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Discussion Starter #1
There is a clunk in the rear end of my '67 Mustang, most noticeable when power is applied to the driveshaft, or when power is suddenly reduced. As far as I can tell the u-joints are ok.

When I block the wheels and place the trans in neutral, there is some play at the differential. Can this be the source of the clunk?

I marked the differential, and then turned the driveshaft to show the play in it. Does this appear to be a normal amount of play? (Does this indicate the amount of backlash on the ring and pinion?)
Thanks, jp

Lined up photo:
759774


After turning the driveshaft by hand:
759775
 

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That U joint look like crap...I would start by replacing them both if you don’t know how long they have been in there. Consider it cheap insurance. If you have a shop do it, then they can inspect the rear end at the same time.
 

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Also check the pinion for in and out play. I had one that the nut had backed out and that caused a clunk when power was applied or reduced.
 

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Since worn u-joints wouldn't have any effect on the rotation of the pinion gear, I'd say it's free-play/wear (or a loose pinion nut). Your measure/marking is a little more than normal.
 
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