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Discussion Starter #1 (Edited)
The owner writes: "...a 2002 LT that the that I suspect the Crown Wheel Bearing has gone bad for the 2nd time. The Bike has 42500 miles and the first one went out at 17000 and was fixed by the dealer."

First failure at 17,000 miles is certainly one of the earliest I have heard of. Then the FD went just 25,500 miles on the dealer's rebuild.

The existing shim was 0.60mm. The new bearing called for 0.30mm and that's on the tight side of the preload specification range. That suggests that the final drive was overshimmed by upwards of 0.30mm. Mechanical engineers I have talked to about this problem say that excess preload will certainly be damaging. I have to wonder if the earlier rebuild included measurements for shim thickness or if they just replaced the bearing using the original shim.

This FD also had the "migrating" inner race of the input pinion needle bearing. I don't know what causes this and don't have any fix other than to press the race back until it is fully seated. I've seen what can happen if this problem is left unchecked so I always check this bearing race, it is actually a pretty common finding; if you plan on rebuilding your final drive I suggest you look for this potential problem.

Pics attached:
The mess inside the FD housing:


The crown wheel bearing:


The pinion needle bearing race before reseating:


The pinion needle bearing race after reseating:


....and from another drive, what can happen if the pinion needle bearing continues to move off its seat:
 

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Due to the cut of the pinion gear, acceleration and deceleration causes axial loading on the pinion shaft. this causes it to move back and forth moving the pinion shaft on the bearing inner race. Eventually this back and forth hammering causes the bearing to drive through the housing.

That's my thought.
 

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I just finished up a first failure at 15K for Wes on an 02 LT. Classic big bearing cage failure. There was some oil leaking at the pinion area and I found the compression ring severely pitted and rusty. The inner and outer races were spalling but the balls were in good shape. Drive was over shimmed by 0.15 mm. There was no creep on the pinion pilot bearing


I did notice that the outer race of the bearing would move up about 0.008 inches so I decided to take that into consideration when doing the Dman method of measuring. I would only do one pull up at a time because I could not push the outer race back down that 0.008. I feel you miss that distance if you just push the crown wheel back down because the outer race is displaced by 0.008. By doing one pull up and then removing the cover and doing another pull up I still got consistent readings. They were also matched to my static measurement method.
 

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Discussion Starter #4
New2rt said:
Due to the cut of the pinion gear, acceleration and deceleration causes axial loading on the pinion shaft. this causes it to move back and forth moving the pinion shaft on the bearing inner race. Eventually this back and forth hammering causes the bearing to drive through the housing.

That's my thought.
Yes, the pinion shaft has forces that cause axial movement. BUT, this movement is normally limited at the rear by the seat in the pinion housing for the pinion main bearing and its associated shim; the pinion main bearing, not to be confused with the needle roller bearing, is a very "beefy" assembly made up of a combination of a ball bearing and a roller bearing. Movement is limited a the front of the pinion shaft by the locking ring.
The needle bearing at the back end of the pinion shaft (the one pictured above), plays no role whatsoever in limiting the axial movement of the pinion shaft under normal circumstances.

The axial movement of the shaft is not designed to be limited by the inner race of the needle roller bearing bottoming out. Under normal conditions, there is never contact with the inner race of the needle bearing with the bottom of its recess in the housing. That is to say, under normal conditions, there is never "hammering" of the needle bearing race on the housing.

The inner race of the needle roller bearing ONLY wears on the housing when it has slipped off its seat. WHY it slips or "creeps" off its seat is the question. Probably the press fit off the race on the pinion shaft is insufficient, but that's just my guess.
 

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Discussion Starter #5
Hi John,

Thanks for posting this, I have not seen the rust and pitting on a compression ring like that before. Interesting, I wonder what caused that?

