Need advise on rearend

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Need advise on rearend

Post by Hs1973 on Tue Aug 23, 2016 2:56 pm

My chevelle needs a new rearend. I have a 10 bolt 8,5 from a Monte Carlo with out posi and a 2:74 gear. I have a chance to buy a cheap rearend from a 1977 trans am with a posi and 3:42 gear.

My question now is will the posi unit and gear from the trans am fit my Monte Carlo rear. I know i need to use my old aksels since the Monte rear is wider. And If it fits, is it at all worth it to spend the cash on a used rear. Or should i rebuild the Monte rear with new parts. Im going to buy a kit with bearings and crush sleve shims and so on no matter what. But what to do with the gear and posi. New or used?

All info are welcome as im not that much into rear ends.

Regards Henrik.
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Re: Need advise on rearend

Post by bracketchev1221 on Tue Aug 23, 2016 3:53 pm

Well here are some ideas. Yes the gears and posi unit out of the trans am 8.5 will go in. However, the gears are used, and setting them back up may or may not work out exact and they may be noisy. But if the posi unit is in good condition, it may be worth giving it a shot and worst case replace the gears later.
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Re: Need advise on rearend

Post by Joe73 on Tue Aug 23, 2016 4:06 pm

If I remember correctly, around 1977 GM messed around with different ring diameters and axle spline count.
I'd double check the trans am rear. It will be either 7.5 or 8.5" ring gears and either 28 or 30 spline axles.
Maybe go to rockauto.com and check trans am listings for ring and pinions. Should give you the info you need.
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Re: Need advise on rearend

Post by chevellelaguna on Wed Aug 24, 2016 3:47 am

I did this very thing, had a spare 2 series (2:73) 8.5 rear, and bought a used 3:42 posi carrier. I actually paid a local guy that built tear ends to set up the gears, just to make sure it was done correctly. I didn't want to buy the pinion depth checker tool. Turned out great, however there was an issue with putting the 3 series gear set in the 2 series rear end. I worked in a machine shop at the time. There was a spacer used behind the 3:42 gearset, he told me how much he needed removed from it to get the proper mesh pattern, he couldn't obtain it with spacers on the carrier bearings alone.
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Re: Need advise on rearend

Post by Hs1973 on Wed Aug 24, 2016 11:42 am

In that case im leaning more Towards buying new parts that fit. I cant tjek the trans am rear before i purchase it. And he wants 300 dollars. Thats alot for a used rear that Might not work, or make noise.
So i Think ill go on with building the Monte rear with new parts.
Any ideas on what kind of posi unit to shop for. Im pretty set on a 3:42 so thats kind of decided. But there is a gizillion makes and models of posi units.

I also read that about 2 gen housings and 3 gen carriers Being difficult. So i that just makes me even more not buying the trans rear. Here in Denmark it would be hard tomfind someone that can set it up, If there is an issue like that. And i cant do it myself.

I need something that i can drive every day without any noise issues. Thanks guys


Last edited by Hs1973 on Wed Aug 24, 2016 11:47 am; edited 1 time in total (Reason for editing : Typo)
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Re: Need advise on rearend

Post by 77ElClassic on Wed Aug 24, 2016 11:51 am

I've used the gears and posi units from 2-wheel drive Chevy trucks from the mid 80s. They seem to hold up well and are easily installed. Also readily available in both 3:42 and 3:73.
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Re: Need advise on rearend

Post by Joe73 on Wed Aug 24, 2016 1:28 pm

I agree, chevy trucks offer a good option. Only thing to look out for is a "gov lock posi". You dont want that unit. They are prone to cracking. I've blown 2 of them and so has a buddy of mine.
Google "gov lock posi" and click images so you can see what they look like. Very easy to identify.
If your going to buy a posi. I'd go with an Auburn for the street. Or you can also find reasonably priced Yukon posi units on the net or ebay. They're good as well and an exact copy of an Eaton posi but a bit cheaper.
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Re: Need advise on rearend

Post by Joe73 on Wed Aug 24, 2016 1:31 pm

BTW, if your going with a used posi with ring an pinion, its best to purchase a cheap dial indicator and measure the gear backlash prior to removing it. And take a pic of the contact pattern on the front and backside of the ring gear so you can match both specs up later.
Since the gears are used they are worn in already. So if you match the backlash and contact pattern you have way better odds of the unit being quiet.
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Re: Need advise on rearend

Post by Hs1973 on Wed Aug 24, 2016 2:13 pm

I Think im going to buy new parts. There is not alot of chevy truck rearends here in Copenhagen. And i am not interested in buying used parts and find out it dosent work as intended. Used parts in Europe are expensive and new parts are not that much more. And i can order it to fit right first time around.

Also i need someone to set it up for me, i am used to doing bodywork and other mechanical things, but a rearend i have not the tools or the knowledge to do myself. And i would hate having to pay a mechanic to find out it is not as i would like. So new parts it is. And Thank you for the good advise, i love this site for all the knowledge and all the helpful guys that are here.

Regards henrik
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