Need help understanding end play on rear axles

[32sbct]

I am just about to finish up my M38 rear axle service. I have read up on how to measure and adjust end play on the rear axles but I really don't understand it. First, I don't have a dial indicator or access to one and I hate buying a tool I won't be using very often. TM9-8012 states that end play should be just perceptible. I don't feel any at all. I was careful to keep the existing shims together for each side when I took everything apart and put them back the same way. So my questions are?

What exactly is end play and why is it important?

How does adding shims increase end play? I don't see what changes by adding or subtracting shims between the axle housing flange and the bearing retainer.

What problems would I have with zero endplay since I can't feel anything move when I pull on the hub/drum?

I really want to understand this. Thanks for helping to educate me.

[RimfireJim]

End play is the difference in length between the rotating parts (axles, bearing cones) and the fixed parts (axle housing, bearing cups). Zero or negative end play would mean that the rotating parts would be squeezed between the two bearing retainers; positive end play means there is a little clearance, or "slop", side-to-side. The two axles ends touch each other where they meet in the differential carrier, so the carrier itself doesn't come into the equation. This also means you can check end play only with both axles installed, not each side separately.

Adding shims is the equivalent of making the axle housing longer, increasing the difference in length between rotating and fixed parts. From an end play standpoint, it doesn't make any difference which side the shims are on.

Here's a crude analogy: the axles are a door, and the axle housing is a door frame. End play is the clearance around the door that keeps it from binding in the frame. Adding shims makes the opening in the frame bigger.

I couldn't go by feel, either, so I used my dial indicator. There was enough "sticktion" in the system that I had to tap lightly on the axle ends to get them to shift to make the measurement. Do you have a dial (or digital) calipers? Feeler gauges? I think you could rig up something to allow you to use either of those. What you would do is tap the axles to, say, the left, establish a reference of the left axle end, then tap them to the right and measure how far they moved. If you already have drums on, you might be able to make the measurement between the edge of the drum and the backing plate.
_________________
Jim M.
1952 M38 son-father project
Discovering more worn out parts, one assembly at a time

[32sbct]

Jim, thanks for for reply and a great explanation! I do have digital calipers, I think I will be able to figure this out now. I did not realize you need to have both sides completed to check the end play. Thanks again.