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Discussion Starter #1
Grettings all,

Today my 3.46 diff arrived and I took it to my BMW Mechanic for the install. He called me a few hours later and said that he could not locate a torque spec/preload figure for the Input Flange that mates to the pinion bearings?

He told me he is very concerned with the warranty of his work and does not want to put the wrong preload on the input flange because it would put either to little or to much stress on the pinion flange.

What should I tell him:hmm:

Much appreication in advance. You guys are great! P.S. I DID search for 2 hours and could not find the answer.

Thanks,:thumbsup:
Tim
 

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Tell him this:
Swapping diff input flanges

Background:
The pinion bearings need some preload for optimal life, about 5000N (~1100lbs) the last time I checked BMW specs. Although that may seem like a lot, you are generating this preload with a large diameter fine pitch thread. Generating 10 times that load is easy. The input flange likes to be held on with a LARGE amount of preload, probably 10 to 20 times the bearing preload. To little load and the input flange will move around on the pinion shaft resulting in fretting, brinnelling, or other forms of wear on the mating splines which leased to spline failure. So, how do you get a huge load under the input flange nut (a.k.a. pinion nut) without overloading the bearings? A crush sleeve serves the purpose. A cylindrical sleeve is used under the flange and provides the majority of the reaction force when tightening the pinion nut. The sleeve is designed to bow outwards (barrel shape) to allow for movement until the bearings start to share some of the load. The trick is tightening the pinion until the bearings see the proper amount of preload. Unfortunately, it is impossible to determine bearing preload by using a torque wrench on the pinion nut since the crushing of the crush sleeve takes up such a large and unpredictable amount of the generated force. We can only find bearing preload indirectly by measuring the amount of torque it takes to spin the pinion shaft. Since you probably don't want to diassemble a diff to the point of just having the pinion in the case, you are best off measuring the torque required to spin the whole differential assembly prior to removing the input flange.

Procedure:

1. Determine the "spinning torque" prior to disassembly. Run one of the driveshaft-input flange bolts into the input flange until it bottoms out. Use a stack of washers or a socket under the head of the bolt if it sticks through the flange too far. Use an in-lb or in-oz dial torque wrench and the appropriate sockets/adapters to see how much torque it takes to keep the pinion (and the rest fo the diff guts) spinning once started in motion. Ignore the initial spike in torque and watch for the constant number after the spike. Write down that value on the side of the diff case. Do the same thing for both your old diff and the new one. You should put the new flange on the old diff so the next guy an use it.

2. Pry out the pinion nut lock plates and remove the nuts (30mm socket). To keep the flange from spinning, have a buddy thread a couple driveshaft bolts into the flange and counter with a pry bar. Keep the nut with its original diff.

3. Pull off the input flanges. The flanges will come off with hand pressure. If not, use a rubber mallet. A 10 ton 3 leg puller is not necessary.

4. This is a good time to replace input flange seals if necessary.

5. Put your original flange on your new diff with the new nut using Loctite 272 (high strength, requires heat to remove). Snug it down and check the spinning torque. If the spinning torque is low, tighten the pinion nut a little more and check again. Repeat until the spinning torque is correct. DO NOT OVERTIGHTEN THE PINION NUT. Overtightening causes the crush sleeve to crush too far resulting in either overloaded bearings or insufficient pinion nut torque and a loose input flange. You will have to replace the crush sleeve and start over.

6. Install a new pinion nut lock plate. Use a socket or punch to drive the plate all the way down. Stake the lock plate into the grooves to keep it fom spinning.

7. Repeat steps 5 and 6 using your old diff and pinion nut and new flange.


Ed

P.S. The reason I though an obscene amount of torque was necessary to get the pinion nut off was because of the last time I rebuilt a diff. I helped a buddy rebuild a Ford 8.8 using new gears/bearings/etc. Getting a new crush sleeve to start collapsing requires a HUGE amount of torque (200-300 ft-lbs).

The above came from a guy on here named teamdfl.
 

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Discussion Starter #3
Thanks, that is the one piece of info that I have printed out. It just seemed a little complicated. It would be nice If I just knew how hard the input flange needed to be tightened down... Anyone?
 

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You should just install it yourself...Its not hard and the satisfaction of a job well done is priceless in my opinion...Food for thought
It appears you did a 3.38 swap which came directly from a 330 auto and you did not need to swap the front flange. This is a more difficult install when the front flange needs to be swapped.

I am also not the type who enjoys major mechanical projects. I'll do oil changes, brakes, ect but I would wrather spend the time doing what I do (Real Estate) and letting the mechanic do what he does.:thumbsup:
 
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