(limited slip axle,i'm not sure how this is called)

[BadAssChevy502]

The only thing I want for this truck is sper (limited slip axle,i'm not sure how this is called) can anyone help me with this ,how I should do this........(it's an 10 bolt axle).

Can someone also tell me if my gears are correct for this engine it's fast on the green light but when i'm on the interstate it runs 60mph with 3000 rpm is this ok or should i change gear ratio when i put in limited slip...... PLEASE ADVISE

Info: 1972 3/4 C20 502 ci with a TH400 rear axle 410 ratio

Thanks

['68OrangeSunshine]

Limited slip is the generic term, "PosiTraction" was GM's buzzword for the product ["POSI" for short] and F@rd called it "TracLoc," I think. Limited slip is a way of transfering drive thrust to both rear wheels instead of just one, as in an open differential. In the Eaton (GM's main posi subcontractor for this period) Posi, a clutch pack in the carrier senses the greater number of revolutions on the wheel outside of the turn and applies proper force accordingly.
Posi is usually full time.You can't shut it off. Lockers are a differential device that transfers thrust to both wheels w/o slip - [big with 4x4 enthusiasts], usually part time-- you have to engage the locker before hitting the mud, snow, sand, what have you. And lockers are usually installed on the front drive axle.
Since you have a C/20 you shouldn't have a front drive axle, right? Also the 14 bolt rear axle was factory standard for a 3/4 ton. Are you sure you have a 10 bolt? 10 bolt rears were not stock until much later. How many lugs per wheel ? Should be 8 for the model 20s, not 5 or 6.
4.10 or 4.11:1 ratio is stock for 3/4 tons. A 3.73:1 rear end ratio (like stock 1/2 tons had) might get you more wheel rotations per engine RPMs, but the 4.11 is better suited to handle the mass of a big truck with a big block engine. And big blocks love high rpms.
If the basic question is: does my posi cause extra drag (resistance) and keep my big block from really winding out? No. Not enough to matter. Most muscle cars of the early '70s were optioned with posi (or whatever the mfr called it). Posi does handle a bit different than a rig w/ open diff. It won't "drift" easily, and it requires you to oversteer (I think). Once thru the turn, a smart tap on the gas pedal and it will straighten out quick.
Hope I have not confused you too much.

[BadAssChevy502]

Hey thanks for your help!!!

This really cleared up things for me
I have attached a photo of my axle I think it’s a 10 bolt (but I’m not sure) the wheels have a 8 lug bolt pattern
And I don’t have a front axle
But the mane reason of my question was that I really smoke up the tires when I floor it and I thought that If I had Posi that would be less (since the truck has so much torque) and I would be faster from zero to 60 mph for example, or am I wrong ,by the way I’m not using this truck for dirt or mud just street ..

['68OrangeSunshine]

I thought you already had a posi. Sorry.
Looks like some PO (previous owner) swapped in a later model rear end. The PO may even have switched in 8 lug axles on a 1/2 ton carrier. Who knows? You're a long way from Detroit. The nomenclature for describing an axle is literally the # of bolts on the inspection pan. 1/2 tons in '67/'72 had 12 bolts, 3/4 tons had 14 bolts. The 10 bolt units came in during the late '70s or early '80s. I'm not really sure. I now the S/10s have them. And I thought the 14 bolts were still standard for 3/4 tons for a lot longer. I'm not a Model 20 expert, I have a K/5, a C/10 and a K/10 sub, but as far as 3/4 ton stuff, I'm guessing. All 3 of my trucks do have posi. 2 factory Eatons and an aftermarket Auburn in the Sub.
So you're considering adding posi traction? Hmmm.
Going to posi would help you smoke BOTH tires, not just one.
Not burning rubber may only be a function of how hard you hit the throttle. When I converted the Suburban to a ltd slip,[circa 1995] it ran me about $1000 US, but I also needed new axles. And the GM plant was on strike at the time so I had to get custom made Moser axles, but I'm glad I did. That also included a rear axe rebuilt kit (bearings, shim & seals) and new wheel bearings. Once I lined up all the parts it only took a local differential shop a day to set it all up. There was no break in. It was good to go. In the mid '80s, when the rear end went out on my '68 C/10 stepside, I lucked out in the junkyard and found a '69 GMC rear with 3.73:1 and Eaton posi. All it needed was a missing axle, which was easier to find back then. You may be far from any promising junkyards, however.
By the way, Nice Rig.
Good luck.