Regarding measurement of axial movement of the inner race with respect to the outer race in the DMAN method, you don't need to push the outer race down to close the axial play "gap" to get an accurate measurement, you just need to push the hub down to make sure the tapered roller bearing is fully seated. If you were directly measuring the outer race during the DMAN method you would need to push down the outer race of the crownwheel bearing, but measurments are taken at the hub. The downward movement of the crown gear assembly is limited by the tapered roller bearing fully seated in its race, not by the outer race of the crownwheel bearing. As long as the outer race is sliding freely in the cover and the tapered roller bearing is full seated, it doesn't matter that the outer race of the crownwheel bearing isn't pushed down to take out the axial play. I had to spend some time thinking about this to "get my head wrapped around it", but I think you'll reach the same conclusion once you sort it out. This was discussed in a thread quite some time ago by someone else (I've forgotten who, but it was a knowledgable poster) but they expressed the same conclusion, i.e. the outer race of the crownwheel bearing does not need to be pushed fully down, only the tapered roller bearing needs to be fully seated.

It is on the "upstroke" that the axial play of the crown wheel bearing must be taken up and it occurs easily. Once the axial play is taken up on the upstroke (a low resistance, and the first movement you'll see on the dial indicator) and the outer race of the crownwheel bearing is pulled up to fully seat in the cover (higher resistance because the race has to slide in the cover, and the second movement you'll see on the dial indicator), you get can get a direct measurement at the hub of the total movement. You'll generally see two distinct movements on the upstroke on the dial indicator. The first, the small low resistance movement is the uptake of axial play. The second, typically a larger movement and requiring more force, is the movement of the outer race in the cover.

You are correct in that the first movement isn't a measure of the total axial play in the bearing, you would have to push the outer race fully down to get that, but it doesn't matter for the purpose of measuring for shim thickness.

In my experience, some bearings will move in the cover easily with the cover at 250F. Others will not, and I have had to heat the cover more. I also grease the bearing seat to make the bearing race move in the cover more easily. But the key is to make sure the tapered roller bearing is fully seated, it doesn't matter that the outer race of the crownwheel bearing isn't fully down on the "downstroke" for the DMAN method.

I have discovered that the crownwheel bearing is "sloppy". Engineers tell the that the "C" designation on the bearing relates to its play. This play makes measuring the axial movement prone to error. I spend some time trying figure out why sometimes the DMAN method and the Service Manual (static) method gave slight different results. I realised that pressure has to be applied evenly all around the bearing or one race will "tilt" with respect to the other. I earlier posted pics of my bearing stabilizer (homebrew BMW special tool) made of a modifed FD cover. Even with that stabilizer in place, care must be made to measure the bearing on opposite sides because the outer race maybe tilted. Only when the same measurement is obtained at opposite sides of the bearing is the measurement meaningful.

In my experience and experimentation, it is the tendency of the outer race to tilt with respect to the inner race that introduces differences between the DMAN and static (service manual) methods. Careful elimination of the tilting by stabilization of the bearing is needed to eliminate the "tilt error".

You don't really need to measure the axial play of the bearing to get accurate measurements using either the DMAN or static methods, so measurement of axial play is sort of academic interest. But if you wanted to get a reasonably accurate measurement of axial play, I think that there'll be errors from the method shown in your pics. You've supported the outer race in two locations for the "up" measurement but your dial indiator is measuring a point 90 degrees to the support points; I'll bet the outer race is tilted, and if you could measure 180 degrees from your current measurement point WITHOUT moving anything you'd get a different reading. You wouldn't be about to do this with the setup pictured because just the pressure of the dial indicator will change things.
In your pic measuring the outer race in the "down" position, the outer race isn't stabilized at all that I can see, again I suspect that the force of the dial indicator may be introducing some "tilt error".

While measuring axial play isn't necessary, discussion of the "tilt error" is important because the tilt error affects measurements in the service manual (static) method. I was not until I was careful to stabilize the outer race and take multiple measurement 180 degrees opposite eachother that I was able to get consistent results between the DMAN and static methods. I've never seen the BMW special tool, only pics of it in the service manual, but I have speculated that it may not eliminate the "tilt error" and this may have contributed to so many FDs being overshimmed at the factory. If the outer race isn't stabilized, downward pressure from a dial indicator or depth micormeter will case the outer race to tilt down, resulting in a larger measurement. This would result in a thicker shim. Think that one over for a while... :)

Sorry for the verbosity.... or, This post was brought to you by the department of redundancy department, which was responsible for this post. :histerica

PS Did you get that ABS computer I sent?

