Differential Ring Pinion Backlash measurement

Posted by 39SixSedanMan On 2011/7/8 7:00:43

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