Build thread - 3.08 to 3.42 front and rear gear change

DaveO9 · Jul 25, 2024

Starting a thread for my gear change. This week I purchased a GU6 14-bolt, Gov-lock rear end from LKQ. Came from an '09 Yukon. I've taken the cover off and removed the axles so far. Everything looks pretty good. No silver in the oil (although it was pretty black) and the parking brake components on both ends look brand new. So do the axle seals. The only downside is the brake backing plates are toast. Dinged up and bent good where it was on the ground. I think I should be able to use those from my 8.5. I already have the conversion u-joint I will need. Should be able to install in the truck in the next few weeks. Hopefully.

At the same time, I also purchased a 3.42 front axle from a different donor/location, but still LKQ. They have since canceled that order and I'm getting a refund, apparently it didn't pass their inspection. There were others available for the same price, so it should be pretty straightforward to order another one.

DaveO9 · Jul 28, 2024

I got the old rear end out today, but I had a significant set back I think I'm going to have to return the rear axle and get a different one. The PS trailing arm mounting bracket is badly bent. Also the stabilizer bar (not sure on official name - it mounts on frame on PS) mount is tweaked as well. Not sure if it fell off the fork lift or what. I don't think I will attempt to straighten the brackets. Probably best to return and exchange.

swathdiver · Jul 28, 2024

DaveO9 said:
I got the old rear end out today, but I had a significant set back I think I'm going to have to return the rear axle and get a different one. The PS trailing arm mounting bracket is badly bent. Also the stabilizer bar (not sure on official name - it mounts on frame on PS) mount is tweaked as well. Not sure if it fell off the fork lift or what. I don't think I will attempt to straighten the brackets. Probably best to return and exchange. View attachment 433849
View attachment 433850
View attachment 433851

Those guys in the salvage yards do more harm than good all too often! The upper bracket can probably be fixed with a 2x4 and a mallet, and a mallet for the track bar bracket. Use your old track bar if the donor is tweaked.

mikez71 · Jul 28, 2024

dont forget to get your tcm flashed with new tune (gear ratio and shift schedule different)

DaveO9 · Jul 29, 2024

mikez71 said:
dont forget to get you tcm flashed with new tune (gear ratio and shift schedule different)

I have a local tuner I use. I've already had ECM and TCM tuned from him, so I was planning on just going back to him and have the gear ratio changed in the tune. He used a pretty standard upgrade for shift schedule previously, and I like the current shift points, RPM-wise. I was thinking I was just going to keep it as-is, but is there something specific I should change?

mikez71 · Jul 29, 2024

Nope, sounds like you got it covered!

DaveO9 · Jul 29, 2024

swathdiver said:
Those guys in the salvage yards do more harm than good all too often! The upper bracket can probably be fixed with a 2x4 and a mallet, and a mallet for the track bar bracket. Use your old track bar if the donor is tweaked.

Thanks for the encouragement to make some "adjustments," James. I did use a long 2x4 to try and lever the upper bracket back into place, along with several well-placed blows with a 2lb machinist hammer. It's better now, but I think it's still going to be challenging to get the upper arm/strut in there. But there is some give on the other end, so maybe it will slide in. That strut is still bolted to the frame, but I may take it off and do some test fitting before I do the install. I also think I may be able to straighten the track bar bracket enough to get it installed too. I didn't get any intact suspension components with the LKQ axle - they were cut with a sawzall close to the brackets. I'll have to reuse everything from my rig.

So am I correct in assuming that as long as I can get these rear suspension components installed in their brackets, without having to tweak them, I'm good to go? Since it's a straight axle, it doesn't seem like there's anything that can cause alignment or handling issues. They're just there to keep the axle in place and prevent lateral and fore/aft movement? This is my first vehicle with live axle but coil sprung rear end, so I'm learning as I go. My '76 Vette with trailing arm independent rear definitely has some adjustments required out back, but that's quite a bit different.

swathdiver · Jul 29, 2024

DaveO9 said:
Thanks for the encouragement to make some "adjustments," James. I did use a long 2x4 to try and lever the upper bracket back into place, along with several well-placed blows with a 2lb machinist hammer. It's better now, but I think it's still going to be challenging to get the upper arm/strut in there. But there is some give on the other end, so maybe it will slide in. That strut is still bolted to the frame, but I may take it off and do some test fitting before I do the install. I also think I may be able to straighten the track bar bracket enough to get it installed too. I didn't get any intact suspension components with the LKQ axle - they were cut with a sawzall close to the brackets. I'll have to reuse everything from my rig.

So am I correct in assuming that as long as I can get these rear suspension components installed in their brackets, without having to tweak them, I'm good to go? Since it's a straight axle, it doesn't seem like there's anything that can cause alignment or handling issues. They're just there to keep the axle in place and prevent lateral and fore/aft movement? This is my first vehicle with live axle but coil sprung rear end, so I'm learning as I go. My '76 Vette with trailing arm independent rear definitely has some adjustments required out back, but that's quite a bit different.

I reckon that as long as the brackets are straight, the control arms will fit inside them and it will track straight down the road. @iamdub @Doubeleive @gooffeyguy @randeez

What does everyone else think about those brackets and getting them straight?

Doubeleive · Jul 29, 2024

the welded on bracket metal is maleable so as long as you can get it straight it should be fine, will take a little work with a variety of instruments. since it is on the driveline I would use a square to be sure it is straight.

DaveO9 · Jul 31, 2024

Doubeleive said:
the welded on bracket metal is maleable so as long as you can get it straight it should be fine, will take a little work with a variety of instruments. since it is on the driveline I would use a square to be sure it is straight.

Thanks for the info. I found more damage tonight - the flange that the brake backing plate/parking brake mounts to is bent on one corner. That seals the deal, I'm going to start my return/exchange with LKQ tomorrow and hope my second attempt is more successful. I'll do a lot more close inspection before I accept the next one.

On a different note, since I couldn't work on installing the new rear end tonight, I thought I'd work on replacing the rear u-joint. But I'm confused. In other threads about this swap, guys have talked about having to use a 1310 to 1350 conversion u-joint or switch driveshafts, but it seems like my stock driveshaft already is 1350. All the bearing caps measure out at 1.19 (1 3/16") dia, which is the size of a 1350, and the yokes on the 8.5 and 9.5 are identical. So it seems I could have left my old u-joint in there, but new Spicer on order now. Can't hurt to have a new rear u-joint. Just trying to figure out why some have had to do a conversion and I don't. Maybe they changed the size in the later years of GMT 900? Mine's a '13.

Build thread - 3.08 to 3.42 front and rear gear change

DaveO9

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DaveO9

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swathdiver

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mikez71

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DaveO9

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mikez71

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Doubeleive

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