By the book, ring and pinion clearance is supposed to be .008-.012, ideally. However, the pinion is located by the bearing spool which is located by .395 bolts in .430 holes. So even if I get perfect clearance of .008-.012 during assembly, as soon as I hit my first pothole, the spool (and therefore the pinion) can/will move .430 -.395 = .035, completely wiping out my hard earned perfect clearance.
Am I missing something?
The spool has a lip that fits tightly in the rear axle housings so therefore it cannot move.
Ahhh, that was the detail I was missing. Thanks, Stephen.
Something else to think about. The parts in your axle are not perfectly parallel, and not precisely machined. If you measure and set it to .008 - .012 during assembly it will quickly wear to a much larger dimension because there is no way you can measure clearance all the way around.
Therefore it is better to fit to .000 - .005" to start with. After the first trip around the block if you were to disassemble and measure you will find it is now .012" after the parts mate to each other.
Makes sense, Royce. I measured a .045 clearance, so I should be able to shim with three .010 and one .015 to make it fit dead on. Or two twenties for a .005 clearance. Thanks for the advice.
You should do your moving with the thrust washers if possible. Did you setup or check the drag on the ring gear/differential installed in the housings first? If you have, check the thickness of the thrust washers, maybe you can swap them around and move the gear closer to center.
I like to set the drag on the ring gear to a snug just able to turn fit without the gasket. The gasket adds about .005 so you get a nice fit when it's installed.
If you are using new gears, you should try to get the clearance as close to spec as possible, however if the gears are used, you need to use some Prussian Blue on them and set up for the greatest contact pattern. That will usually be greater than the original clearance because the gears have worn in and mesh together.
Quietest rear ends are set up with .015" lash
No ring gears are perfect they are all inherently out of true even the gears today are. They have to be checked at four different locations and set to the closest part of the gear. When the assembly is bolted up tight there should be no binding at all four quadrants. There is no gasket the thin paper components are shims. The more you put in the worse it can get. In some cases to get the required\desired clearance a shim may be required behind the crown gear to move it in closer to the pinion. At the factory up to .040 was deemed in spec. Cars today may be .002 - .003 but a Ford diff was never intended to be that close. The differentials were filled with 1.5 pounds of #2 grease and .030 - .040 was ideal for that type of lube. Most people today use oils of different weights and the clearance is relative. Many people have their own beliefs as to the appropriate clearance based on the type of lubricant used. It could vary from .005 - .040.
There are as many theories as there are T mechanics but here is what I do. First of all, you need to have the differential housing standing upright. You can never get a true reading otherwise. I check the bronze thrust washers for thickness and being in plane because a lot of them are off as much as .010. I true them up, take the thickest one and start with it. Assemble the center differential carrier, put the thrust washer on and drop it in, make sure I have a tight roller bearing in the end and put the pinion/drive shaft setup in and bolt it tight. If there is no clearance I change the thrust washer to a slightly thinner one. (I actually have a set of split washers I made years ago that are easy to change) Nominal is .200 I have .195, .190, etc. If there is a little clearance I run a couple feet of thin solder through the gears. It is easy to see tight teeth, what the clearance is, etc. Too tight? Thin that washer down. Too loose? Thicker washer. I like .015-.020 clearance, especially with a Ruckstell because the ball bearing will never change clearance. When I am happy with the clearance, I put the other housing on with a thrust washer I think will be the right thickness, reach in with a long feeler gauge and measure the clearance. I like not more than .005. When I get that I goop up the parting line with Copper gasket maker, let it set for a few and bolt it together. No centerline gasket. Just goop. Never had one leak, it's quick and easy.
If you put your rear axle together like Ford says to, you don't need to check it. It will be ok if done correctly.
I'll bet in the factory they could do these blind folded and I would also bet there was no testing, they were assembled with assured clearance(up to .040) and out the door. Their life expectancy was about 5 years.Today we try to make them last for ever.
IMHO the reason they used babbitt thrust washers at the factory is to allow for the tolerances needed to assemble them. When you think of the number of machines cutting the inside of the housing for the boss the thrust washers run on there had to have been some variations. Babbitt thrust washers in the rear end would allow them to just tighten the bolts and get it out the door. With 100 years of wear on them those housings most certainly have a lot of variation now.
I prefer to do them with care and gauges rather than just slap them together and put it on the street. But then, I've only rebuild 70+ Ruckstells in the last 30 or so years and so probably have been doing it wrong all this time and just didn't know it.
The older I get the more I find that out.
After studying the Ford rear axle parts drawings, and noting the tolerances the engineers designed into the machined surfaces, I have reached the same conclusion you have. Thrust washers made out of softer materials were used because the would squish to provide the proper fit. The folks working in Ford's rear axle assembly department did not have the time (and possibly not the skill) needed to custom fit rear axle parts, and to set clearances to the extent that we do today.
One other point: in Accession 94 The Walter Fishliegh Files at the Benson Ford Research Center, there are a collection of drivers test reports on new Model Ts as they came off the line at Highland Park in late 1926 to early 1927. In reading through them, the most common complaint I noticed was that the rear axle on these brand new cars was "noisy".
In some instances, our standards today are higher than the original factory standards as to what was acceptable.