C3 differential preload

[Bill Stephens]

I'm setting the preload on a 1969 automatic small block differential. I don't have an adaptor for my torque wrench to do it on the bench, so I'm going to do it on the car, but I don't understand the manual instructions.

The manual says to take off one wheel and tighten the front nut on the differential until it takes 40 foot pounds to turn the half-shaft on the opposite side of the differential from where the torque wrench is turning the hub. What I don't understand is that the manual says to keep the opposite tire from turning while doing this. I don't understand how the half-shaft will turn if the wheel is held because the half-shaft can't turn independently of the tire.

What am I missing? I'd appreciate any help. Thanks.

[Duke W.]

Re: C3 differential preload

I don't understand what you're trying to do. Half your post sounds like you are trying to adjust the internal differential preload/drag torque, and the other half sounds like you are trying to check Positraction torque preload.

What service manual are you reading - title and page number?

Duke

[Michael H.]

Re: C3 differential preload

I'm confused too Bill. I think what you're trying to do is set the preload for the Positraction, correct? If you have one wheel off and the other blocked so it can't rotate, all torque inputs to either the side yoke or pinion will be THROUGH the Positraction unit clutch pack instead of rotating the entire differential assy. With the driveshaft removed, rotating the pinion flange (cw) will force the input rotation to go through the resistance of the Positraction unit which has a desired torque value. If this value is more or less than the 40 ft lbs that it calls for, the value has to be changed within the Positraction unit. (new clutch plates/springs etc) It has nothing what so ever to do with tightening the pinion nut. That's an entirely different operation which has it's own specification. (usually about 15 INCH lbs to rotate) If you haven't removed the pinion gear or changed the position of the large nut that retains it, don't. That setting should only be set during overhaul.

The problem with trying to set the clutch pack rotational resistance is, there's really not much that can be done about it if it doesn't fall within specs.

If I completely missed the target on this and you're actually trying to set the pinion preload, let us know. That's a completely different operation.

[Bill Stephens]

Re: C3 differential preload

Thanks guys. I'm the one who's confused. I'm just tightening the pinion nut and misread the manual. My mistake.

[Duke W.]

Re: C3 differential preload

I'm not aware of any correct procedure to "tighten the pinion nut" on the car other than to mark its radial location and install it back to the same position if it has to be removed along with the yoke to replace the pinion seal.

Tighening the nut to achieve correct drive pinion bearing preload can only be properly done on the bench before the differential case assembly is installed.

You will get better advice if you state what it is you are trying to accomplish. Right now my concern is that you may be attempting something that is ill-advised.

Duke

[Roy B.]

Re: C3 differential preload

If you've replaced the pinion seal and need to tighten the nut. I use a air impact for about 3 seconds when tight and call it done. Done this for over 40 years many times and never had a problem.

[Terry M.]

Re: C3 differential preload

Roy,
Most text books caution against using an impact to preload bearings due to the chance of indenting (called bernelling, although my spelling of that term may be off) of the rollers. I am not aware of any service manuals that call for the use of an impact for the same reason.
Maybe you are very skilled in the use of an air gun, or maybe you have been lucky, or maybe the danger of this is over-sold. I just don't know for sure, but I prefer to check the pinion pre-load per the service manual -- measure the drag with a good inch/pound torque wrench. It is way too much work to be that far into the differential to take a shortcut and risk a failure. Just my deuce centavos.