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.