[BadAssChevy502]

Thanks ,but you know what is stange this is my second 3/4 ton truck ,the first was a 1971 and it has the same axle and also no posi.

and junkyards are realy hard to find here almost none

but thanks '68OrangeSunshine
for the info,if anyone has any other ideas let me know

[Woody]

68OrangeSunshine really explained it pretty well. But I have a few things i'll add. First the 4.10 rear gear is a low gear. Yes it helps you take off fast but you are cranking the RPMs at highway speeds. With the 502 you have enough power to run a higher gear. There were a couple of gear ratios that came in these trucks and suburbans. There was a factory 3.54 axle that was in big block C/20 suburbans. The 4.10 (or 4.11) and a 4.57 were in others. I don't know if 3.73 (which was used in the C/10) ratio was in C/20's or not. I have a 68 C/20 and it has 4.57's in it. Way too low, but it will smoke one rear tire! I bought a 3.54 that i"m going to install to help lower highway RPMs.

[BadAssChevy502]

So let me get this right ,the lower the gear number the faster the truck will run (instead of 3000rpm at 60mph I will run 2400 or less rpm at 60mph) did i got it right !!!!!!

thanks very much guys !!!!!!!!!!

['68OrangeSunshine]

Thanks, Woody. I knew there was a hole in my "lecture" somewhere. That is: numerically higher gear ratios are mechanically lower (and slower). I had a 454 on my '67 K/10 suburban, and it had 3.73 gearing.[TH350/NP205] It would cruise at 80 mph. Only problem was that when the carrier bearing went, it would crunch U-joints for lunch.
I'm still puzzled by his rear differential being a 10 bolt. Could it be some off-brand for export? Ford 9"? Dana?
<<502>> was your truck originally a military cucv (civilian utility comercial vehicle) ? Maybe the Army changed the rear ends on those trucks?

[Blazer1970]

Originally posted by '68OrangeSunshine

I'm still puzzled by his rear differential being a 10 bolt. Could it be some off-brand for export? Ford 9"? Dana?
<<502>> was your truck originally a military cucv (civilian utility comercial vehicle) ? Maybe the Army changed the rear ends on those trucks?

His rear axle is the stock Dana 60 that came with the truck. Dana 60s do have 10 bolts holding the cover on, but it is not what is normally referred to as a "10 bolt". There are limited slip units available that can be installed in that axle, and that would be a good upgrade.

[BadAssChevy502]

Here's a view of my VIN tag

[BadAssChevy502]

Here's a view of my VIN tag

[BadAssChevy502]

Thanks Blazer1970

i have posted my VIN tag
But can you also tell me where to turn to to get this "posi" differential and maybe the numbers/company for it ,if you got any.

I would like to thank everybody for there repley,this board is really helpfull

[CPNE]

3/4 ton BB trucks with automatics came with 3:54 Dana 60 rears from the factory. Your truck is equipped with the optional (for that drivetrain) 4:11 gears.

4:11 would have been the standard RPO for small block 3/4 tons with autos.

I'm curious about the statement that BB like high RPM's. I was under the impression unless they were specifically built and balanced that big blocks did not like high rpms due to the weight of the reciprocating mass.

[BadAssChevy502]

Thank you also for your reply but now I’m getting a bit confused on the high RPM’S
In the spec sheets for 502 big block’s GM says they can handle up to (I’m not sure) 5800 rpm
But this is peak level I’m guessing, and not for long distances, and also you will burn more fuel with higher RPM ‘S, now that’s not an issue otherwise I should shut up and drive a small car with better mileage but it is better for the engine to drive at lower a RPM.

[DUTCHMAN]

Give these guys a call, they know a lot about it.

http://www.progear.nl/start.htm

['68OrangeSunshine]

I'm curious about the statement that BB like high RPM's. I was under the impression unless they were specifically built and balanced that big blocks did not like high rpms due to the weight of the reciprocating mass. [/B]

Gee, maybe that's why mine blew up. LOL Sorry I didn't recognize the Dana 60. My experience is limited to 12 bolt [GM] Corporate rear axles.
I got my Auburn from Pro Gear in San Diego. They were cool to deal with. You might also consider the TH700R4 transmission with an overdrive of 0.70:1 in final. Or did I just open another can 'o worms?

[adamls]

There are many rearend gear companies out there. DTS, Moser, as well as ProGear. Right now with 4.11's you are running 3000 RPM with 3.54's you will run closer to 2500 at 60 and with a OD transmission and 3.54's you will run close to 1800 RPM's at 60. If you do nothing bu change to an OD and keep the 4.11's you run at close to 2100 RPM's at 60. As you can see an OD would be the best way to go to help the cruise speed and still maintain that low end grunt with the 4.11's but your 502 should have enough torque that you will still have fun with the 3.54's. Also if you go to a bigger tire it will lower your top end RPM's but it takes more torque to turn a larger tire. The calculation for RPM is

(MPH*Transmission gear*Rearend Gear*336)/Tire Diameter

Your tire diameter is probably close to 28 inches with the numbers you gave.

[burb71]

Did you or a pre owner import your truck to Europe, or was it built for the European market?
If so what are some of the differences?