Best Regards,
Curtis


jzeiler said:
I just finished up a first failure at 15K for Wes on an 02 LT. Classic big bearing cage failure. There was some oil leaking at the pinion area and I found the compression ring severely pitted and rusty. The inner and outer races were spalling but the balls were in good shape. Drive was over shimmed by 0.15 mm. There was no creep on the pinion pilot bearing


I did notice that the outer race of the bearing would move up about 0.008 inches so I decided to take that into consideration when doing the Dman method of measuring. I would only do one pull up at a time because I could not push the outer race back down that 0.008. I feel you miss that distance if you just push the crown wheel back down because the outer race is displaced by 0.008. By doing one pull up and then removing the cover and doing another pull up I still got consistent readings. They were also matched to my static measurement method.
 

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CharlieVT said:
I've never seen the BMW special tool, only pics of it in the service manual, but I have speculated that it may not eliminate the "tilt error" and this may have contributed to so many FDs being overshimmed at the factory. If the outer race isn't stabilized, downward pressure from a dial indicator or depth micormeter will case the outer race to tilt down, resulting in a larger measurement. This would result in a thicker shim. Think that one over for a while... :)
It is fundamental for any accurate thrust clearance or preload measurement of a bearing pair, that all races be accurately held in alignment. CharlieVT's fixture analysis and procedures should be studied and understood by everyone (especially BMW) working on FDs.
 

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Discussion Starter #7
niel_petersen said:
It is fundamental for any accurate thrust clearance or preload measurement of a bearing pair, that all races be accurately held in alignment. CharlieVT's fixture analysis and procedures should be studied and understood by everyone (especially BMW) working on FDs.
Hi Niel,

Nice vote of confidence there, not sure how well placed it is. ;)
I think the BMW engineers understand it all quite well; I have always though that the problems were in manufacture and assembly. Essentially a quality assurance problem, not an engineering problem.

I have to say that I can't hold a candle to the amount of knowledge and experience that John Z has.
John, my observations in response to your posting of your recent FD rebuild were only meant to be thought provoking and helpful.

I've been doggedly trying to understand the final drive and my FD "hobby" has continued over almost 10 years now. I have always appreciated thoughtful posts from folks like you, Jim B. (Realwing), David S., and others who have chimed over the years with information and suggestions. There are many, many folks who understand the mechanics and engineering of the final drive better than I do... they just don't blab about it on this site like I do. :)
 

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I have struggled with the way to stabilize the race to get that measurement. I did set that up for a photo op but I had placed multiple "finger tips" evenly under the race to get the same reading. Tried to ensure I didn't tilt the race.

This drive was hard to measure with the Dman method. Even with the cover at 250 degrees I could get one "up" movment and then it would not go down and up again. When I unbolted the cover from the housing and lifted it out the bearing slid smoothly out. Looks like there is some offset (which may not be good for the bearing) in this assembly.

Now back to my 0.008 reading. My thought was this does come into play. You stated that it did not matter that the outer race would not go down. I think it does. Lets see if I can verbalize this.

You have the bearing (and outer race) down all the way when you first put the cover on. Then you perform a "lift" you get the radial play first then the race come up to contact the cover. That distance is "x" assuming you zeroed the dial indicator. Now you push the hub down and some radial play then the race moves down in the cover but it does not go as far as it was in the first place (x- radial play). Now if you re- zero the dial indicator and do subsequent lifts your measurement still has radial play but the race is now at a point that is the original starting point - the radial play distance. I guess that is what I was trying to point out. I seem to remember you re-zeroed the dial indicator in the video. To get the true distance you need to consider the radial play as the shims will pre load the race in that direction.


If you don't re-zero the dial indicator you are OK for subsequent lifts. Does that make sense?


UPDATE: I was drawing myself a picture to help prove the point and I realized it does not make any difference. The hub movement will always be the same each time you lift. The outer race never goes back to the original position but that does not matter since the hub movement will always be the same. Don't know why I didn't see that earlier. Thick skull I guess.
 

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Discussion Starter #9
jzeiler said:
....This drive was hard to measure with the Dman method. Even with the cover at 250 degrees I could get one "up" movment and then it would not go down and up again. When I unbolted the cover from the housing and lifted it out the bearing slid smoothly out. Looks like there is some offset (which may not be good for the bearing) in this assembly......
Interesting observation, I've had several drives where Dman's method was diffcult due to tightness of the bearing in the cover, even with good heat. Never considered that the machining might be off a bit. Considering all the other machining errors (loose tapered roller bearing on some 05s, the slipping aluminum hub in the carbon steel crown gear, the "creeping" pinon bearing race) it seems pretty reasonable that other machining errors might have occured in the FD machine shop.

