Build thread - 3.08 to 3.42 front and rear gear change

[DaveO9]

Ok, here's the post where I ask what else I should do when I have both diffs out. For the back, so far it's just the evap charcoal canister. I've read too many posts here lately about those failing so I bought a GM OEM and have it now, will install this weekend. (I know it's probably not much more difficult, if at all, to replace with the rear diff in, but I've got it in the air, spare tire out, tools out, etc. so best to get it done now) I was also going to do rear shocks, but there's no sign of leaking and they appear to have the proper damping action, so I'm holding off for now. Pretty sure they're GM, but not original. My rig has 200k, I don't see how they could be original.

OK, so now for the front. Original plan was to drop the pan and do the oil pump pickup o-ring and new pan gasket. BUT..... my oil pressure seems good and the very small amount of leaking I get seems to be coming from the oil cooler block off plate - I know those gaskets fail over time. More on that in a sec...

Here are my typical oil pressures: hot idle, about 18-19 psi. 1500 rpm, 35 psi. 2000 rpm, 38-40 psi. 2000+ it doesn't go much higher than 40. Any concerns with any of those values? I just don't think it's worth it to drop the pan at this time if they're OK.

Regarding the oil pump cooler block off plate gasket. But, I had long range plans to add the OEM cooler lines and replace the radiator with one with that functionality. Worth it to just add the oil cooler now? To sweeten the pot: I actually already have the lines. I would just need to drop the $200 or so for the new Denso radiator.

Last question is about the motor mounts, especially DS. Is there anyway to check them for signs of impending failure? Looks OK and very clean up there to my eye, but I've read about many failures.

 

[mikez71]

The one test I keep hearing about for motor mounts, is revving the motor in drive with the brake on.
(Helper watches to see if the motor lifts while revving) I figure the motor mount can go anytime at our age.
Mine seemed fine, until driver side tore/leaked ~8 months ago. Then I could feel clunky acceleration/shifting. (2012, 210k miles)
You will probably have to do them eventually. How bored are you?

Your oil pressure seems pretty normal to me.. (?)

Since you already have the cooler lines, my vote is throw a new radiator in and be done with it!
(Of course you'll probably replace other cooling bits once you do the radiator!)

 

[j91z28d1]

motor mounts is probably bad.. only good one is the 300$ hummer solid. it's annoying but it what it is. check the motor mount sticky for video of what a bad mount looks like. they do have a cross bar design, so it stops the engine from tq over so far it breaks stuff, why they doesn't keep the rubber from tearing up, I don't know but it's metal to metal and I bet the knock sensors hate it. mine had the weirdest sound at idle, like light timing chain slap or loose tq converter bolts. since mine doesn't have a converter and ls timing chains aren't known to fail I just waited to see if it got worse. after changing the Dr mount, I haven't done the pass side yet. the noise went away. I'm guessing it was metal to metal in the mount.

 

[mikez71]

Yup, driver mount here too.

If you got 10 years, 200k miles out of your originals, then another stock mount should be fine.?
Especially if you want smoothness. Just be ready to replace them again in.. 10 years?
If you tow or drive like Doubeleive, I imagine you would want the H3 mounts.

I wouldn't be surprised if the few degrees of knock I used to log was due to the bad driver side mount... (-1 to -4 degree occasional blips)
Nothing like loose/missing motor mount bolts, resulting in -16 degrees knock retard!

PRO TIP: Tighten your motor mount bolts. Don't ask how I KNOW!

 

[Foggy]

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.

According to my research the 8.6" 10 bolt rear and the 9.5" 12 bolt rear
use the same rear driveshaft (and U joints).
But Yeah, do it once... do it right !!

 

[Foggy]

motor mounts is probably bad.. only good one is the 300$ hummer solid. it's annoying but it what it is. check the motor mount sticky for video of what a bad mount looks like. they do have a cross bar design, so it stops the engine from tq over so far it breaks stuff, why they doesn't keep the rubber from tearing up, I don't know but it's metal to metal and I bet the knock sensors hate it. mine had the weirdest sound at idle, like light timing chain slap or loose tq converter bolts. since mine doesn't have a converter and ls timing chains aren't known to fail I just waited to see if it got worse. after changing the Dr mount, I haven't done the pass side yet. the noise went away. I'm guessing it was metal to metal in the mount.

I noticed zero knock sensor extra activity when I installed both H2 motor mounts...
I did it like 3 years ago..
I still have my OE GM low mileage mounts in the boxes still !!!

