The Stovebolt Site Forums The Shop Area Driveline Clunk when letting off the gas

In my 51 GMC I have a '59 chevy truck 6 cylinder drive line. I have a pretty loud clunk when I let off the gas. Checked the u-joints and tehy are tight. carrier bearing is tight as well. If anyone has any suggestions, what direction should I head to in next?

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What is your:
Truck model
Engine
Transmission
Rear end
Drive shaft

The drive line is a '59 Chevy 235
'59 4 speed granny low
'59 chevy rear , don't know the ratio
2 part drive shaft with center carrier bearing

The drive line is a '59 Chevy 235
'59 4 speed granny low
'59 chevy rear , don't know the ratio
2 part drive shaft with center carrier bearing
also it's a half ton GMC long bed

** I would first look at the rear engine mounts & go from there. **

It is very common that you can't tell a bad Ujoint by hand. Often times you can't tell with all four wheel off the ground and motor running, etc because there is no "load". Most of the time you can hear the general area of the noise. If you don't find anything wrong, it's the Ujoints. Maybe the carrier bearing too. If you replace them, make sure you know what "phasing" means.

Thanks for the hints I will update as soon as I have investigated more

Here's what I do when I have an issue similar to yours.
First, once you determine that something isn't about to fall off, just drive it.
The problem will likely get worse over time. Once this happens, it should be easier to diagnose. Whatever is making the clunk will become more loose, and show itself.
Carl

Jack the rear of the truck up and place jack stands under each side. Sit on the floor beside the wheel and rotate it back and forth as hard as you can. Make sure the trans is in neutral. Have someone watch under the truck and listen for the noise or see where the slack is. Primitive to say the least but you might just get lucky.🛠

It probably has a lot of slack in the rear end spider gears. With the trans. in neutral put on the emer. brake & see how much the drive shaft turns back & forth. Its a good idea to pull the cover off the rear end & check it over & replace the gear lube.

George

Another primitive way is to get on a level surface, car in some high gear, rock it back and forth on the level surface while looking under and listening for noise, a helpful friend that can be conned in to do the rocking makes the job easier.

Ed

I had that on my 68 after having it worked on at a local garage. I had put some shims on one side of each U bolt that holds the back axle on to correct the pinion angle. The mechanic saw them and didn't know what they were for so he took them out. He neglected to tell me about it and he also neglected to tighten the U bolts back, leaving slack that allowed the axle to rotate a little under torque. Got the clunk every time I shifted. Once I figured it out it was an easy fix.