14 bolt axle

jamesgmc · August 23, 2018

Hi community,

seems like there is alot of information out there about the various GM 14 bolt axles, how to try and ID them from the outside and what model year/trim trucks have what. I have a 2014 GMC Sierra 2500 crew cab, standard bed, Z71 with a 6.0 gas. I know most of these come with 4.10s but the RPO code (GT4) exsists on my tuck indicating it has 3.73s. Funny enough the RPO references online seem a bit out of order as my truck has the L96 code which according to some indicates I have a 2.0L I4 lol.

So I’m going to re-gear and ditch the G80 for a truetrac. To my knowledge there are no RPO codes that tell you weither you have a 10.5 or 11.5aam axle? Anyone have any insight as to which I have or how to find out without taking the diff cover off and measuring? I know you can take it to a dealer and they will tell you but they also told me the truck had 4.10s when I brought it to them after I bought it used from a CarMax.

Also pretty sure upfront I have a GM 9.25 unless anyone knows better?

Edited August 23, 2018 by jamesgmc

jhplak325 · August 23, 2018

2500’s with the 6.0 have the 10.5 axle. 2500’s with the Duramax and all 3500’s have the 11.5 axle.

Up front is the 9.25.

jamesgmc · August 23, 2018

Lol just to add comedy to this post, I called my local GM dealership and asked for a quote on the 4.10s just to compare. The parts guy told me I can’t just change the ring and pinon in the diffs, I would need to bring the truck in and have them do a whole drivetrain swap (axles, transmission, transfer case, drive shafts, yokes) off another 2500 that has 4.10s... I just said thanks with a chuckle and hung up.

jamesgmc · August 23, 2018

36 minutes ago, jhplak325 said:

2500’s with the 6.0 have the 10.5 axle. 2500’s with the Duramax and all 3500’s have the 11.5 axle.

Up front is the 9.25.

Thanks for the info

Edited August 23, 2018 by jamesgmc

SierraHD17 · August 24, 2018

What you have been told is correct as far as what axles you have in the truck. The rear gear set in the 10.5" is the same since 1973 as is the carrier of whatever you choose. The bearing set is the same since around the mid 1980's when GM increased the pinion bearing size.

Front is the same gear set in the 9.25" since 1988 and the bearings will be specific to the Salisbury style 9.25" that came out in 2011.

I went to 4.56's and a Detroit locker in my 2017 2500HD which mechanically is completely identical to your 2014. With a 34" tall tire it's the best thing I have ever done to do the truck. My truck started life with 3.73's and the old 6 liter wasn't having any of that. I wouldn't even bother with 4.10's.

Here is my install thread.

Edited August 24, 2018 by SierraHD17

jamesgmc · August 24, 2018

Thanks SierraHD17. I want to make sure this project is getting done correctly. When you said what I've been told is right, were your referring to the parts clerk at the Chevy dealer when he told me I need to replace my entire drivetrain? That seems to counter what I've read and watched from hundreds of people talking about changing gears in a diff.

My understanding of my project is I need to change my rear ring and pinion to a 4:10 or maybe consider 4:56 with the appropriate Truetrac carrier. No one seems to make a reputable locker or LSD for the front GM 9.25 so I'm planning to just install matching gears to the rear. Of course each install will be done with a master install kit with the appropriate bearings, seals, gaskets and silicon. As for my actual axles, drive shafts, transfer case, transmission, and yokes I'm not planning on touching those.

Edited August 24, 2018 by jamesgmc

SierraHD17 · August 24, 2018

No...I guess nobody else did say anything about changing parts so no you were not told anything correct in that regard. Dealership guy was an idiot and clueless.

Just change the parts I said in my previous post lol. You need two gear sets (front and back) bearings and a Trutrac if that's the carrier you want. Again....the GM dealership was clueless.

The only time I change the yoke is if its a higher mile truck because it's just insurance against a pinion seal leak. I did on both axles on my 90 K5 Jimmy and my 05 Sierra as example. My 17 had like 15000 miles on it when it was regeared so I didn't bother lol.

The carrier break in the rear end is at 4.56 as well. So if you want 4.56 gears you need a 4.56 carrier or a thick cut gear set. I have the thick cut gears in mine because the carrier I used is for 4.10 or numerically lower gear ratios. It's all explained why in my post I linked. Since you are buying a new carrier and new gears it makes no real difference in cost whatever ratio you choose. The rear carrier bolts are different lengths too depending on the carrier. The 4.56 and higher numerically uses longer bolts(about 2.1")to attach the gear than the 4.10 and numerically lower does(about 1.80"). Replacing the bolts is a norm so just make sure you get the right ones.

The front is just gears regardless of the ratio you go with.

With the rear locker in mine and the front open the truck goes wherever I need it too which is basically deep snow covered gravel roads at work. Because of design you honestly dont want a limited slip or full time locker up front... the truck will drive funny in 2wd as the drivers axle is always turning in the carrier while the front actuator disconnects the passenger side axle shaft. With a locker or limited slip in there you will now force the pinion and front driveshaft to stay loaded and spin too. That's why there are only selectable lockers for up front like arb. I wouldn't really want to test the limits of the ifs with a locker up front anyway.

