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I am considering whether the ring & pinion in my 39 Six are correctly adjusted, but am not sure how to measure. The manual and every other piece of original literature I have found specify a backlash of "0.003-0.005" inches; but it is unclear to me where this measurement is made. I expect this is one of those obvious types of measurements that any worthy mechanic understood; therefore, clarification of the method was not needed in the literature. Is this measurement spec referencing perhaps a pin gauge somewhere in the gear tooth engagement?

Jselph wrote: " Many years ago, when I was "turning wrenches" for a living, I found that the most common use for a dial indicator (in the auto repair business) was for checking/setting the backlash for the rear end ring and pinion setup..." That would indicate to me perhaps backlash is measured as the difference between two relative measurements.

I have seen gear backlashed measured in degrees of free rotation of one gear member while the other is held stationery, but that is clearly not the metric here. The reason I am considering pulling the rear end apart is that I get an occasional high frequency vibration/groan at about 45 mph from the rear end and while poking around under the car, I noticed about a 10-20 degree free play of the pinion input shaft. However, I don't know what amount of free play in degrees is acceptable.

If this has been addressed in a thread I couldn't find, please point me in the right direction.
Pat

Just out of curiosity does the 39 service manual really use the term "backlash". Usualy it is refered to as GEAR lash.

Either way, gearlash is the clearence between the teeth of the pinion gear and the crown gear. Set up a dial indicator so that the plunger of the indicator is resting against one of the gear tooth faces. Usualy a crown gear tooth face. The gear tooth face is the surface of the tooth that mates with the pinion gear tooth. i.e. the surface of the tooth that would wear.

PackardV8: I expect you are correct in that it is gearlash, not backlash. That is what we call it in my engineering department.

THanks also for the info; I think I understand; would it be best to place the indicator at the center of the contact region?

There is a little more to it, but not much. There are a number of web sites dedicated to diff rebuilding and they have some very good information on particular diffs - I suspect none of them Packard. The 4WD guys do a lot of this sort of thing as you have to change gear ratios when fitting bigger tyres and they also commonly fit diff locking mechanisms.

One good site I have used quite a bit isgearinstalls.com/

If you sort through the numerous articles here you will probably find the more general information you are looking for.

Something else to keep in mind is that the 'backlash' you described when turning the prop shaft under the car can vary quite a bit. Try checking it as you have then roll the car forward and check again then repeat a few times until the wheels have done 1 full revolution. If you find any significant difference in the 'backlash' you feel it may be caused by a bent pinion or crown wheel not running true. The vibration you describe might also be caused by a bearing on the way out.

I have also been told that some of the Packard diffs are extremely fussy when it comes to the grade of diff oil used. I don't know any details on this, but heard it from a reliable source - though it may have been in relation only to the low pinion/offset diff I have in my '34 chassis.

Good luck with the car.