 

[buckwild27]

Here's another motor mount option for you.

https://atomicfabandperformance.com/product/07-14-gmt900-poly-motor-mounts/

 

[j91z28d1]

I noticed zero knock sensor extra activity when I installed both H2 motor mounts...
I did it like 3 years ago..
I still have my OE GM low mileage mounts in the boxes still !!!

I was thinking with my stock mount that when lifted up was metal to metal.

I think the hummer mount is good it's just pricey

 

[Foggy]

I was thinking with my stock mount that when lifted up was metal to metal.

I think the hummer mount is good it's just pricey

Ah, Yeah ! Definitely would cause knock sensor with broken OEM mounts..
MikeZ71 Just had a long thread and successful fix that ended up being loose/missing
motor mount bolts that caused "knock sensing"

 

[swathdiver]

I think the hummer mount is good it's just pricey

I got some off Amazon for under a $100 a couple winters ago.

 

[mikez71]

Here's another motor mount option for you.

https://atomicfabandperformance.com/product/07-14-gmt900-poly-motor-mounts/

Similar looking unit from Summit.. https://www.summitracing.com/parts/sum-7731112
Looks almost identical except..

1. Summit has a couple extra gussets on the engine side mount.
2. "S" cutout instead of "AFP"
3. bolt holes are slotted in opposite directions bottom and upper vs. AFP's all slotted the same direction
4. Summit mount has bent ears for the frame mount, AFP is welded tabs.

To me, the Summit part looks a little more refined.

---------------------------------------------------------------------------------------------------------------

Regarding H3 mount prices..

https://www.amazon.com/gp/product/B010GPIUBO

$271 currently! I doubt the price will drop...

Of course I think that about everything. Spend your money NOW!

 

[DaveO9]

Well I ordered a Denso 2219514 radiator AND a single GM OEM motor mount from RA today. More projects and extending my down-time for the Tahoe by a couple weeks. While I’m there syndrome…..

On the motor mount I think a GM OEM will be sufficient for my 5.3 and fairly benign driving habits.

I also ordered my second try for the 9.5 rear today. Less than 100k miles on this one. Hoping for nothing bent or broken on this one. Front 3.42 on order as well.

 

[buckwild27]

Similar looking unit from Summit.. https://www.summitracing.com/parts/sum-7731112
Looks almost identical except..

1. Summit has a couple extra gussets on the engine side mount.
2. "S" cutout instead of "AFP"
3. bolt holes are slotted in opposite directions bottom and upper vs. AFP's all slotted the same direction
4. Summit mount has bent ears for the frame mount, AFP is welded tabs.

To me, the Summit part looks a little more refined

To bad they don't have a set for my 2014 and 2018 sierra.

 

[5StarCustmSolutns]

Well I ordered a Denso 2219514 radiator AND a single GM OEM motor mount from RA today. More projects and extending my down-time for the Tahoe by a couple weeks. While I’m there syndrome…..

On the motor mount I think a GM OEM will be sufficient for my 5.3 and fairly benign driving habits.

I also ordered my second try for the 9.5 rear today. Less than 100k miles on this one. Hoping for nothing bent or broken on this one. Front 3.42 on order as well.

Dang.....just missed getting this to you before you ordered another from LKQ. Curious what $$$ range LKQ has them at now? Last April/May they were in the $550-650 range + core return of the 10 bolt that came on most of our vehicles.....which they were selling for $12-1400.

The math didn't add up to me, so I (and this is what I was going to suggest to you) went the salvage yard route. It was not for the faint of heart though! By that I mean: you have a better chance of convincing a Grizzly to share a spawning salmon it snagged out of mid air while perched on the edge of a waterfall than you have of getting anyone at a junkyard to physically walk out to the axle and confirm it's a 14 bolt with spring perches! The only way to do it is to learn the VIN codes that indicate it's a 14 bolt 9.5" rear. Then use a couple of the free VIN search sites to find out everything you can about the donor vehicle. In my case that took about 10 days or so.... But once I had the VIN codes figured out (thanks @swathdiver @Geotrash) I found a 14 bolt w/92,000 mi on it, out of a RWD '12 Yukon XL, 3hrs away for $212.00 and NO ($0.00) core charge! Saving that much money allowed me to put an Eaton Trutrac carrier in it and spring for high end bearings. I still have the stock G80 that came out of it if anyone reads this and needs/wants it?? Plus the 3.08 Ring/Pinion and the 10 bolt rear end complete with axles....