Even if I stayed on stock tires I would run 4.56. My trucks tires are about an 1.5" larger than the stockers and in 6th I turn 2100 rpm at 70 mph with 4.56. My mileage on the highway actually marginally improved from 3.73 haha.

Edited August 24, 2018 by SierraHD17

jamesgmc · August 24, 2018

Thanks again for the info Sierra!

Edited August 24, 2018 by jamesgmc

jamesgmc · August 24, 2018

So currently I’m pegging about 1800 rpms at 70mph cruising. That seem high for 3.73s with 34.5s if you’re getting 2100 rpms at 70mph with 4.56s and 34s. 2100 rpms isn’t bad at all for 70 with 4.56s, I would have thought you would have been closer to 2600-2700 rpms. I generally cruise at 60 so I’d be under 2100 aswell.

Any insight on strengthening the 9.25 IFS? I’ve seen aftermarket CV shafts but not really sure those need upgrading.

SierraHD17 · August 25, 2018

Your tires probably aren't 34.5" tall overall much like mine aren't actually a true 34". And I am assuming you have corrected and checked your speedometer because I tweaked mine to get it dead on via a GPS. Really thats about where my truck sat when it had 3.73's at 70 mph ( 1750 rpm or so). People never bother to calculate the actual overall effect of axle ratio changes and just speculate. 2600 would be running the truck in 5th gear lol.

For fun calculating my own truck using a 33.5" tall tire with 3.73 my calculated rpm is 1728 rpm and with 4.56 it's 2113 rpm. My truck mirrors that very closely in real life lol.

Strengthening? I have tie rod sleeves and likely will do a limiting strap up front on my 04 2500 which is has about the double the hp my 2017 L96 does. I am not putting a limited slip in it though... it's fine for traction on the street as it is with a detroit locker in the back.

You want to wheel with your truck and go places...... cut the ifs off and put a good solid axle under it. It's built for ride comfort for the average pavement pounder. If I went in wanting a truck to use offroad any amount I would have bought a Super Duty myself... Well, that's a lie because I don't need it and have a 1990 K5 with a trutrac in the front and a detroit locker in the back if I want to screw around.

Keeping your cv's flat you should not break them.... angle ruins them and it's why I have a diff drop in my 17. I also don't drive the truck into big mud holes, or rock crawl with it. Trail ride is accurate... the roads I run at work are more trails than roads, just wide lol. I won't even drive my 17 there in the summer as the mud just ruins paint and trucks overall. it's this sandy clay snot that gets into everything.... I still run my 05 for that... which also has an open 9.25" front and a detroit locker out back haha. Yes I have 4 trucks with Detroit lockers... I practice what I preach haha.

newdude · August 25, 2018

18 hours ago, jamesgmc said:

So currently I’m pegging about 1800 rpms at 70mph cruising. That seem high for 3.73s with 34.5s if you’re getting 2100 rpms at 70mph with 4.56s and 34s. 2100 rpms isn’t bad at all for 70 with 4.56s, I would have thought you would have been closer to 2600-2700 rpms. I generally cruise at 60 so I’d be under 2100 aswell.

Any insight on strengthening the 9.25 IFS? I’ve seen aftermarket CV shafts but not really sure those need upgrading.

IFS 2011+up is way beefier than 01-10. Not sure you will really need to do much if any beefing up front.

newdude · August 25, 2018

Here is an archived post on this site with a picture of the two diffs for a visual difference of 10.5" vs 11.5":

Biggest giveaway for the 10.5" is the ring gear is on the right and the driver's side tire is the drive tire. Most diffs tend to be the opposite, ring gear on the left, passenger tire is the drive tire.

Edited August 25, 2018 by newdude

SierraHD17 · August 25, 2018

1 hour ago, newdude said:

IFS 2011+up is way beefier than 01-10. Not sure you will really need to do much if any beefing up front.

No its not.. not for what he is asking about. Putting a locker in that front and it still has the same issues as it is not designed to run like that. It's why you only have selectable units and even then... you still blow cv axles apart and whatnot. The front end parts are a nice improvement for sure minus non greaseable inner tie rods but the rest of it is still an aluminum housing with cv's hanging off it.

Edited August 25, 2018 by SierraHD17

jamesgmc · August 25, 2018

I did use gps to adjust for my speed. I havent actually messed with the speedo but if I’m highway driving I use a gps app. If my speedo says 60 im actually at 62 give or take.

I upgraded to CST tie rods when I did my lift. And I did drop the diff to straighten out the CVs. I’ve accepted it’s going to be an open carrier but just wondering if the factoy CVs themselves are a weak point with an aftermarket “fix”.

I don’t rock crawl or mud with it either. Pretty much just logging roads, fire roads and other un maintained dirt/sand type trails in California.

Formulabruce · September 5, 2018

" Dealership guy was an idiot and clueless " The Dealer would address the tire size and speedo and sensor issues for cruise, AND SHIFT POINTS of the Transmission

IF the tire size was even within spec range.

I would suggest you get a Transmission programmer at Least to fix your speedo, and shift points.

Remember the dealer has to warranty their work and Driveability is JOB#1 , and you are well off that mark, and off road !