Jim - I've had a diff rebuilt and various other transmission and related work done on different trucks/cars at Hi-Tech over the years. IMO they know their stuff and specialize in this sort of work, and tend to skip the BS. In fact, they saved me a bunch of money once when I was ready to rebuild a trans and they discovered it was another problem that was FAR less expensive. The last diff I had done was for an E-350 diesel van. Another ordinary mechanic wanted to swap it for a reman unit, but Hi-Tech convinced me that rebuilding it with factory parts was both cheaper and it was preferable to keep the original case/parts with the chassis whenever possible since slight variations over model years can sometimes cause problems.

http://www.httrans.com/

Dammit, You beat me to it

Thanks everybody for the helpful comments. I did call both places recommended and have spoken to a total of 3 shops. The estimates for labor to install a new diff, and replace the pinion bearing, pinion seal, and carrier bearings from two shops @$600, and one shop @ $775.

Nobody wanted to touch reubuilding the factory limited slip, except for Convenience Car Care (Timmy Tirrell), for a very reasonable price.

There are 4 basic solution to my problem (getting drive force to both rear wheels of my F-350, so I can have a reasonable plow vehicle). I don't plan any doing any real offroading.

1) Rebuild the original limited slip (primarily labor and will likely wear out in 3 years, so the cheap up front cost is a short term solution)

2) Install a manual locker rear diff - (Very expensive parts, more complex mechanism so higher likelihood of failure, most suitable for serious off roading) (I know there is an automatic locker, but I have no interest in that as it can make driving in the rain very hairy).

3) Install an aftermarket clutch based limited slip (will still wear out, especially if doing a lot of towing, which I do)

4) Install an aftermarket Torsen based limited slip (no parts to wear out, but if one wheel is in the air or has zero traction, requires the parking brake to get the vehicle going).

Convenience Car Care gave me the best advice, and that was they could install anything, but don't know what is best for my needs. They said to call Randy's Ring and Pinion (http://www.ringpinion.com/) find out what is best for my needs from Randy's, buy the parts from them, and they will install it for a fixed labor fee. Randy's Ring and Pinion is very famous in the U.S., gave me lots of options, and felt that it was a no brainer to go with the Torsen limited slip (Detroit Truetrac by Eaton).

They told me that $600 for labor was pretty damn pricey for the install. That's what I thought, and that's why I called around, but that was the cheapest price I could find. Oh well, the privilege of living in the DC area.

Anyway, I need to get some numbers off my differential tag, order the parts through Randy's Ring and Pinion, and then have Convenience install them.

Thanks again!

Why would removing the ring gear reset the pinion depth? Is there something different about the E30 diffs (I've never opened mine up)? The ring gear should still be centered on the axis of the diff when you put it back, so how would the pinion/ring gear alignment along the axis of the pinion change? The Porsche manuals say that you don't have to reset it if you replace the differential housing - implicit in that operation is removing the ring gear and bolting it to the new one. New ring gear and pinion, yes. New housing or side plate, yes (although side plate is borderline IMO).

Removing the ring gear and reinstalling it on the SAME carrier does not require resetting the pinion depth. Removing the ring and putting it on a DIFFERENT carrier requires all the special tools and pain of resetting the depth.

Its not the ring being colinear w/ the carrier axis of rotation thats the problem (thats inherent when the carrier is machined on a lathe). Its the side-to-side location. If the ring moves further from the center line of the pinion, the pinion must move closer to the centerline of the ring to keep mesh constant. And visa-versa. Depending on machining depth, bearing thickness, preload, etc the carrier can be moved side to side. That needs to be compensated for.

You say the porsche manual doesn't require resetting it when swapping carriers? (Like non-lsd to lsd.) I don't see how that can be. Every lsd install requires resetting the R&P.

I'm still missing how the side-to-side affects the pinion depth setting... The pinion needs to be positioned in 3 dimensions (well, really 2 since the up/down is dictated by the case). The backlash/preload setting positions it along the axis of the carrier (perpendicular to the pinion shaft). The depth setting positions it in the other dimension, and is independent of the side-to-side position. How am I changing the depth if I am just moving the carrier along the driveshaft axis? The face of the pinion shaft is still the exact same distance to the centerline of the carrier axis.

As you said, the machining depth (of the carrier bearings), bearing thickness, shims, preload move the carrier side-to-side, and are compensated for with the correct combination of shims under the carrier bearings. This is set using the backlash and preload measurements.

The pinion depth does however affect the backlash setting. It's almost a 1:1 relationship between the pinion depth change and the backlash change because of the way the gears mesh. The Porsche workshop manual has you setting the depth first, then the other measurements.

Here's the table from the 993 manual:
Attachment 21521

"Drive set" means the R&P. The older 911 turbo manual has the same table, except they think you need to reset pinion depth when you change the side cover. Maybe tolerances weren't as good with the earlier covers?

Cool chart.

Sorry, It doesn't directly. But you have to adjust it if you move the carrier side to side to adjust mesh (on some transmissions.)

The two can slide inversely anywhere along the bevel angle and still mesh correctly (though I don't know if hypoids follow that same relationship - though I suspect they do in some crazy mathmatical proof that would give me nightmares.) You want the center of pressure to be in the middle of the faces for strength so you have to shim both R & P to do it perfectly. IIRC, my ZJ jeep rear axle had no provision to shim side to side - I rebuilt that one twice:( So everything was done w/ the pinion. I don't remember any shims near the E30 carrier bearings either. But you don't have to take that one fully apart to replace the clutches, so maybe we just didn't dig far enough? I've never taken a 915 diff apart so its kinda cool to know it can be adjusted in both directions. A PITA to setup but cool:) I'd bet w/ modern CNC you can hold carrier tolerances enough to eliminate the side-to-side variate enough to just use the pinion to set backlash - and the contact point is 'close enough' to the middle. So if you replace the carrier, you re-shim the pinion.

Since this is more than I've talked about gears since last christmas: on a semi-related gear note, I picked up a small horiz mill (thats 3 for the folks keeping track @ home:)) to fix a gear in the apron of the sheldon lathe. The shaft is worn ~0.010" undersized so after I hardface it, I'll have to fix the spur gear its sitting on (see the pointy teeth - thats bad). The current plan is to weld up the teeth and recut them so they look like the other involutes.

Before you order any parts or have Convenience install them, give County Transmission in Vienna a call. They build all of our race transmissions and do all of our COD trans overhaul work.

http://www.countytransmissions.com/

I want to thank everybody for the advice on the F-350 differential. I ended up calling Randy's Ring & Pinion, probably the best known shop and Internet ordering place for rear end components for trucks. They recommended the True-Trac torsen limited slip rear differential which I purchased. I also went with their install install kit (replaces all bearing in the diff). After a lot of calling around, I went with Convenience Car Care for the install. They charged the same as the other shops, plus provided pick-up and delivery service from my house. Can't beat that.

The torsen limited slimt rear end is incredible. Just the slightest bit of slip and the rear end instantly locks up and transfers power to the side that's not slipping. I'm very happy.

Also, for Christmas, I got myself an on-board air compressor and air horns for the truck, just for fun. They are loud. Unfortunately, they took signifcantly longer to install than I ever imagined (need to mount switches, pressure gauge, air compressor, air tank, air intake, air solenoid, and 2 horns).

Wait, you put those horns on your truck? And people call me a redneck. :)