School me on replacing gears...

[Chrispbrown36]

This isn't a project I am planning anytime soon....just thinking ahead.
I have an 87 V10, TBI 350 auto with factory 3.08's and a locker. The gears are pretty lack-luster and to be honest I haven't paid attention to whether it really helps with the gas mileage...first of all because it's a new truck to me so gas mileage takes a back seat to just driving my new toy and second because the only thing I have to compare it to was an 86 2wd with unknown gears and a 305 in it that I had around 13 years ago. I do think that the motor needs some work to get it to perform better, but i really think the gears are dragging it down too.
Anyway, as I said school me on gears. I keep reading that 3.42's seem to be the sweet spot with these trucks, but can I switch out the 3.08's piece for piece or will I have to change some things up? Also with the factory locker will I need to put a new one in or can I keep the original in place with the new gears? Anything else I need to know? Brands? Prices? What about having a shop do it....prices on that?
Thanks in advance.

[special-K]

You have the same carriers as 3.42s. I would prefer no taller than 3.73s in a 4wd, personally, with overdrive. 3.42s are a good ratio to go either with or w/o overdrive. That's how my little 2wd shortbed will be set-up (w/M22). At some point I want to install a Gear Vendors O/D. I did have what I assume were 3.08s in my '90 Blazer 4spd with 33/12.50s and that was actually a good set-up for a family car. It was like having overdrive on the hiway, it pulled off the line just fine in 2nd, and there was always granny gear for certain circumstances. It still had the original clutch in it when I sold it at 193k

[andrewmp6]

If you stay stock size tires that should be good but if you go bigger then you need different gears.And remember being a 4x4 you have to re gear both axles front and rear so budget that.If you never done it i would pay a shop to do so you don't mess it up lol.As far as prices that verys shop to shop you could spend 300 bucks to 1200 call around get quotes.

[JIMs70GMC]

I bought front and rear axles for $200 total with 3.42 gears. I am going to freshen up the seal and brakes. A lot cheaper than re-gearing. Just an option. These are for my 86 K5 and the axles came from an 87. I have a SM465 tranny too.

[Mike C]

Buy 3/4 ton truck with 3.73 gears. Swap front and rear axles in your truck. Spend all that extra $ on cool new 8 lug wheels and tires. Put 1/2 ton axles in truck and re-sell.

[RIDE-RED 350r]

One thing I urge you to keep in mind about buying used axles, especially axles that were alrsdy pulled and sitting for an unknown period of time, particularly outside: Be prepared to find out you need to rebuild the axle anyway. Condensation does form after a while and will cause corrosion on the ring gear and possibly other components depending on how it was stored. Sure, corrosion will wear off the gear, but when its excessive I do not believe it will play very nice with the carrier and pinion bearings as it wears off and gets suspended in the gear lube. I say this because I experienced it firsthand. I needed a particular axle and I had only one option. I ended up replacing all of the internals except the posi unit. No complaints though, I have peace of mind that I can expect my rebuilt axle to got 100k plus with no issues.

Obviously, this is not nearly as big a deal if you buy axles from a running and driving truck.

There are places you can get away with cutting corners here and there, axles generally are not one of them.

[Chrispbrown36]

I have considered an axle swap but that seems like a much harder job than the gear swap....I guess it's just a matter of scale.
Ride-Red, I have had that same thought on a few occasions on parts from pick and pull places many times...with all those cars and trucks just sitting there where has the rain dripped into and rusted out something internally that you can't see without tearing it apart. In our area floods are fairly common so I also wonder about mud being packed into places that it wouldn't under normal conditions.
So with a locker is it transferable to a new gear set?

[RIDE-RED 350r]

Axle swap is MUCH less involved than a gear set change! There is more heavy lifting un-bolting U-bolts and whatnot for sure... NOTE: NEVER re-use U-bolts. Always replace with new.

But the REAL work starts when you begin to tackle pinion depth, backlash, pinion bearing pre-load, and finally, carrier bearing pre-load. It's very meticulous time consuming and at times VERY frustrating. I have only done one to date, the one I mentioned above. The first day of work on setup I got nowhere after 14 straight hours playing with it. I got advice online and from a family member who owns a repair shop that used to do rear diff setup. You spend a ton of time learning what you are supposed to be trying to get and how to get it there.

