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I'm slowly re-building my diff with new 3.73 gears (the last set from Tom's until '09, I believe!:thud:).
As part of the re-build I'm tuning the posi unit as per Gary Ramadei's paper. :thumbsup:

Reading various posts here and on other forums, I noticed some people asking how much drag was enough when shimming the side gears to pre-load the clutch plates.

This is my first Corvette differential but I've rebuilt three clutch type LSD's for UK vehicles (2 for a '79 Escort RS2000, and 1 for a Sunbeam Lotus), so I think I can make an educted guess as to how much slip I'm looking for for tractable street use.

With this type of build, having no springs to pre-load the clutches, I wanted to be sure that both sides produced exactly the same amount of drag. So I hooked a spring balance on to the ring spanner, and rotated the unit, keeping the balance at 90degs to rotation, and reading the "weight" produced.

This was after I had finally selected 035" shims in both sides, and was happy with the hands-on "feel" of the unit.
It confirmed the drag was the same on both sides.



The drag measured was 8lbs with the balance 10.25" from the centre. This (by my calcs) equates to 6.3ft-lbs of drag.


Another point of note that my be of interest: The clutches were left; coated with GM diff aditive, installed in the carrier and preloaded, for two weeks (while I scooted off to Azerbaijan for some offshore work!! :down:)
When I returned I found the action of the diff had very noticably smoothed out. Prior to leaving the diff had felt notchy regardless of how small the shims were, and I spent my working time convincing myself that the springs were going back in.
After two weeks covered in fluid and crushed up together the notchiness had gone, and I was back in the springless zone :laughing:

I haven't spoken to Gary about this specific finding, but thought it may be of interest to anyone else maybe wrestling with a similar problem.

A final point - the drag I set for the previous cars, if I would jack-up one drive wheel and leave the g/box in neutral. The raised wheel could be turned by hand but took some effort.
Out of curiosity, I re-installed the Corvette springs, with 020 shims, and checked the feel - I could not budge the damn thing!
The pre-load is immense, no wonder standard diffs clunk and grind on corners. :crazy:

I will post another thread once the diff is back in the car. To confirm whether this method of tuning really does work, and whether 6.3ft-lbs is too much, too little or bang on the money ;)
The only "while I'm at it" items I am undertaking, is to replace the drive shaft UJ's and to notch the diff crossmember to raise the diff 1/2" to impove my driveshaft angles.
 

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Interesting test, I haven't done that but if I can dig up a scale here maybe I'll do the same. I'm starting on a posi now.
Did I remember to tell you to rotate the posi 50 times in each direction and each side? That will seat the clutches. Not sure why the 2 week sit did the same. Be sure they're not too loose. 035 is about the "normal" range but it will vary.

If you have a slight notchy feel but smooth you're good. Then check the play in the side gear by putting your index finger and thumb on them and moving. There should be only slight movement if correct. If too loose here the side yoke will move the side gear.

Once you get them set or close and then compare to the stock setup there will be a big difference in feel.

good job:thumbsup:
 

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Hi Gary,
You did mention the 50 rotations for bedding-in the plates, and I went through that exciting step a couple of times just to make sure, but as I mentioned in my last PM, I couldn't seem to get even drag on both sides and the gears really felt like they were binding too much.
The two week "soak-in" really seemed to make a difference though. As I said, I was all for throwing the towel in and going with the springs.

I would be interested to know what drag you come up with if you can find a spring balance and duplicate my agricultural method :D

I spent a good couple of hours swapping shims to experiment with the different feel and pre-loading, and I'm pretty happy with the two 035's.
I've now got the ring gear and the carrier bearings installed. I start on pinion installation tomorrow night (family permitting).

Cheers,
Douglas
 

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Will do Doug.

Save the old pinion bearing and hone or polish the ID and use it for a setup bearing. Then when you have the pattern where it's good then install the new brg and re-check. You may find the pattern off a little with the change but you'll save the new bearing from being pressed on/off.

Start with a 026 shim with those 373 gears it should be very close.
 

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I know this is a really old thread, and I've tried to absorb all the newer threads Gary has advise on about tuning a posi, but I can't find much info on how to know when you have the spider gears shimmed properly. I watched the late Tom Watt's video on this a couple times and it seems its just by feel. I know Gary has mentioned when shimmed properly (and balanced), you will not be able to "feel" gear lash or any movement in the gears, but you will be able to rotate easily in either direction.

My question is has anyone come up with techniques (like Doug above) to measure the rotational torque as a target setting to know the spiders are "tight enough", but not too tight? I guess I'm referring to what we all do now with spring-filled posi's with the 40+ ft/lb rotational torque method. I imagine this torque with a tuned spring-less setup is gonna be much less than 40 ft/lb, but how much less as a rule of thumb? Bill
 

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Tuning a posi for the first time is challenging why do you think rebuilders don't offer it- time and experience.
You are not looking for a drag rating since the spiders are not machine lapped like a ring and pinion, so they will vary through 360*.
Follow Tom's video and try it. There should be no endplay or lash in the posi spiders when done, not any binding. It should be smooth, snug, and have no lash. It has to be balanced between the sides and again Tom's video covers it all.
 

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Tuning a posi for the first time is challenging why do you think rebuilders don't offer it- time and experience.
You are not looking for a drag rating since the spiders are not machine lapped like a ring and pinion, so they will vary through 360*.
Follow Tom's video and try it. There should be no endplay or lash in the posi spiders when done, not any binding. It should be smooth, snug, and have no lash. It has to be balanced between the sides and again Tom's video covers it all.
Awesome Gary. More art than science. I'll give it a go once I get my case polished.. Bill
 
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