rear diff rebuild questions?

Considering complete re build of the rear axle.  10.25, 3.55.  Its a standard diff as far as i know. Tag says, V126A 3.55 10 2 5D30.  Axle code = 39.  Clunking when shifting to drive and reverse.  Just changed all the u joints and driveshaft center support bearing.  Did total backlash test as per shop manual and appears to be at max allowance (1 inch)

Use it for picking up building supplies and towing 5th wheel and skid steer, cars, etc.  7 or 8,000 lbs.  It is not a daily driver.

Stick with the stock setup or change ratio, limited slip, posi (if its even possible)?

Best pricing and store for kit? I live in Canada.

Thanks

I don't know about best pricing.  And, I've not owned a diesel in these trucks so don't really know that much about proper gearing for them.  However, I would ask a couple of questions.

First, how does the current combo do getting a load started?

Second, how fast do you tow and for what distances?

I ask those because any increase in gear ratio will help get the load rolling but hurt the performance and MPG at higher speeds as well as make the engine sound very busy.  So if you tow at 65 MPH plus I'd be cautious about increasing the gear ratio.  But if you tow at 55 MPH and need more torque to get the load rolling, or struggle pulling it up a hill, then a gearing change might help.

Ford's spec's on the Engines/IDI Diesel page show max torque at 1800 and max HP at 3300 RPM.  So the engine is going to be comfortable cruising at 1800, but won't want to rev much past 3300.

You can play with the calculator (Driveline/Calculators) to see what different rear axle ratios do to your RPM.  But with the 3.55's you are probably turning ~2800 RPM @ 65 MPH currently running light.  But when towing the torque converter in the C6 is going to be slipping more and the R's will go up slightly.

If you go to 4.10 gears you'll be turning 3100 RPM at 65 MPH and the engine is close to being wound out.  But at 55 MPH you'll be turning 2700 RPM, which is much more relaxed.

6.9-250 wrote

....  Stick with the stock setup or change ratio, limited slip, posi (if its even possible)?....

I don't disagree with anything Gary said about gear ratio, but if you want to know what Ford said, the GCWR for a 6.9L diesel with auto trans and 3.55 gears in 1985 (according to the pages from the owners manual that Gary has filed for us ) is 12,000 lbs.  Going to 4.10s ups it to 14,000 lbs.  So if you care about ratings, taking 6000 - 7000 lbs (a guess) off for the weight of the truck, 3.55 gears are rated for a 5,000 - 6,000 lb trailer, and 4.10s a 7,000 - 8,000 lb trailer.

As far as diff goes, if you drive in snow (and depending on where in Canada you are from I'd guess that's likely) I would strongly discourage you from going with a clutch-type limited slip (like the Eaton positraction that GM mostly used, or Fords TractionLok).  Any time you go around a corner the tire traction needs to overcome the friction in the diff, otherwise the tires will skid.  It can make you spin out even when coasting slowly around a corner.

A stock open differential is the most stable driving on ice and snow, but obviously the worst at getting you started.  If you think you need more traction there are options that won't hurt stability as much as a clutch-type limited slip (and likely will help traction even more).

If you don't want an open diff, my first choice for a truck that will tow would probably be a TrueTrac (or other gear-type limited slip, there are a few other brands out now as well, but that's the best known).  It doesn't have clutches to wear out and it doesn't try to drag tires around corners like a TractionLok.  It doesn't help a LOT when one tire gets really bad traction (it's easy to spin one tire if starting with that tire on ice, but it does usually get you moving even then), and it will make it easier to spin out if you get on the gas too hard.  But overall it's a really good compromise between street manners and traction.  (I have one in the rear axle of my F-250)

A selectable locker is another option.  ARB is the best known, but OX and e-locker and probably a few others are also out there.  They give you an open diff most of the time, but lock the two tires together with the flip of a switch (or pull of a lever).  That gives you the best on road manners and the best traction when you need it.  But they are more expensive and a little more complicated to install.  And you need to engage and disengage them as required for traction and turning.  (I have one of these in the front of my Bronco).

An automatic locker is another option.  Detroit is the best known, but Grizzly arguably has a stronger product, and there are lots of others as well, including "lunchbox lockers" that don't require a gear set-up to install.  They offer great traction with no need to mess with engaging and disengaging.  However they have the worst manners on the street of these three, and will spin you out very quickly if you get on the gas too hard on in a slippery corner.  They also only drive one tire when you are turning (allowing the other to roll ahead faster).  This is the source of the bad manners, and it's particularly an issue when towing, especially with a lighter tongue weight.  Personally I don't mind the on-road manners that much (I've driven them through several Minnesota winters), so if I value the performance I'm very willing to live with them.  But personally I wouldn't choose it for a truck that is primarily for towing, but I know there are others who still think it's great for that.  (I have this in the rear of my Bronco, and had them in the rear of a CJ5 and an F-150).

