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#725191 - June 30th 2017 9:49 am 1985 Chevy with 3:42 (GU6) rear axle, want to change to 4:10  
Joined: Sep 2013
Posts: 519
tuner4life Offline
tuner4life  Offline

Joined: Sep 2013
Posts: 519
I do a bit of towing with my 1985 G20, it's no secret that the 305 struggles with pulling anything over the van's own weight around. My original plan was to use the 305 until it showed signs of dying then swap to an LS, but after nearly 3 years of use, mostly towing a trailer with a car on it, the engine still seems to be in good shape. I figure I might as well try to make it's life a bit easier. Plus it will be nice to have that already taken care of when I do swap to LS power down the road.

The van currently has a 3:42 (RPO code GU6) rear gear and I want to swap it to a 4:10 with posi. I've done a bit of research, but I am used to the imports with independent rear suspensions and the differentials are much simpler to set up (less of the marking/painting/crush spacers/etc). The old GM rear ends seem a bit more complicated to set up plus I have no clue what the best parts are to buy.

So, my questions:

-What should I buy to convert my rear end to 4:10 with and without Posi? Trying to see what all the options are, but what brands should I look at vs avoid, etc.. What do you recommend?
-Also, what is a link to a good guide for setting up a rear gear like this one? I've read a few different "instructionals", but looking for something more technical with actual torque specs and such.

Thanks in advance!

Last edited by tuner4life; June 30th 2017 9:50 am.

[Linked Image]
1985 Chevy G20 305ci/4spd Manual
#725380 - July 05th 2017 8:03 pm Re: 1985 Chevy with 3:42 (GU6) rear axle, want to change to 4:10 [Re: tuner4life]  
Joined: Mar 2013
Posts: 94
DC79 Offline
DC79  Offline

Joined: Mar 2013
Posts: 94
Lake Isabella Califronia
I have a 84 G30, 350 with a 700r4 3.73 with 9.50R16.5. Ive pulled a u-haul auto trailer, the one with all 4 wheels of the car off the ground. I ve had a 2001 corolla and a 1997 Jimmy (small) on it. its ok on flats at 65mph, but any slight hill and its 3rd gear. up a hill its 2nd and under 55mph. I have thorley try-y's for it but have yet to put them on.
I think saving for the LS or for me a 383HT is the way to go.

1984 G30 Chevy
#726326 - August 01st 2017 7:31 am Re: 1985 Chevy with 3:42 (GU6) rear axle, want to change to 4:10 [Re: tuner4life]  
Joined: Sep 2009
Posts: 1,039
Bullet Offline
old hand
Bullet  Offline
old hand

Joined: Sep 2009
Posts: 1,039
Bristol, TN
Tuner, I would suggest a Detroit TruTrac or similar true limited slip, not a locker. Lockers are not pleasant for everyday driving and dangerous in slippery conditions. Indiana sees its share of snow so stay clear of the lockers or the "lunch box" lockers like Spartan and such. They do good for what they are designed for but you need a limited slip for your purposes. The best bet is to buy a 'kit' which will include gear, bearings, seals, crush sleeve, ring gear bolts, etc. I usually end up buying an extra crush sleeve, sometimes two before I get everything set back up to my satisfaction. Your diff is a C-clip arrangement meaning to pull the axles, you must first pull the cover, remove the cross shaft locking bolt, slide the cross shaft out, then shove the axles all the way in, use a small pencil style magnet to remove the clips, at this point your axles will slide out. The axle bearings are in the housing at the end, not pressed on the axle itself. Check your axles over very well, especially on the very end where the c-clip goes on. Unfortunately there seemed to be some issues with the hardening of the spline end of the axles which rears its head on some GM axles in the 80's-90's. If it is nice and flat with no wear, you should be good but buy new c-clips, they are only a few bucks. If they are dished or mushroomed, get new ones, it's a lot better than having an axle exit especially with a trailer on back. You can get new axles in Dorman brand from Rock Auto for about $150 but that price includes new bearings, seals, and wheel studs where most replacement axles do not. As for removing that lock bolt, when you put a wrench on it and try to loosen, if it doesn't come loose with the force you would think, stop, get some brake clean and clean the grease out of the diff as best you can, get a torch, propane will work, and heat the carrier where the lock bolt goes in. They are 'supposed' to be installed with red Loctite and if it was installed correctly (I've seen a lot just come loose on their own), the only way you will get it out in one piece is by heating the area just below the head of the lock bolt.