All said and done, just in axle tear-down and rebuild I have about 2o hours of my time in it. Not including hours of research online and seeking advice from other people I know. This does not include the time I spent pulling the old blown axle and installing my freshly rebuilt axle.

I am not trying to scare you away from tackling it. But you should know it's not a 2 hour job to re-gear a diff. There is a learning curve and it takes patience and true understanding of some things that may be new to you. My cousin used to charge 8 hours labor to set up a rear end at his above mentioned shop... I told him it is a bargain!

Yes, the locker is transferable so long as you aren't changin ratios too much. To put it simply, series 3 carriers are for gear sets 3.42 and numerically lower. Series 4 carriers are for gear sets 3.73 and numerically higher. You CAN however run certain "thick" series 4 gear sets on a series 3 carrier. My truck is running a GM 12 Bolt truck axle, 3.73 gear set with "thick" ring gear on a series 3 Auburn Posi carrier. I can elaborate more on this if needed, but for now this post is running long.

Carrier is the part that the ring gear is bolted to. Your carrier is what houses your diff and drive axle splined ends. When you buy for example an Eaton Posi unit or Detroit Locker, you are getting the complete carrier assembly. If your locker is a GM Gov Loc it's nothing terribly special and has a bad reputation for failure earning it the nickname "Gov-Bomb".

Do you KNOW what locker you have??

[Chrispbrown36]

It is a factory locker....that is all I know.

[jeffahart]

I agree that axle swap is easier; with this caveat, you need to locate axles that you think you can drop in. For me, it turned out to be an exercise in futility. All axles I looked at were dubious and I figured I would have to rebuild anyway. So, it's a tough call. If you locate what you think are good axles in your time frame then great. But if you cannot and you want to move the ball forward then you may need to take on the gear swap or buy take outs from know source.

It's a double edged kind of a deal!

I say 3.73 or 4.10 it's a truck!

[RIDE-RED 350r]

Copy that..

One other point I would like to elaborate on concerning already pulled axles.

The axle I got was pulled and sitting outside on 2 large logs. Naturally, the axle will rotate around and the pinion will be hanging down pointed at the ground.

This axle looked good externally. No rust holes in the diff cover or anywhere else. BUT, apparently it had been sitting awhile. I pulled the cover and was delighted to find the ratio I needed which was 3.73. But, the inside of the cover showed the tell-tale signs of some heavy condensation, as did the part of the ring gear that was not in the oil. Fairly heavy, flaky rust.

I got the axle for better than a fair deal all things considered. I needed the housing as mine cracked when it grenaded.

When I got it home and drained the old oil, I got quite a bit of water with it. Of course water being heavier than oil, where do you think it was... That's right, right in the pinion housing, particularly the outer pinion bearing. The bearing was basically sitting in water.

The carrier bearings had a little corrosion on them as well and felt gritty when spinning the carrier with no axles or pinion gear in place.

The only thing useable was the carrier itself and the axle housing.

New gear set, master rebuild kit (all bearings, shims, seals and pinion bearing crush sleeve) and she is doing great!

Very satisfying job once done! One of my post proud accomplishments. I have always been a wrench head, but mostly on old Honda ATC's. This was my first rear end rebuild.

[Mike C]

I bought a 3/4 ton '86 Suburban with the optional 4.10 gears for $300 because they had broken the motor. Had brake drums turned in the back and put new rotors on the front. Used my existing calipers and replaced the parking brake cables with my original. Re-welded perches and used original 67-72 shock mounts. Total investment, less than $400 up front. Oh, and I sold parts off the Sub. Lots. Net result is 8 lug swap was free except for the wheel and tire thing.

And this is even easier on a square body truck is all the parts just bolt in. One of the advantages of buying a complete vehicle is that it is better protected from the weather and will be obvious it rolls ok...

[jeffahart]

The Mike C route is a great plan. In retrospect I should have done this. If you have a yard that you can house a transitory donor truck.... then.... what he said. A few years ago I bought a take out rear D60 for a 64 F250. I had trouble with the read end when a ran it; the yard gave me another, only cost me some extra labor.

