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Old 02-05-2010, 11:00 AM
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GM posi rearend gear change

I have a mid 70's gm 12 bolt posi rear end w/ a 4.88 gear and drum brakes. I want to change the gear to a 3.42 or 3.73. If I change to a 3.42 r&p, do I need to change the spider gears? If I change to a 3.73 r&p can I stay with the spider gears I have?

Because I have never disassembled a rear gear before, should I leave the switch to the professional or can a rookie backyard mechanic tackle the project?

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Old 02-05-2010, 11:30 AM
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If you have a 4 series differential then you are going to have to change it to a 3 series. If you have a 3 series then you probably have a ring gear spacer or a set of thick 4.88 gears.

Do you know how to set the pinion depth, bearing preload and backlash? Do you know how to read the contact pattern so that you will know what adjustments need to be made to the pinion depth? Do you have a dail indicator with a magnetic base, a press, bearing seperator and other tools needed to do this job? If not then take it to someone who does. If you feel like learning how to do this yourself then you are going to need a lot of patients. You have to assemble and disassemble everything several times before getting it right. It's a lot of work and requires precision adjustments within +/- .001 inch.
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Old 02-05-2010, 01:28 PM
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Save your money and do it yourself! HAH!

Quote:
Originally Posted by big gear head
If you have a 4 series differential then you are going to have to change it to a 3 series. If you have a 3 series then you probably have a ring gear spacer or a set of thick 4.88 gears.

Do you know how to set the pinion depth, bearing preload and backlash? Do you know how to read the contact pattern so that you will know what adjustments need to be made to the pinion depth? Do you have a dail indicator with a magnetic base, a press, bearing seperator and other tools needed to do this job? If not then take it to someone who does. If you feel like learning how to do this yourself then you are going to need a lot of patients. You have to assemble and disassemble everything several times before getting it right. It's a lot of work and requires precision adjustments within +/- .001 inch.
Hi, I am a first time hot rodder, but I am not willing to listen to stuff like this! I did my 8.5 inch GM ten bolt, bought all the stuff and gave it a shot for about a month, maybe longer....no lie. I made some parts so I could get the pinion bearing on and off to try various spacers, I built a yoke holder so I could get the pinion nut off, I built a bearing puller to get the wheel bearings out....I bought an inch ounce torque wrench to set the pinion pre-load(?) and on and on and on! I spent about 1200 clams on parts not counting the tools...got the rear end about ready to put back in...then I thought about all that time and money....So I took it to a rear end shop and gave the guy a C note to see if my set up was right, just to make sure. It was CLOSE he said! CLOSE!

When I bought the rear end THAT guy had it close too. He went about fifty feet, then BOOM! (Wrong spacer behind the ring gear.)

I bought the thing from him expecting to just put in a ring gear and save some money. HA HA! I had the wrong carrier for that....then I started learning the hard way.

Take the thing to a rear end shop! I'm stubborn and pig headed, but I'm not crazy. I hear from others around this place, automatic transmissions, engine rebuilds, rear end rebuilds, this is where the hobby types like me draw the line. I don't mind doing the work, I like doing the work, that's what is so rewarding, doing the work. But, hey, I can't afford to make a 1200 dollar mistake.

Just my two cents

R
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Old 02-08-2010, 06:52 PM
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Thanks guys! Nothing like a good dose of reality to straighten me out. I'll be taking it in for sure.
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Old 02-09-2010, 11:59 AM
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Quote:
Originally Posted by jpd37
Thanks guys! Nothing like a good dose of reality to straighten me out. I'll be taking it in for sure.
To answer your question, No you don't have to change the spider gears unless they're bad. I'm assuming your dealing with a car diff. If you do have a 4 series carrier there is a "thin" 373 ratio gear set that will fit on the carrier.

X2 on having it done at a shop. It could very well save you money in the long run. Just do some price checking on the shops.
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Old 02-09-2010, 12:33 PM
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Just be sure that the shop you take it to also knows how to set up the gears. I've seen a lot of shops that said they could do it, but didn't have a clue how to do it right. I talked to a GM mechanic with 20 years experience about a set of gears that he installed in my firends '87 GN. I asked him how much pinion shim he used and he said "What shim? I didn't put any shim in it." I tried to explain, but he wasn't listening. He just wanted to brag about his $5000 Snap On tool box. My friend took his car straight to another dealer with the gears whining all the way to get another set of gears put in it. Ask questions about pinion depth, backlash, bearing preload and contact pattern and see what they have to say.
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Old 02-10-2010, 09:15 AM
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A LOT depends on how bad you want to learn something. This has to be tempered by your skills and the information you have available.

Nothing beats hands-on experience, guided by a professional or serious hobbyist who can walk you through the procedure.

That said, pre-interweb days (for me, anyways- 1990) when info wasn't so easily obtained, I sat down w/a 12-bolt car diff that came w/4.56 gears and a Chevrolet factory Overhaul Manual for the diff, and successfully changed the gears out for 3.31's that I traded a set of ladder bars for.

This is the exact same thing I did when I rebuilt the TH400 for the same car, right after doing the diff.

My posi was fine (the one w/the 4.56) and needed no new bearings. The 3.31 pinion got a fresh bearing on final assembly, another bearing was relieved on its ID to slide on and off during set-up.

I used an adjustable solid 'crush' collar. The axle bearings were knocked out of the tubes and new ones installed w/seals. My axles were also OK, no bad wear on them from having served as the inner race to the damned axle bearings- which has got to be the worse set-up going!

I started out w/the original shims that were installed as a starting point, and didn't have to vary them much when setting up the 3.31.

The bottom line for me is- anyone w/the tools, a manual and the desire CAN do it. But if you have to ask how, you probably shouldn't attempt it on your own, unless failure IS an option.
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Old 02-10-2010, 11:07 AM
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Wow guys! thanks for the advice and background. I am now more convinced than ever that I will take the car in to have the gears changed. I don't have much money to spend on my hobby, but when I need some serious work done, like the rear end, I can only afford to do it once. So my final decision is to have a pro do the work. And I have a few very reputable local sources that specialize in rear axel assemblies. Thanks again for the advice. This is by far the best site I have ever used when looking for advice.
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