My theory was the 14 bolts are few and far between and likely RARELY EVER blow out, they basically only become available when the truck they were under was totaled. So they're simply not in demand, henceforth the low cost. The 10 bolts however have a VIO of 10million+, do blow up, and therefore are in demand, which is how these places justify the ridiculous pricetag for them.....smh! My plan was to rebuild the 10 bolt, coat it with POR15 and sell it for a few hundred less than the JEGS, Summit etc... Now if only I would follow through with that rebuild.....

As far as the four letter codes for the front diff?? I'm not 100% sure but I believe those are axle ID codes? Not gear ratio codes but axle identification. I was not able to find what they meant in about 15min of searching just now but did find this in the description of a Zumbrota front diff for sale:
front differential. With 3.42 Ratio. GM 8.25" ring gear, Independent Front Suspension (IFS), ratio option GU6, ID: AAAP, AAAZ, AABR, AACA, AACJ, AACU, BAAM, Hollander #440-01472, 440-01507. ***Call Zumbrota Bearing and Gear at 1-800-658-2537 ext. 125 for customer service, availability and technical assistance*

And I would have to think your logic is correct on which to choose based on it's similarity to your existing front diff axle code......HOWEVER this also is something I did not know existed, as I bought a GU6 front diff and called it a day......which turned out to be an AWD and you know the rest.....so at worst case, I would hope you would be able to Frankenstein any GU6 front diff with your existing front diff and make what you need. Which still leaves Frankenstein's twin, the front diff assembled from the parts you didn't need, in a fully functional and sellable condition...

-or-

One call to AAM or Zumbrota or Dorman etc.. would likely get you the answer to the AARB BAAB AABT question too. I'll be looking forward to the answer lol

 

[DaveO9]

Dang.....just missed getting this to you before you ordered another from LKQ. Curious what $$$ range LKQ has them at now? Last April/May they were in the $550-650 range + core return of the 10 bolt that came on most of our vehicles.....which they were selling for $12-1400.

The math didn't add up to me, so I (and this is what I was going to suggest to you) went the salvage yard route. It was not for the faint of heart though! By that I mean: you have a better chance of convincing a Grizzly to share a spawning salmon it snagged out of mid air while perched on the edge of a waterfall than you have of getting anyone at a junkyard to physically walk out to the axle and confirm it's a 14 bolt with spring perches! The only way to do it is to learn the VIN codes that indicate it's a 14 bolt 9.5" rear. Then use a couple of the free VIN search sites to find out everything you can about the donor vehicle. In my case that took about 10 days or so.... But once I had the VIN codes figured out (thanks @swathdiver @Geotrash) I found a 14 bolt w/92,000 mi on it, out of a RWD '12 Yukon XL, 3hrs away for $212.00 and NO ($0.00) core charge! Saving that much money allowed me to put an Eaton Trutrac carrier in it and spring for high end bearings. I still have the stock G80 that came out of it if anyone reads this and needs/wants it?? Plus the 3.08 Ring/Pinion and the 10 bolt rear end complete with axles....

My theory was the 14 bolts are few and far between and likely RARELY EVER blow out, they basically only become available when the truck they were under was totaled. So they're simply not in demand, henceforth the low cost. The 10 bolts however have a VIO of 10million+, do blow up, and therefore are in demand, which is how these places justify the ridiculous pricetag for them.....smh! My plan was to rebuild the 10 bolt, coat it with POR15 and sell it for a few hundred less than the JEGS, Summit etc... Now if only I would follow through with that rebuild.....

As far as the four letter codes for the front diff?? I'm not 100% sure but I believe those are axle ID codes? Not gear ratio codes but axle identification. I was not able to find what they meant in about 15min of searching just now but did find this in the description of a Zumbrota front diff for sale:
front differential. With 3.42 Ratio. GM 8.25" ring gear, Independent Front Suspension (IFS), ratio option GU6, ID: AAAP, AAAZ, AABR, AACA, AACJ, AACU, BAAM, Hollander #440-01472, 440-01507. ***Call Zumbrota Bearing and Gear at 1-800-658-2537 ext. 125 for customer service, availability and technical assistance*

And I would have to think your logic is correct on which to choose based on it's similarity to your existing front diff axle code......HOWEVER this also is something I did not know existed, as I bought a GU6 front diff and called it a day......which turned out to be an AWD and you know the rest.....so at worst case, I would hope you would be able to Frankenstein any GU6 front diff with your existing front diff and make what you need. Which still leaves Frankenstein's twin, the front diff assembled from the parts you didn't need, in a fully functional and sellable condition...

