Let's start with basics. Is this the right way to measure?
As Glen suggests in the MTFCA book, I measured, turned the ring 90º, and measured again. I went around three times, and each set of measurements was slightly different depending on where the indicator met the gear tooth. I got .008, .012, .011, .002; .006, .010, .0120, .005; and .009, .013, .011, .005.
As I understand it the difference within each set indicates run-out of the ring gear, but I'm not real clear on what to do about it. Do I want to put some brass shim stock under the part of the gear that has the most play?
Your clearances appear to be right on all the ring gears run out that is the way they are made. The ring and pinion assembly do not have any adjustments. When they were assembled up to .040 was acceptable the they were filled with grease and sent on their way. By to-days thinking and standards most people aim for a more refined clearance of around .005 at the closest point. If you are not happy with .010 - .015 you can shim the ring gear with paper shims to .005 at the closest point and don't worry about the rest of the ring.
A simplified method of determining an acceptable clearance is to use strips of paper .002-.003 and run the paper strips through the ring and pinion gears and note the amount of "cut". Number the strips 1,2,3,4 and mark the corresponding quadrants. the paper should have a firm crimp and not be cut. There is a high percent chance that there will be no adjustments required as that is the way they were designed, any adjusting can only make the gears closer and at your closest point you are .005-.006 that is where it should be. You will have no problems with that clearance.
Are they new gears? If so you are just fine. If they are used, be sure that they run smoothly and use some prussian blue to be sure that the mesh is good over most of the tooth. Worn gears will have high places worn into the teeth and they should be set so that all the pressure is not placed on one small point of the gear or you will break teeth. Usually the best setting for used gears will be approximately where they were before you disassembled it. This is especially necessary for the location of the pinion gear.
I don't think the paper test will tell you much of anything unless you fully assemble the unit to do it. There will be enough side deflection from the paper going between the teeth to give a false reading if you did it without the carrier being locked between the two housings. The bluing test might be the same, until the axle is assembled the ring gear has some float but I think between the bluing and dial gauge both will get you in the ball park.
Make darn sure the pinion tooth is in the center of a ring valley when you are measuring the clearances. Also try to insure the assembly remains perfectly centered in the axle housing when you take various measurements. Because of slop between the diff carrier and the axle gears and the axle and roller bearing you could be experiencing unwanted movement of the diff, hence the ring gear, as you turn the assembly. Also make sure that the indicator is stable and the probe hits the side of the tooth in the same place each time you measure. Your indicator appears to be too vertical. Try to get a more horizontal measure.It is common there is run out as you have observed. Yours is a little more that I'd like to see. The clearances that bother me are those that are closer than .008". See if you can stabilize things and get closer measurements. Strive for 12 to 15 thou clearance with little variation. My quietest RE was set at 17 thou with 3 thou variation.
Just as important, and possibly more so, is, how do the gears roll with one another? In other words, when you turn the driveshaft by hand, can you feel the gear teeth engaging, or is it smooth rotation with no "pulse" felt when teeth engage? You're looking for a clean, smooth mesh.
Steve, I always had trouble obtaining consistent reading with a plunger dial indicator like yours. I like using a finger type indicator like this one. I seem to be able to get better readings with it.
Run out can be caused by many things. A slightly bent lip on the Diff Carrier. Dirt under the ring gear and machine tolerances. Run out does not hurt anything as long as you have clearance, When you loose clearance that is when the gears start to make noise. As long as there is some clearance they should be quiet. However , just as important as ring and pinion clearance is diff Side play. There should be no more than 0.005 side play in the diff. Side play allows the ring gear to move sideways and directly affects ring and pinion clearance.
Very good. As long as I have .005" minimum I'm calling it good enough for now. I'm going to tighten and wire the ten bolts, check clearance again, and move on to the next part.
Steve... I like the fixture and know for a fact that you are compensating for torque tube sag. If the tube ass'y weighs lets say 50lbs, you should relieve the weight by 50% by the U joint. Its a crap shoot but don't let it just hang out its' own out in space. ws
Steve that variance could just be wobble. You have nothing supporting the upper axle.
My friend, Andy Loso, came up with a good idea several years ago and cut the tube off of a right differential housing and cut a hole in the casting near the pinion gear to gain access for the dial indicator. With his setup (that I had to copy), everything is stable because the axle is supported by a bearing and thrust washers, much like it would be In the car. So, you can get much more accurate readings and ultimately get a better mesh.
Steve has a plentiful supply of axle housings from the batch that he rescued from the scrapper over the winter:
Somebody posted a picture of the housing with the window cut into it a while back, but I can't find it, can someone post it again so that we can study it again? Thanks!
In a stock T rear axle the ring gear sits on a differential assembly that is supported by two large steel and one bronze washers. The assembly is not going to be displaced by a free floating axle. So why worry about it? In a Ruckstell Axle the differential assembly and gear are supported by a large ball bearing. True you might be able to displace the ring gear if you apply enough force to it but it just runs true on the bearing if you do not. Runout is always a bent shoulder on the differential or dirt or debrie under the ring gear or small manufacturing tolerance errors in the ring gear that are very small. The most important thing is that you have the pinion gear properly centered on the ring gear and that you give the gears about 0.010 clearance.
The ring and pinion gear clearance is controlled by the driveshaft front bushing automatically. Just read the Ford Service Book. There isn't a dial indicator anywhere.
I had a differential on my 27 that ran very quiet and smooth until one day I had a panic stop and the pinion completely stripped its teeth off because of excessive clearance. On examination the lead washers were non existent to a point there was almost no mesh. Up to that moment the diff ran fine with possibly 1/4" clearance so there is no problem with .005 - .020 clearances. Too close is problematic as Glen says dirt, machine burrs and bent parts are your biggest issues. I believe that if everything is clean with the appropriate thrust washers and nothing is bent the housing should assemble with out any issues as there are no adjustments except shimming the ring gear closer.
Runout is inherent with the manufacturing if the gears as they are to-day, they are cut dead accurate however the hardening and tempering process after distorts the gear some more than others. These assemblies were designed to accept the tolerances possibly up to .040. The appropriate thrust washers were installed and the unit was assembled and that was about it and with 1.5 pounds of grease it was sent out. To day I think there is too much fuss over clearance when there is not much that can be done about it. It is not possible to increase the clearance you can only make it closer by shimming.
Of course you can increase the clearance. Just machine a few thousands off the thrust side bronze washer.
I have a set of split washers that I machined to -.005, -.010, -.015 that I use to set clearance. It takes no time at all to do it that way. I don't like shimming the pinion out more than two or three gaskets. IMHO it is far better to take a little off the thrust side washer if the gears are running too tight.
My cars all have big drum Ruckstells. When I do one I leave the brake lining on the left (Ruckstell side) have some blocks I made, set the housing on them check for plumb then install the axles (making sure the shafts are straight) check for plumb. If so pick up the housing and axles as an assembly install a standard ford bearing (sleeve pre installed) drop the assembly back on the blocks, check for plumb if all is good then go for checking pinion/ring gear clearance.
I have 12 plus feet of ceiling height and set up a light winch for the grunt power , no bumping, banging or knocking things out of line works great.
Matter of fact long story short about a month ago, had to use heat on right housing in horizontal to straighten, when vertically in place needed a little more tweaking, being everything in position I tweaked the tube with heat two more times to point satisfied. Work great. The little winch being the key.
Steve, I was taught to set rearend gears per Glen's method, and have never had a problem, modern or antique. The important dimension is pinion to ring placement, it must be centered. If the lash, min - max, measured in 90 deg. increments is within tolerance, you should be fine.