[Chrispbrown36]

This morning I was doing a little looking around online checking on gears, posi's, lockers, etc. Got to looking at pictures of lockers and posi's and I have come to the conclusion I have to pull the diff cover off and look around again because I think I might have a posi after all. I am about 90% sure I saw the four big springs (I know its only been a few days ago and I should be able to remember that but we have had child drama and that takes up a lot of mind space ha ha). The reason I though I had a factory locker is because the invoice from the dealer lists it on there. But now I am wondering if maybe the marks on the inside of the diff cover were maybe from the original locker blowing up and then it was replaced with a posi at some point. I have noticed that from time to time I hear a pretty pronounced "clunk" coming from the rear end, which from what I have read is common with posi's.....and just as I have read it certainly makes the heart stop for a minute!
The only thing left to do is open it up and make sure...besides if I do have a posi I need to put the additive in there since I only refilled with 80W90 dino lube. Are the springs the best thing to look for to make certain or is there something else I should look for?
Thanks again.

[RIDE-RED 350r]

The springs are a good indicator that you have an aftermarket posi unit. The Gov-Loc offered by GM had been around for many years and it is very possible your truck left the factory with e Gov-Loc installed. That is why it indicates the locking diff on your spec sheet.

And as our trucks are getting older, the likelihood of finding one that the factory Gov-Loc has been replaced is only going to grow higher as more time passes. Even with trucks equipped with open diffs per the spec sheet warrant a check to see what you really have. Plus it's never a bad idea to do a complete powertrain fluid change on that "new-to-you" square body truck that just followed you home anyway.

Posi/limited slip units require the limited slip additive.

Open diffs and to my knowledge "lockers" do not.

The reason is that a Posi unit has for lack of a better term, a clutch of sorts inside. The additive is for protection of the clutch discs.

Open diffs have no such clutches to be concerned with.

Lockers have no such clutches either. They lock the diff mechanically. (as I said, to the best of my knowledge).

Being that the Gov-Loc works mechanically as well, I do not see why it would need limited slip additive either.


But by all means, if you end up getting yourself an aftermarket posi or locker, seek the advice of the manufacturer of whichever you end up with to determine the best lube and/or limited slip additive.

Here is a pic of a Gov-Loc unit as you would find it when you remove the diff cover. Just remember the window may not be facing you when you remove the cover, so rotate the axles as needed to get a full view of what you have for a diff.

It doesn't take much looking around in Google to find alot of bad things said about the Gov-Loc. But if you find out you have one, draw your own conclusions on that subject.

[Chrispbrown36]

Ya I definitely do not have that set-up in there. The more I think about it the more confident I am that I saw a posi in there....just didn't know what I was looking at. Tomorrow I am going to open it up and make 100% sure and try to get some pictures posted up. I was impressed with how clean the rearend looked....kinda wondering if it hasn't bee that long ago that it was changed out. The guy I bought it from made no mention of changing gears....in fact he bragged about the 3.08's being in there, so maybe the guy before him (the original buyer) did the work or had it done.

[RIDE-RED 350r]

Well, having an aftermarket locker or posi (if the clutches are in good shape) gets you several hundred dollars ahead of the game right out the gate!

Working with a stock axle, the diff/locker/posi is your most expensive single component in the rear end. Always good to find a nice upgrade like that. Like finding $500 in your sock drawer you forgot you had...

See if you can find any identifying numbers on diff/carrier. I did not know what my posi was until I took it out and looked closer. It wasn't the numbers so much as the brand logo stamp that allowed me to figure out that I have an Auburn Posi. So make note of, or even take a good cell phone pic of any numbers or logo markings on the carrier and start googling.

Auburn is not the best posi simply because it is not rebuildable once the clutches wear out.. I can't figure out why a person wouldn't buy a rebuildable Eaton Posi for the same money as a non-rebuildable Auburn... Oh well, it's still better than a Gov-Loc. I just figure I'll have to get an Eaton Posi or Detroit Locker someday. But that will probably be awhile as I don't drag or mud race my truck. Just my summertime daily driver and ATC hauler and I don't flog the old girl too much, she's as old as I am! But that Auburn will probably last me 30 years the way I drive my truck. It's not a big concern for me at all at this time.

Also, I don't recall what ratio you said you were running. You would be smart to roll your carrier around till you see the numbers on your ring gear (usually tooth count of ring and pinion). This will tell you without a doubt what ratio you are running. You may need to wipe the ring gear with a clean rag moistened with a little brake cleaner or other similar fast drying solvent to make the numbers more easily readable.

