Reverse snapping

[Jfmarkham]

My 1923 has started to occasionally make a snapping sound when its under a heavy load,it does it when backing up a incline or on soft ground. The noise is coming from transmission not rear end,any ideas where to start? Both forward gears work fine

[Daisy Mae]

One possibility, rear end problems can 'transmit' the sound to the tranny.

Mine had the same issue, banging in reverse, but not forward, (backing out of my shop rolls into sand), sounding like it was thumping under my feet.

In my case the driveshaft roller bearing (among other things such as thrust washers) was shot causing the pinion to pop out of mesh with the ring in reverse. It acted fine going forward.

[Scott_Conger]

Kurt is right

nearly certain it's either pinion roller bearing failure or differential thrust washer failure

stop driving the car NOW. The ring/pinion are undoubtedly being damaged and the longer it's driven the worse the damage will get (including loss of brakes and further related damage).

[TXGOAT2]

You might want to block up the rear end and run the car in high at about 15 MPH for ten minutes or so, then shut it off and pull a sample of the rear axle lube. A suction gun works well for this. You will probably find excess metal debris in the lube. Rotating the wheels by hand, and running it in low and reverse while the vehicle is warmed up and on the blocks may reveal more evidence of what may be wrong. Whatever it is, it won't fix itself.

[Mike Silbert]

If you stick your finger in the rear axle hole it should come out smooth and slippery and not silvery and chunky.
Silver and chunky rear axle lube says - thrust washer failure.

These tests are easier and clearer when someone else is helping and listening and feeling for the issue while one is turning and wiggling parts.
To check the drivetrain, chock the front wheels, put the transmission in high and release the e-brake.
Then put the rear axle on jack stands and start checking things.
Grab the tires and lift (or pry) up and down, front to back, and in and out. It should have have no slop or sounds on these tests.
Rotate the tires each direction and there should be little to no slop and certainly no noises.
It will turn some before the the tire starts moving on the other side, more than a modern car does, but not really far.
If the slop is a lot then that is starting to point to where the issue is.
What is happening is it is rotating until the slop is taken up all the way to the crankshaft and then the tire on the opposite side will start to turn.

Next turn the starting crank and see how the drivetrain feels.
If you remove the spark plugs the engine becomes easier and smoother to rotate.
It should rotate nice and smooth without a lot of slop or strange noises.
To go further, shift the transmission through the gears and see what happens and how it feels and sounds.

I have seen all kinds of issues that could possibly be close to what you describe ranging from transmission cracks, bushing issues, loose rivets, u-joint issues, pinion issues, thrust bushing issues and more.
The trick is to narrow it down to an area and and possible cause.
Mike