After you have all the above done, mark your carrier caps, which side and which end is up, remove them, roll the carrier out without mashing anything fleshy if you can, I usually can't. Be sure to keep any shims that come out on the side they come out of. Next you need to try to get a reference measurement on your pinion depth. I have a piece of 1/4" steel that's been ground flat and parallel, 2" x 6" or so long that will rest across both flats where the carrier bearing caps bolt. I also use a set of calipers with a foot so they can be used to check hole depths. I get the measurement from the bearing cap flat (which is centerline of the diff) to the top of the head of the pinion. Remember to subtract the plate thickness. There are other ways to do this, it is mainly to help give you a starting place to place the new pinion gear. You can remove the pinion after you get that measurement. You don't HAVE to have that measurement but it will be helpful. Knock out the bearing races in the housing, clean it all up and then we'll start putting it back together. I'll try to post some pics to help, this isn't a difficult job but it does take a little patience, a good dial indicator and magnetic base, the pencil magnet referenced above, a good torque wrench and a good beam style or digital torque wrench when setting the pinion pre-load (not a clicker). It helps to have bearing race drivers and you'll need a slide hammer and bearing puller to get the axle bearings out of the housing. You can rent those from most auto parts places but the other stuff, you may have to purchase. The tool aspect is why a lot of people end up paying someone to change their gears, if you don't do many gear changes, the tools are a bit pricey for a single use or two. A pretty good torque wrench will set you back at least $75 and the digitals go all the way up into hundreds if you buy off a tool truck. I'll post more later, gotta get back to work, lol.

'85 GMC Shorty
'99 AWD Astro
'98 Suburban K2500
'77 Pontiac Astre (future Big Chief)
'95 Jeep Wrangler
"I thought you weren't allowed to talk to guys with vans??"
#726363 - August 01st 2017 9:02 pm Re: 1985 Chevy with 3:42 (GU6) rear axle, want to change to 4:10 [Re: tuner4life]  
Joined: Mar 2011
Posts: 250
jcd74 Offline
jcd74  Offline

Joined: Mar 2011
Posts: 250
Can you change your intake, carb, and install headers and still pass inspection? Have you done this already? Your lucky to have 3.42's. When my 84 G20 was running with carb, intake, and stock manifolds, single exhaust it had no problem running 1700 lbs. of junk to the dump and that was 3.08 with 28" tires.

Last edited by jcd74; August 01st 2017 9:07 pm.
#726365 - August 01st 2017 9:08 pm Re: 1985 Chevy with 3:42 (GU6) rear axle, want to change to 4:10 [Re: tuner4life]  
Joined: Jun 2010
Posts: 12,479
frscke1 Offline
frscke1  Offline

Joined: Jun 2010
Posts: 12,479
Van Diego California
I put a Eaton posi in my white van w/4.10

'93 CHEVY G30 454 4X4 SPORTVAN EXT 146" WB
#726369 - August 01st 2017 10:13 pm Re: 1985 Chevy with 3:42 (GU6) rear axle, want to change to 4:10 [Re: tuner4life]  
Joined: Sep 2005
Posts: 8,075
CatFish Offline
carpal tunnel
CatFish  Offline
carpal tunnel

Joined: Sep 2005
Posts: 8,075
Georgia, USA
One place to look:
Complete kit for 8.5" with 28/30 spline axles for about $520.
Includes the Eaton carrier, gears and all parts......

Last edited by CatFish; August 01st 2017 10:17 pm.

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