Transmission/Differential Free Play

[Hladun]

How much rotational play should there be at a rear wheel when the transmission is in gear and one wheel is locked (on the ground)? To do the test I jack up one wheel, put the transmission in gear, release the handbrake and then turn the free wheel. On my 78SC I’m getting about 1.5" at the edge of the tire.

[beepbeep]

Hard to tell, as you are also including freeplay in the tranny. With other words, differentiall could be brand new but you could be picking up gear-slack...

[Hladun]

Originally posted by beepbeep
Hard to tell, as you are also including freeplay in the tranny. With other words, differentiall could be brand new but you could be picking up gear-slack...

You're right, but that's exactly what I'm measuring...total play in the driveline.

[ChrisBennet]

Don't forget the play in 4 CV joints...

[Geary]

This amount sounds about right, but you can't really tell much from it.

[Hladun]

Originally posted by Geary
This amount sounds about right, but you can't really tell much from it.

It may be crude, but it's the only measure of total driveline wear. Have you got a better way of doing it?

[Geary]

It isn't going to tell you if your ring & pinion backlash is the appropriate .006-.008" or double or triple that. So in that sense, it is of little value in measuring the most critical play in the driveline.

[Hladun]

Originally posted by Geary
It isn't going to tell you if your ring & pinion backlash is the appropriate .006-.008" or double or triple that. So in that sense, it is of little value in measuring the most critical play in the driveline.

So how do you measure ring & pinion backlash easily?

[Geary]

There is no simple method when the transmission is already in the car. However, a transmission builder might be able to get *some* indication of something amiss in an LSD-equipped car by draining the oil, dropping an axle, grasping the bare output flange, and lightly twisting back and forth.

It would be fruitless for a layman to go through these motions, as there is a fine line between overcoming diff bearing preload, feeling a barely perceptible backlash (and knowing the difference in feel between .006" and .012"), and then overcoming pinion bearing preload.

[Dantilla]

Every car I've ever owned has had about that much movement when in gear.

[Hladun]

Originally posted by Dantilla
Every car I've ever owned has had about that much movement when in gear.

Thanks, that's the answer I was looking for.

[Geary]

Originally posted by Geary
This amount sounds about right, but you can't really tell much from it.