3/4 ton limited slip diff?

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10-25-2004, 15:33
Mark Kw

I have an '85 chevy C-20 3/4 ton, 8600 GVW w/ 6.2 diesel & 3 speed turbo 400 auto trans, 2wd. The best I can find on identifying the rear is that it's a "GM-9" 14 bolt with the odd gasket pattern. I've never fooled with a limited slip diff and can't seem to find any info on this particular one so I hope someone here can help or at least point me in the right direction.

The problem is gearing, OEM set-up is a camper package with 4.10 ring & pinion ratio. Works great for pulling but trying to run at highway speeds above 50 Mph has the 6.2 screaming which scares the heck outta me. If I did the math right, changing the R&P to a 3.42 ratio should give me 10 Mph difference which would let me run at more reasonable engine rpm's over the road. I don't need to be quick off the line, it's my small welding service truck and just needs to get me there.

There's the background, now for the questions. All the info I've found says nothing about the wire type lock on the right side (as seen facing cover) that looks like it's holding an adjusting nut/locking ring of sorts with holes around its circumfrence. The wire is held in with a single bolt threaded into the center of the right bearing cap...what is this?

If I remove the diff shaft and allow the diff gears to come out, are the clutch packs and associated items on each axle for the limited slip also going to come falling out when I remove the axle retainer C clips to slide the axles out?

I appreciate any help on this.
Thanks
Mark
10-26-2004, 10:25

Mark,
You'd probably get more exposure and help in the "members area". This is a good subject.

I have a friend that has a suburban that hauls a tagalong camper. He had the same problem, and got the Gear Vendor's overdrive. He loves it.

good luck
10-26-2004, 11:32

What Morgan said. The Gear Vendor unit is a good option, pricey, but good. The other thing you could do is go down to your local wrecking yard and grab a 4L80E trans and swap it in to your rig. You will need a controller box as the 4L80E is electronic. The Gear Vendor unit and the US Gear Aux Trans are both about 2500 bucks plus installation, you might get away with swaping the trans for around $1500. Also, I don't think you will like the 3.42's, your rig will feel very sluggish and you will lose MPG around town. My $.02 worth.
JP
10-26-2004, 11:41
G. Gearloose

I wouldn't wory about the engine buzzing, it can take it. EGT's stay lower too.

Just get some taller tires to take the edge off.
10-26-2004, 14:50
Mark Kw

My TH-400 is freshly rebuilt and in good shape, I considered the OD unit add-on but declined on the price.

Got the biggest diameter tires I could get and maintain the high load range.

Highway running over 50 mph she sucks fuel like there's no tomorrow empty or loaded it doesn't matter. No tach but the engine has to be pushing 3500+ rpm at 65 Mph from the sound of it. Staying below 50 mph and the fuel milage goes up considerably which is another reason why I'm opting for the lower rpm's. Once we move, more than likely this truck is going to see more highway than in-town. Not trying to argue here, I'm asking for your input but want to make sure you have all the info too.

Heartbeat Hauler, by sluggish, exactly what do you mean? I'm not one to jack rabbot start off the lights because it wrecks what little organization I have in my tools. I'm usually pretty easy, "driving like an old fart" as my kid puts it.

Gearloose, call me stupid but what are EGT's? I can build a heck of a custom truck body or anything else for that matter but I ain't too well versed on engines & drivetrains.
10-27-2004, 04:49
G. Gearloose

Exhaust Gas Temp.

Just comparing your numbers with an old '84 K20 I had, 4.10, 6.2 and MC465 (1:1) and 235 tires
It screamed on the highway but still managed 20-22 mpg, at 60-65 mph

Since you have a C not a K, Its all either the TH400 or state of tune of your engine.

Did your rebuild have a new gas or diesel tconverter?
10-28-2004, 05:47

Quote:

Originally posted by Mark Kw:

Heartbeat Hauler, by sluggish, exactly what do you mean? I'm not one to jack rabbot start off the lights because it wrecks what little organization I have in my tools. I'm usually pretty easy, "driving like an old fart" as my kid puts it.

What I mean is that the acceleration will suffer with the 3.42's. I, like you, am not into the stoplight street fighters, but you will probably end up using more fuel around town while the engine is trying to rev to an RPM that can get the truk moving. Case in point, my wife drives a 04 2500 Sub. It has the 6.0 and 3.73 gears. When accelerating it seems as though you have to wait a second or two before it starts moving. That's because the torque is made at high RPMs and with 3.73s it takes a bit for the RPMs to climb. So in your case by going to the 3.42's it will take longer for your truck to reach the RPMs where it makes torque to get you going. Keep in mind this is not some catastrophic event, it's just disappointing. I can't imagine what is going to happen when you try to pull your camper... :eek: Dropping gear ratios sounds like it would just give you lower rpms when pulling, but actually it changes the performance of the whole truck. I would still consider changing to an Overdrive transmission, i.e. 4L80 or 4L80E. You get a lower first gear and a .75 Overdrive in 4th. One last thing, if you do put the 3.42's in and your truck can't pull in drive it will shift down until it finds a gear that puts the engine in the torque range to pull the load. This means you will have 3.42's, but you might be in first gear = high RPMs and low fuel mileage. (I think someone mentioned this already).
JP