Thanks.
 

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Curtis, John and others, in answer to the comment regarding the "C" rating for bearings I can advise as follows. Rolling Bearings are assembled with a range of internal clearances to accomodate designed loads and speeds in their intended applications. The system used to identify the clearances is - C2 - less than nominal internal clearance, C0 - Normal internal clearance, C3 - greater than normal internal clearance, C4 greater than C3 clearance. To obtain the values for these clearances in respect to different sized bearings, and different bearing types, you need to refer to documentation such as the SKF general catalogue, FAG standard Programme etc. Important to note is that if a bearing is of a standard clearance range it will normally not be marked, but if it is of less or greater than normal clearance it will have that denomination etched into the side of the outer raceway (c2,c3,c4). And of course, as the internal radial clearance increases, so too will the axial displacement of the outer race in relation to the inner race. Bearings in our FD assemblies would be normal clearance I expect. Hope this helps, Dennis.
 

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John Z - Measuring axial preload is not possible with a single dial indicator on the outer race, & is even of mediocre precision if two simultaneous indicators are used on opposite sides of the outer race. Any alignment canting allowed to occur with any of the four races (the two inners plus the two outers) will make the axial preload measurement invalid.

Unless I am missing something in your measurement scheme? Actually as an alternate for the setup challenged, one of the better means would be to leave out a shim, reassemble, and then use three or four equally sized and spaced very small solder bits & measure the final thickness with a good micrometer.

I think it would be unlikely that the bearing housing diameters and thrust faces would have concentricity problems from manufacturing as the part is quite rigid & is most logically all done at one chucking. On the other hand, who says there's logic........ :wave
 

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Or use this product, readily available in Aust, assume it can be found in the States. Though it is quite obvious to me that Charlie VT has the job nailed as he has never had a return. He'll be getting mine if I have a need, and I'm in Tasmania, Aust.
Flexigauge is precision manufactured plastic strip.

It is available in four sizes and is used to check clearance of main and connecting rod bearings, and is suitable for Automotive, Marine and Industrial applications.

The four sizes available together cover clearances from .001 in to .016 in, or .025mm to .406mm and as follows:

Reference: Covers Clearance Between:
AG-1 - (RG-1) (Green Pack) .001 - .003in or .025 - .076mm
AR-1 - (RR-1) (Red Pack) .002 .002 - .006in or .051 - .152mm
AB-1 - (RB-1) (Blue Pack) .004 .004 - .009in or .012 - .229mm
AY-1 - (RY-1) (Yellow Pack) .008 - .016in or .203 - .406mm

Flexigauge strips - 12 in long - are packed in individual envelopes, which are marked in appropriate graduation - one side in thousandths of an inch and the other side in millimetres. Ten strips in their envelopes are packed in their appropriate coloured carton.
 

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Getting ready to check clearance on two drives - 2002 and a 2009

On the 2002 I just replaced the main bearing and seal a day before CCR2011. Bike has 58000 miles and I caught the bearing failure very early. This was a quickie and all I did was replace the bearing and took out 2 shims totaling 0.65mm and put back in just 1 shim - 0.50mm. Did not have time to take the drive off the bike - just pulled the main gear and bearing. Put 6000 miles on the bike and now I am going to officially measure the clearances. Drained the oil three times and looks extremely clean and clear. I am going to sell the bike and want to make sure its properly set for the new buyer!!

On the 2009 I bought the bike a month ago with only 12,800 miles on it - WOW. Drove it 2,000 miles home. I am now hearing either a gear or bearing noise from the rear drive?? Run the bike up to 35 mph pull in the clutch and coast - no acel or decel - and I hear a noise. Thought it might be the tires - had a cupped Metzler on the front - thought it was coming from the front. New tires still have the noise. So I put the bike on the center stand and rotate the tire: 1) in the normal direction I hear / feel a noise 2) in the reverse direction no noise - nothing. I am thinking maybe the u-joint is the source!! Or is this normal for a low mileage 2009?

