Working on my Ford truck’s rear brakes the other day, got to wondering if there’s an easy way to measure the differential play caused by the ring gear not perfectly engaging with the pinion. It seems like you could assess this readily by putting transmission in neutral, jacking one rear wheel but leaving other on the ground, and measuring the distance you could turn the jacked-up rear wheel. Would that work, or are there complications?
You remove the center section and measure the back-lash. 
Tester
Yes, I know how to do it that way. But that way isn’t a very fun & easy way.
Sorry George.
I thought you were looking for the correct way.
My mistake.
Tester
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But it is the right way…
Trying to measure backlash from the axles will add in all the slop of the axles
Let sleeping dogs lie.
Trying to use just the wheel rotation will/might only tell you maybe if something is really, really, really wrong. I leave differential work to the pros.
What if you jack up both rear wheels, and you have two people do it. One turns the wheel one way, and the other person goes to the other side to turn the wheel the other way. This eliminates the play in the other gears. Now turn a bit harder to overcome the force from the other person until you feel the pinion gear engage or you see the drive shaft turn. Then have your assistant turn his wheel harder to move it the other way and do the same thing.
But this also measures the play between the planetary gears and the carrier.
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Fair enough, but since it is very easy to do, why not make use of that info? If it is increasing over miles driven, you’d have a heads-up you have some wear happening. MM, you probably have some cars with Ford 9 inchers. Next time you have one rear wheel jacked up, how much movement do you measure? In degrees of movement? I’m doing a rear brake inspection on my truck in the next few weeks, so I’ll make the same measurement, and we’ll compare.
For perfectionists who want to measure the differential backlash the correct way, I have no objection. Suggest to wear old clothes … lol …
No nine inchers in my house, just an 8.8. I take the cover off and measure it directly.
The splines will wear as well so there is no way to separate that from backlash. Besides, the bearings will wear out long before the ring and pinion will.
The only way that will give useful information. In my experience the slop in all the other parts of the rear axle will swamp anything from the ring and pinion.
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I don’t think it is possible to do it that easily on a 9 incher. IIRC the entire 3rd member has to be removed. A race-car hobby friend of mine did that measurement for me when I had to remove third member to replace the limited slip clutches about 20 years ago. He told me he brings several 3rd members with him to the track and installs the one that is configured best for that day’s track conditions.
Doing it that way you are feeling the spider gear play as well as the ring and pinion play… You are better off leaving the tires on the ground with the trans in neutral and grabbing the pinion (at the u-joint) and see if there is any up/down/side movement/play in it, that will let you feel your outer pinion bearing for play, also check your u-joint while there… Now rotating the pinion CW/CCW easily until you feel it grab the teeth will let you know if there is a lot of slop in it or not… HOWEVER, if you don’t know drivelines very well and have an experienced feel for what you are feeling for, then this does you NO good!!!
As already stated, the ONLY way to check it the proper way is to check it the proper way…
Look here Differential (mechanical device) - Wikipedia
Scroll down to Ring-and-pinion design. There is a black and white diagram with numbers. The names for the numbers are below. Is the wikipedia page incorrect?
Every mechanic I know calls them side gears.
https://www.google.com/search?client=firefox-b-1-d&q=differential+side+gears
Tester
#1 is the axle
#2 is the ring gear
#3 is the differential side gear (big spider gear in set)
#4 is the spider gears (small gear in set)
#5 is the carrier
#6 is the pinion
not shown are the carrier bearings and pinion bearings nor any of the many shims/s[acers… in this picture that is called an open differential… spider pin is missing also and depending on how the axle is held in, the C clips are missing…
What makes a planet gear set is the planet gears housed in a cage of some sort that rides between the sun gear and the ring gear, the sun gear is the center gear
I really don’t care if the internet is right or wrong, call Summit, Jegs, Dr Diff, any USA based company that builds Diffs and ask them for the planetary gear set for a Ford 9", Dana 60, Mopar 8.75, Chevy 12 bolt yada yada etc etc and they will ask you WTF are you talking about…
It depends on what you are interested in measuring. If you want to know the amount of play from the pinion to the rear wheels, measuring the pinion/ring gear backlash only gives you one part.
That’s the question you asked.
Now you’re going to change the subject because you don’t like the answers you got?
Nice!
Tester