-or-

One call to AAM or Zumbrota or Dorman etc.. would likely get you the answer to the AARB BAAB AABT question too. I'll be looking forward to the answer lol

Thanks but slightly different story for me with LKQ. In an earlier post, I advised others they should order from their local LKQ rather than LKQ online. I am backtracking that advice now - LKQ online is the way to go if you are doing an upgrade, e.g. 8.5 to 9.5 rear end. The reason - NO CORE CHARGE! I didn't realize this until I tried to order the replacement rear from LKQ Portland - their price was slightly lower than what was listed online for the same stock number, but there was a $200ish core charge. And of course they wouldn't take my 8.5 as a core. So I called the LKQ online number and bought both the replacement rear and front at the same time. It had to be two different orders, since they came from different locations/vehicles (I believe all the trucks with 9.5 rears were RWD only). Rear was $380 (86K miles) and front was $290 (134k miles). Add $50 to both of those for a transfer fee to Portland, OR and I'm at $770 - I'm thinking that was a pretty good deal for both diffs. I looked at car-part.com a bunch of times and couldn't find anything close to $400 for a 9.5 rear at the yards closer to me. (Some were listing one for as much as I paid for both front and rear!) But I do totally agree with your assessment of why the 9.5s are cheaper, at least from a big outfit like LKQ: the few that have them as stock never need to replace them! And yes, when I was looking at 8.5s they are much more expensive. If not for this site, I would have never known the 9.5 was direct bolt-in, and probably would have been thinking about having the 8.5 rebuilt, probably with a Trutrac like you. (I'm sticking with the G80 in the 9.5 though.)

Fingers crossed on the front diff that I got the right letter code. It came out of a 2014 suburban and LKQ calls it a "4x4 front axle" so I'm optimistic. but I will see and report back.

 

[DaveO9]

I was thinking with my stock mount that when lifted up was metal to metal.

I think the hummer mount is good it's just pricey

Dumb question: could you do a hummer mount on DS and stick with Tahoe OEM on Pass side? I'm not going to do this now, but something I've been thinking about if I DO have a failure on DS some day, or if I do some motor upgrades.

 

[Geotrash]

Dumb question: could you do a hummer mount on DS and stick with Tahoe OEM on Pass side? I'm not going to do this now, but something I've been thinking about if I DO have a failure on DS some day, or if I do some motor upgrades.

Yes, no problem. This is how my 2012 is currently.

 

[DaveO9]

Yes, no problem. This is how my 2012 is currently.

Awesome! Glad to hear that works. I may consider that in the future.

Picked up 2nd try axle today at Portland LKQ. This one much better regarding suspension attachments - all brackets are nice and straight. I was a little disappointed to see some surface rust on the diff flange where the ring gear mounts, but I will do some light cleaning with scotchbrite and then flush out with diff oil. Everything inside the diff looks OK and the axles looked good at the bearing surfaces. Pic makes it look worse than it is. I think this was sitting around in a yard for a few years.

Thought I'd share my method for moving these monster 9.5s around and in and out of the trailer. Cherry picker with a sling. And sling is repurposed straps from child car seats. Our youngest is 14, but years ago when it was time to get rid of all those items I took out the straps and saved them. They've come in handy a few times now. I even used them to move around a 4L80e from my '91 suburban.

 

[Geotrash]

Awesome! Glad to hear that works. I may consider that in the future.

Picked up 2nd try axle today at Portland LKQ. This one much better regarding suspension attachments - all brackets are nice and straight. I was a little disappointed to see some surface rust on the diff flange where the ring gear mounts, but I will do some light cleaning with scotchbrite and then flush out with diff oil. Everything inside the diff looks OK and the axles looked good at the bearing surfaces. Pic makes it look worse than it is. I think this was sitting around in a yard for a few years.

Thought I'd share my method for moving these monster 9.5s around and in and out of the trailer. Cherry picker with a sling. And sling is repurposed straps from child car seats. Our youngest is 14, but years ago when it was time to get rid of all those items I took out the straps and saved them. They've come in handy a few times now. I even used them to move around a 4L80e from my '91 suburban.

That rust is curious. How could it possibly have rusted unless it was left exposed to the elements. And if that was the case, I’m worried about the bearings. Thoughts?

 

[swathdiver]

That rust is curious. How could it possibly have rusted unless it was left exposed to the elements. And if that was the case, I’m worried about the bearings. Thoughts?

They ought to be thoroughly inspected. I would also rotate the pinion with the carrier removed to check for binding from any corrosion it may or may not have.

Mine got rained on after we put the new yoke in the 2nd time and got the crush washer in and rotational torque to where it's supposed to be. We drained out the rainwater and poured 2 quarts of gear oil into the case and let it sit for a few days. All seems well.