So now to jump into the conversation with CharlieVT and John - further distilling your dialog looks like I am to measure the movement of the hub - while the bearing is there and along for the ride it is the hub movement we are targeting. Certainly plan to make sure the bearing is set properly.

One question - what are your thoughts regarding pulling a bearing - will it destroy the bearing and once pulled if nothing is wrong with it would you drop it back in or would your replace with a new one? In my case the 2002 has 6000 miles on the bearing and the 2009 has 15000 miles on its bearing.
 

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Discussion Starter #14
Dennis, thanks for the post.
The bearing supplied by BMW for the final drive is a German made FAG 61917.C3


K100Dennis said:
Curtis, John and others, in answer to the comment regarding the "C" rating for bearings I can advise as follows. Rolling Bearings are assembled with a range of internal clearances to accomodate designed loads and speeds in their intended applications. The system used to identify the clearances is - C2 - less than nominal internal clearance, C0 - Normal internal clearance, C3 - greater than normal internal clearance, C4 greater than C3 clearance. To obtain the values for these clearances in respect to different sized bearings, and different bearing types, you need to refer to documentation such as the SKF general catalogue, FAG standard Programme etc. Important to note is that if a bearing is of a standard clearance range it will normally not be marked, but if it is of less or greater than normal clearance it will have that denomination etched into the side of the outer raceway (c2,c3,c4). And of course, as the internal radial clearance increases, so too will the axial displacement of the outer race in relation to the inner race. Bearings in our FD assemblies would be normal clearance I expect. Hope this helps, Dennis.
 

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Discussion Starter #15
Use of Pastigauge has been discussed here before and tried by me. It does not lend itself well to this application. I assume Flexigauge would have the same problems as it is a similar product.
Problems: the space between the outer race of the crownwheel bear and its seat is small, unlike measuring a bearing journal which the Plastigauge products are designed for. It is difficult to place the material and get it to say in place. The size of the compressed section that needs to be measured is very small and difficult to measure.
And, most significant, the BMW service manual recommends heating the cover to remove and install the bearing. This reduces the chance of damaging the bearing, Problem is the heat melts the Pastigauge!

Dan Martin on this site has done a few drives using fast setting epoxy instead of Plastigauge. I'm not sure how accurate the method is but in theory the method should work and Dan reported he was getting good results with it.

I think that any new method should be repeatedly checked for accuracy against the "standard method" which is the method described in the BMW service manual. Problem is, the BMW method requires a special tool, and is fraught with the potential for measurment errors. This is why I have made an effort over a number of drives to compare Dman's dial indicator method with the method described in the service manual.

Dman deserves credit for introducing his method to this board. It is a relatively simple and elegant method. However, as John Z. noted in an a post above, not all final drives will be as easy to measure as others due to resistant of the bearing to slide in the cover during the measurment process.


K100Dennis said:
Or use this product, readily available in Aust, assume it can be found in the States. Though it is quite obvious to me that Charlie VT has the job nailed as he has never had a return. He'll be getting mine if I have a need, and I'm in Tasmania, Aust.
Flexigauge is precision manufactured plastic strip.

It is available in four sizes and is used to check clearance of main and connecting rod bearings, and is suitable for Automotive, Marine and Industrial applications.

The four sizes available together cover clearances from .001 in to .016 in, or .025mm to .406mm and as follows:

Reference: Covers Clearance Between:
AG-1 - (RG-1) (Green Pack) .001 - .003in or .025 - .076mm
AR-1 - (RR-1) (Red Pack) .002 .002 - .006in or .051 - .152mm
AB-1 - (RB-1) (Blue Pack) .004 .004 - .009in or .012 - .229mm
AY-1 - (RY-1) (Yellow Pack) .008 - .016in or .203 - .406mm

Flexigauge strips - 12 in long - are packed in individual envelopes, which are marked in appropriate graduation - one side in thousandths of an inch and the other side in millimetres. Ten strips in their envelopes are packed in their appropriate coloured carton.
 