Being that it seems you have an aftermarket locking/limited slip, it's a reasonable possibility you may have a gear ratio other than what you think or have been told by the PO. 3.08 was a pretty common ratio found on alot of C-10's. That being the case, often times a gear change is done along with a locker/limited slip. But I'm not sure exactly what you have as in your OP you indicate you have a V10! LOL! I am guessing you have a C-10 as "V" is next to "C" on the keyboard and you possibly suffered from a little bit of fumble fingers as I often do.

[Chrispbrown36]

1987 GM trucks were designated V's for 4wd and R's for 2wd...I have heard that it was that way for a few years but I have only seen it mentioned pertaining to the 87 model year. So essentially it is a K10 I just prefer saying V10 because the guys at the auto parts place give you this high and mighty you have no idea what you are talking about until they look it up. It always makes me chuckle!
The original invoice lists 3.08's but I am with you in that if the limited slip was added then there is a possibility that a new ratio was put in. I will be checking tomorrow as long as the rain holds off and will try to get some good pictures.

[Mike C]

I guess they were preparing for the new '88 model trucks. I wasn't aware they did that with the K10-V10 but I did know they did with the K30-V30 because the produced the 2 simultaneously until '91. And in '87 they obviously switched the square bodies nomenclature before the platform change.

[Chrispbrown36]

Finally got the rearend tore apart again and some pictures. Definitely a limited slip of some sort, but I did not see any makers mark on it. I was also able to find the gear marks and I do have 3.08's on it.
[IMG][/IMG]
[IMG]

[/IMG]
[IMG][/IMG]
[IMG][/IMG]


These were the only markings present on the carrier housing and they were worn off. I know they are numbers but couldn't really tell which ones.

[71swb4x4]

That's an Auburn limited-slip: http://www.spohn.net/support/questio...ntial+types%3F

[Chrispbrown36]

So not rebuildable?

[RIDE-RED 350r]

Originally Posted by Chrispbrown36

So not rebuildable?

Unfortunately no, it is not rebuildable.

But, if you don't flog the you-know-what out of your truck at the mud runs or drag strip and change the rear end oil now and then, it should give you many years of trouble free service.

When the time comes that you decide to re-gear just remember: If you decide to go with 3.73 or numerically higher you will need to get a gearset with the "thick" ring gear.

[Chrispbrown36]

I learned a long time ago that playing in the mud can get very expensive...very quickly. So I won't be doing too much playing in the truck....the occasional run through a field but mostly the 4wd is for emergencies and the cool factor.
The 3.08's are staying for now and who knows I may end up liking them. I had read that 3.42's were the way to go for a nice middle ground, so eventually I will probably go that route.
Thanks for all the help.

[RIDE-RED 350r]

I hear you there.

So, you have the overdrive auto then?? I have been told it's basically the non-electronic 700-r4...

3.08 seems aweful high for my taste in a later K-10 with OD Auto trans...

What size tires are you running??

My brother has an '85 K-10 with the OD Auto trans and 33" tires. He just had his re-geared to 4.10 from 3.08 and likes it much better being as he hauls a 19' Fiberglass I/O boat. In OD if I remember correct, he is turning about 2000 RPM at 60 MPH. His truck had 3.08 in it when he got it and had all sorts of trans shifting issues, particularly with converter lock-up at highway speed. It would kick down if he hit more than a 2mph head wind on flat ground! Turned out, the 3.08's were just too high a ratio for his transmission and tire combination. TOTALLY different truck after the switch to 4.10 gears!

If you are still running stock tire size I think 3.73 might be a better fit for you and give you a more noticeable kick in the pants in power to the ground.

My '78 with 4spd, 3.73 and 35" tires on the other hand... bout 2400 RPM at 60mph. But I am very happy with it for what I use my truck for.

You definitely need to figure out what quality is most important to you.... pulling power or fuel economy. Where your compromise should be is really to be determined by you and how much value you place on those 2 things for what you do.

Quick question: Have you confirmed your front axle ratio?? I ask because I don't think that 3.08 gears are common original gears on the 4x4 trucks. I could be wrong and will stand corrected if so. But regardless, someone has played around in the rear end, I would make sure your front axle ratio matches before using 4x4. You may laugh, but I have seen this with 2 old K-10s in the last couple of years. People at some point decided they needed a re-gear for their intended every day use and only re-geared the rear diff not touching the front... Very interesting seeing a truck attempting to use 4x4 with mismatched axle ratios!