Bob - Good thoughts.

On the TrueTrac, as you know I have them in both the front and back of Big Blue.  And I've towed a bunch with it and never had a problem.  In fact, the only problem I had was too much gas on a wet corner, which caused the rear to step out.  But I've put down 4 patches of perfectly-matched rubber on the concrete when trying to pull down a tree, so they do work well.

As for the towing specs, here they are.  And for the first time ever I see that a manual transmission has a slightly higher tow rating than an auto.  

AOD is not anywhere near as strong as a C6. On limited slip, I have the Ford Traction Loc in the rear of Darth, he will lay 4 nice stripes from the duals if I stomp him from a standing start. Once the new engine with more compression is in, He will probably do even better. Stock 460 torque peak is around 2200, which if I drop the E4OD to D (third) that is 55 mph in lockup, 2700 in unlocked (500 rpm slippage).

I redid my Sterling 10.25 with my granddaughter's help. I took a differential, ring and pinion from the 1990 parts truck and installed in a 1993+ housing that had a water damaged open 4.10 in it. This gave me the same 3.55 gear I had but limited slip and the later dually 3 1/2" wide brakes. Unfortunately, it wasn't new enough for the updated rear hubs.

Gary Lewis wrote

....  On the TrueTrac, as you know I have them in both the front and back of Big Blue.  And I've towed a bunch with it and never had a problem.  In fact, the only problem I had was too much gas on a wet corner, which caused the rear to step out.  But I've put down 4 patches of perfectly-matched rubber on the concrete when trying to pull down a tree, so they do work well....

For people that are familiar with amplifying electrical signals, the difference between a TractionLok (or other clutch-type limited slip) and a TrueTrac ( or other gear type limited slip) is that the clutch types have bias and the gear types have gain.

For everyone else, clutch-types have a set amount of friction between the two sides, so they can always send that amount more torque (the torque it takes to slip the clutches in the diff) to the tire that has traction.  If the right tire supports zero torque the diff can send up to zero plus the clutch friction to the left tire.

Gear types multiply instead of add.  In the case of a rear TrueTrac I believe the multiplier is 3.5 (2.5 for a front TrueTrac, I don't know about any other brands).  So if the right tire supports zero torque a TrueTrac will send zero times 3.5 = zero to the left tire.  That's why I say TrueTracs work well as long as all tires get some traction, but they aren't as effective when one tire gets very little traction.

In my one winter of having it in my pickup I've found it only a little better than an automatic locker at not spinning out if you're on the gas too hard in a corner, and quite a bit less effective than an automatic locker at getting you started with one tire on ice.  But it is a lot more effective than an open diff at getting you started with one tire on ice, it has no bad manners (other than being easier to step out if you're on the gas in a slippery turn), and the manners don't get worse when towing.

So personally I think I prefer an automatic locker over a TrueTrac for winter driving, but I do see why many would not due to the automatic lockers manners, and I'd lean pretty strongly toward the TrueTrac for towing.

Good analogy, Bob.  I understand it.

And I know you've not forgotten as you told me, but for the others you can bring a TrueTrac into play by touching the brake.  That raises the "traction" on the spinning tire above zero so the multiplier has something to work with.

1993 also gets you more support with the long pinion, doesn't it?

Only if I kept the 1993 pinion, the difference is the pinion and yoke, housing didn't change. Since I used the 1990 innards I ended up selling the yoke to an FTE member who needed it.

Definitely a lot to consider.  

1.  Truck seems fine to get loads started.  No issues there.  

2.  Last tow was at 100km/hr with 7,000lbs on, rpms at about 2600.  Rpms are at that 2600 loaded and unloaded at 100 km/hr.  I should try driving a little faster and see if the mileage would improve. Not sure where the sweet spot is for this truck.  About 9.3 mpg last trip loaded.  I did not take good notes on the unloaded mileage but seemed to be the same?

Winter driving seemed fine, no traction issues at all in 4x4.  

Sound like the current setup is working well for me, so I may just keep it the same then.  

UPDATE:  clunking noise is pretty much gone or very little now.  The only thing I did was unbolt the very rear ujoint and ujoint off the transfer case.  One of the bearing cups on each was not purging grease.  Used a c clamp to hold cups (lightly) and was able to purge grease on all 4 now.  Cleaned the yokes (hopefully thats the right name) better and reassembled.  
Perhaps the ujoints were not properly seating and there was some binding?  Its been good since then though.  Rear axle rebuild project may be delayed.  

Thanks for all the info everyone!