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In reply to the question of Dan Finazzo regarding re-use of a bearing which has been pulled from its mounted position - if any procedure used in either mounting or dismounting a bearing transfers forces through the rolling elements, then the bearing will almost certainly be damaged. Known as false brinelling, the condition presents itself initially as a series of minute, even microscopic, dents in both recaways, spaced at the pitch of the rolling elements, be they balls or rollers. These impressions are the start of a bearing fault and eventual degredation of the running surfaces on the raceways. The same goes for assembling sub components with bearings already mounted. Striking the end of a shaft with a hammer to drive it into a housing will certainly damage any bearings as the impact forces are transferred through them. Bearings are tough when installed correctly. Damage them out of ignorance and you will pay the price. Hope this helps.
 

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Discussion Starter #17
dfinazzo said:
.....One question - what are your thoughts regarding pulling a bearing - will it destroy the bearing and once pulled if nothing is wrong with it would you drop it back in or would your replace with a new one? In my case the 2002 has 6000 miles on the bearing and the 2009 has 15000 miles on its bearing.
If you heat the cover to remove the cover from the bearing, it can be done without stress to the bearing. You can then measure for shim thickness of the existing bearing. There is no need to remove the existing bearing from the crowngear hub to check it, and I don't know how you can remove the bearing from the crowngear hub without excessive (damaging) forces on the bearing. I would never pull the bearing from the crowngear hub and then reuse the bearing. Engineers tell me that the forces involved in pulling the bearing will cause micro fractures in the balls and races setting the stage for future failure.

I would say if your new bearing in the 2002 measures okay for the for the shim thickness you have in there, you would be fine just reassembling it. HOWEVER, if the shim thickness that you installed isn't correct, I would consider the bearing potentially damaged and I would replace it.
 

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Curtis, the bearing you quote (61917 C3) has external dimensions of 85 x 120 x 18 mm. The SKF catalogue states that this bearing in C3 clearance (bore size greater than 80 mm and less than 100 mm), which means it has an internal clearance prior to mounting ranging from 80 to 100 microns, or 0.08mm to 0.10 mm (0.0035" to 0.004"). The catalogue states that when mounted, deep droove ball bearings "operate with internal clearance virtually zero". So this means that the fit for the inner ring is critical in reducing the pre-mounted bearing clearance by the designed amount. There will be a tolerance range for the journal (in this case the gear carrier) but it is unlikely that it would be released by BMW. It can however be calculated based on the bearing bore size.
If we all contribute on the technical side I'm sure this whole issue will eventually be extremely well understood, which always seems to shrink the problem in my experience.
 

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Discussion Starter #19
jzeiler said:
....This drive was hard to measure with the Dman method. Even with the cover at 250 degrees I could get one "up" movment and then it would not go down and up again. When I unbolted the cover from the housing and lifted it out the bearing slid smoothly out.....
I have found some drives are definitely harder to get easy, consistent results using Dman's method due to resistance of the outer race to move in the cover.

Things to try:
1) Sometimes the aluminum of the bearing seat in the cover is scored from the bearing spinning. Other times the aluminum is roughened on the upper side of the bearing seat, which I have assumed to be the result the the weight of the bike being carried on this surface. In these cases I might use a very mild abrasive, steel wool, green pot scrubber, or the like, to lightly polish the rough areas in the bearing seat in the cover.
2) You are probably doing this, but remove the cover O-ring during the measurement process.
3) Grease the inside of the bearing seat.
4) I generally use only 4 of the 8 cover fasteners during measurement to speed installation. The cover cools quickly, especially when the workshop is cold.
5) Tighten the fasteners only loosely at first, move the bearing up and down, then do the final tightening of cover fasteners before starting measurements.

Reheating the cover, #6 or #7:

6) Loosen the fasteners, reheat the cover with a heat gun or propane torch with the cover still on the housing, then retighten the fasteners. I have an infared surface thermometer handy to check temps when doing this. This works sometimes but doesn't work as reliably as does #7 below as the bearing will take up some heat as well as the cover. #7 below takes a little longer, but works more predictably.

7) Heat the cover hotter in the oven at the start; I've gone as high as 350F. And move fast. :)
 

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Could you over shim the new bearing, assemble (no hardware - weight) measure case to cover clearance, subtract from shim to give zero clearance, add pre-load spec to come up with the proper shim?

I did this for many years and we used a standard pre-load shim, solder, or expandable rings.

I haven't been into an LT final drive so I'm asking. Seems to be over complicated as is.
 
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