the new carrier fit the same as the old one, so I was able to use the old shims to get an idea of how tight it should be. The shims place a pre-load on the bearings and hold the carrier from moving side-to-side. You should be able to tap the shims in without a lot of force, but they should not pull out without some effort. The thickest shims should be placed against the housing and the bearings, with the thinner shims in the middle. This is a good time to get hold of a caliper or micrometer so you can measure each shim. I found it easier to place one shim on each side of the housing and drop the carrier between them. Then I added the thin shims, and left the last thick shim as the one I tapped in. The reason is, if you try tapping on the thin shims, they will bend and it will throw off your adjustment. There are usually several .100Ē thick shims in each pack, use those to tap in.

Before you start tightening down the bearing caps, try turning the pinion yoke. If it binds you probably need to take everything apart and remove some shims from the pinion shaft, this will lower the pinion in the housing and give you more room. Itís a good preliminary test that may save you a little time.

When youíre happy with the feel of the yoke you can tighten down the caps and try turning the yoke again. If when you turn the yoke you hear it grinding (but not binding) the carrier is off and the shims need to be adjusted. If you hear no grinding and no binding, but there is too much backlash (distance where the ring and pinion do not meet) the carrier shims need to be adjusted the other way. Trial and error my friend. The carrier shims should always be adjusted evenly from side to side. If you remove a shim from the left side, an equal sized shim should be added to the right side. Backlash will need to be measured with a dial indicator. They arenít expensive and will save you a headache if you donít bother to verify the correct distance.

Now you can use gear marking compound to see what kind of pattern your gears are making when they mesh together. The pattern will determine if you still need to do any more tweaking with your shims. A proper pattern should be roughly centered on the gear teeth and have oval, or rounded, outside edges. Once the pattern is verified you can take it all apart for the last time, have your rear pinion bearing pressed on and put it back together. I would suggest a good cleaning of all the parts before reassembly to be sure they didnít pick up any stray garbage from your work area.


 

This is by no means a complete set of instructions. There are way too many variables for me to have included everything. But this should be a good guide to get you started if you feel the urge to try this yourself.

Here again are links to the 2 websites I used:
Setting the Pinion Depth (Richmond Gears website)
Installation Instructions (Chevy High Performance website)

Good luck.

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