HO52 swap to 3.73 posi - go with 10 bolt or 14 bolt?

[customc20]

I have searched this out but have not found exactly what I'm looking for. I currently have a 72 C20 with a 350 / TH350 with an HO52 rear end with 4.10 gears and 31" tires. It has coil spring rear suspension.

I am going to switch to a 700R4 tranny with 3.73 posi (limited slip, etc) and 28-29" tires. Since I can't get to this gearing using the HO52, I'll need to change rear ends. I've been looking all over and have found multiple 10 and 14 bolts from 73-87 trucks.

I know I'll have to weld the spring mounts and panhard mounts on both, so that's neither here nor there.

I'll be switching the axle out to a 5x5 lug pattern and disc brakes as well. Each of these rear ends has different ways of achieving this swap (I believe).

My question is, which axle would be better to build up for street use. The 10 bolts (5 lugs) I've been finding are generally 3.73 (sometimes 3.42's) but non-posi. The 14 bolts (mostly 8 lug I think )I've found are 4.10 SF non-posi. What would be the best option, and which would be the cheapest option, to get to a 3.73 rear posi? Should I buy the 14 bolt and switch gears and third member, and potentially 5 lug axles? Or buy the 10 bolt 3.73 and put in a posi third member/center section?

The rear ends generally seem to be about the same cost from the people I've contacted around here.

Hopefully this makes sense. Thanks for your help

[Beat68CST]

GM 12 bolt buddy!


They're common and easy to get, or you could go with a Dana60

[my67c20]

So i did some research and here is some interesting numbers to ponder.

Compare the following, assuming a 1:1 final drive at your transmission.

4.56 gears @ 65mph with a 29" tire your RPM's are roughly 3112
4.11 gears @ 65mph with a 29" tire your RPM's are roughly 3095
3.73 gears @ 65mph with a 29" tire your RPM's are roughly 2809

Now lets take a look at the same numbers but changing your transmission to a 700r4 with a final drive of .75:1;

4.56 gears @ 65mph with a 29" tire your RPM's are roughly 2576
4.11 gears @ 65mph with a 29" tire your RPM's are roughly 2321
3.73 gears @ 65mph with a 29" tire your RPM's are roughly 2107

So by "simply" changing the transmission you reduce your RPM's by almost 700. Essentially to accomplish the same thing would be the equivalent of changing to a set of 3.09:1 rear gears. Overdrive advantage.

Of course this is all quick calcs so if anybody sees any errors please correct.

[special-K]

3.54 posi Dana60 coil rear from a 67-72 GM Truck.

[customc20]

my6720, you are correct, however, a lower drive rpm is not the only reason for switching. I would like a posi/limited slip rear end, which is not available in the HO52 chassis. I would get a detroit locker (or another mfger) for my current rear end, however, I don't want a locking rear. Therefore, the only way to accomplish that with my current setup would be to switch rear ends. Since I'd be switching, I figured I would change ratios as well. The tranny swap will happen at a later time. I figured by switching gears (with a posi) and tranny's I'll have plenty of low end power and still be able to cruise the interstate at better RPM's, all while helping the gas mileage a little bit.

Special-K, you are correct a 3.54 posi Dana could be a great way to go. Should bolt right in too (I think), however, I have not been able to locate one of these. WEre teh 3.54 posi Dana's used in anything other than the 67-72 trucks?

Thanks for the help.

[special-K]

Well,3.54 is a Dana ratio for Dana 60s and Dana 60s were used in Fords,Dodges,and Internationals. What you can hope to find is the rear from a Big Block C/20. It seams they are harder to find in coil than leaf. The main thing is to come across any Dana 60 coil housing. You can always swap the differential internals. There is a posi Dana 60 in the parts area right now,I believe.

[RUSHNBOBO]

Originally Posted by my67c20

So i did some research and here is some interesting numbers to ponder.

Now lets take a look at the same numbers but changing your transmission to a 700r4 with a final drive of .75:1;

4.56 gears @ 65mph with a 29" tire your RPM's are roughly 2576
4.11 gears @ 65mph with a 29" tire your RPM's are roughly 2321
3.73 gears @ 65mph with a 29" tire your RPM's are roughly 2107

Of course this is all quick calcs so if anybody sees any errors please correct.

Your real close for a "quick" calculation.........the 700r4's final drive is actually .70:1

So its more like this for a 29 inch tire,

4.56 gears @ 65mph=2408 rpm
4.11 gears @ 65mph=2171 rpm
3.73 gears @ 65mph=1970 rpm

[customc20]

If I end up getting a 10 bolt and building it up, what are some of the better limited slip/posi diffs available? As I said before, I don't want a locker since this will be a street/weekend cruise truck. I know I've read that a well built 10 bolt should withstand a moderately built up 350. What classifies a 'well built' 10 bolt?

Also, If I go with the 12 bolt, what are some of the better internals for this rear end?

[RUSHNBOBO]

Powertrax no-Slip, Auburn, Eaton Posi, TrueTrac ,etc.......all of these are avail for 10 bolt 8.5..30 spline and 12 bolt 8.875..30 spline. All have their advantages and disadvantages.....check link for descriptions of each.

8.5 10 bolts are plenty strong for a mild 350, I've heard of 500hp+ on the strip with this rear, no problem

http://www.drivetrain.com/

http://www.chevyhiperformance.com/te...all/index.html

[Shaky]

Originally Posted by customc20

my6720, you are correct, however, a lower drive rpm is not the only reason for switching. I would like a posi/limited slip rear end, which is not available in the HO52 chassis.

HO52 and HO72 were available with "No-Spin" limited slip. These are available used from time to time.

[customc20]

I have looked into the "no-spin" center sections, however, from what I understand this is the same thing as a Detroit locker, which I don't want to use for street driving. Plus, I don't like the 'clunking' associated with lockers. Am I mistaken about the 'no-spin' being a locker?

[RUSHNBOBO]

HO52 and HO72 came with open or "no spin".....this is a misnomer because "no spin" is a Detroit locker; thats just what they called them at the time....see attachment.