Clunk when stopping and starting. Transmission?

[Ozark Tim]

My 72 F-100 has had this problem for some time and it hasnt gotten worse but it is anoying. I am hoping one of you all have experianced something like this to point me in the right direction. The truck is a 72 F-100 with 360 & C6.
When you come from road speeds to a stop at a light etc. if after you have come to a complete stop you momentarily let off the brake enough for the truck to move forward a bit it clunks like it never shifted all the way down to first when stopping.
When you start from a stop it clunks also and if you baby it you can keep it from doing so all together. You can also baby it for up to midway into 1st gear and if you accelerate harder at that point still feel the clunk.
All motor and transmission mounts are good, and all three u-joints and carrier bearing have been replaced and it did it before and after.
Personally I think it does some odd delayed downshift, but I have never owned a C6 in this vintage truck to have experianced this problem.
Any suggestions?
Thanks, OT

[68F250]

I've been trying to find my "clunk" too but I can tell you it's not the transmission. It happens when the driveline goes from loaded to unloaded and vice-versa. Not the U-joints or differential, it's either the carrier bearing rubber support, the splines themselves or maybe the motor or tranny mounts because I can feel it in the gas pedal when it happens. Kind of a cross between a clunk and a shutter.

Hey when you find it let me know!

[390Nut]

Check your drivers side motor mount. Even if it's not broken, it may be just a touch loose, or broken internally. I chased a "clunk" for several years on a car once, and could never figure it out till after I sold it. Seems the mount was loose internally (the bolt that went into the frame was loose inside the mount itself).

[FORDification]

Are you sure it's not the rearend? If there's excessive backlash on the ring and pinion gears (or they're just worn) you'll get that clunk.

[72hiboy4x4]

Just what I was thinking!

[68F250]

Yes, I agree but this is a different kind of clunk, it sounds and feels different than a rearend clunk and seems to originate from somewhere under the cab or nearby. More of a thump than a clunk, I gotta look at it some more. Not that it's been bothering me , just adds personality.

[DuckRyder]

Try greasing the splines on the driveshaft slip joint. There is now a special grease for this.

On the off brand stuff we used to have to cut a small piece of hose and put it in the slip joint.

[fordman]

when my c6 is low on fuild it will do that clunk in and out of gear.

[Ozark Tim]

I have been watching to see if any response to my inquiry sounded like a solution, but I have to say I am not sure. It is not back lash because I can take off gently which means I have power side gear contact and at 10 mph or so accelerate mderately and it does it then just like it I accelerate normally from a stop. It is almost like the suspension is bound and the acceleration vs. deceleration causes something to jump back in place. I looked at it this weekens, but I dont see any tell tale signs of a leaf spring or shakle problem. Also I just had the whole driveshaft rebuilt including lubrication of the slip yoke by a drive shaft shop. I suppose I could try loading it up with sone type of grease to see iif it is binding, but I have to tell you that it acts identical with all new driveshaft rebuild as it did before. You can brake torque the truck in forward or reverse and there is no sign of motor mount trouble. Again I wouldnt say that isnt it because I just dont know. This F-100 does have the rear spring system with the cantilever rear shackle as a overload device. I cant remember what it is called, but I looked close at them and I cant see any troubles. Oh yea. Flexomatic I think. I will post my eventual find and until then any other suggestions are appreciated.
OT

[steveheer]

The only reason why I am going to say check your motor mounts AGAIN is that I just had the same thing happen to me last week. I have newer mounts but the passenger side worked loose and started clanking around, (Not a lot of engine twist either, just enough to make noise). Also if I babied it on acceleration or decel no "Pop" .

Good luck!

[FORDification]

Maybe check the transmission bellhousing bolts and the torque converter bolts? Though unlikely, it's remotely possible that loose bellhousing bolts is causing the tranny to 'spin' on the block...or that ovaled-out holes on the flexplate causes the torque converter to move around.

[Hotwhlz85]

I've asked this question recently at another forum and the answer I got makes sense. Duckryder was pretty much on target with his suggestion too. What I was told is that this is the famous "Ford clunk" (google it, you'll get plenty of links to look at). What happens is as our trucks get older and the rear springs lose some of their temper the rear axle actually rotates forward (loads) when coming to a stop. This pushes the driveshaft forward and the slip yoke up into the rear of an auto tranny. Taking off causes the reverse to happen. The rear axle rotates back (unloads) and pulls the slip yoke back out to it's normal position. This sliding back and forth puts sharp edges on the splines where they meet and causes it to grab hold in both directions until the force overcomes the grip and it slips (clunks). This makes sense to me because I never had a problem until I changed from a 4 spd tranny to a C6.

I've heard of two remedies; the first is greasing the splines on the yoke as mentioned before. There is a special grease just for this I'm told. The other, more permanent fix is to add a helper leaf to your rear springs. Something stiff enough to stop the axle from rotating forward.

As of yet I haven't tried it, waiting for better weather.

[Ozark Tim]

O.K. Here is the answer to this dilema, and the reason that you have to go behind everyones work to make sure it is correctly done.
It was the slip yoke. Apparently when drive shafts unlimited installed all three new u-joints and carrier bearing they left the slip yoke dry and didnt properly lube it. I even ask the manager about it when I picked the truck up and he said it looked fine etc. So last night as an attemt to conquer the dilema I bought some of that good red miledium ( ? ) grease and filled it as well as unscrewed the ring at the front and coated the spline there as well. That was it, and the clunk is gone.
Thanks to all for your suggestions, and this is one of those cases where it shouldnt have been because it was supposed to be taken care of, but as I say you really have to double check behind most shops work and I should have known that.
OT

[68F250]

Lubed up the splines, clunk is gone! I knew it was in the driveline somewhere, thanks guys!

I recall reading a Ford service bulletin about this clunk. Their remedy was to lap the splines, will try to find it tonight and scan it for ya.

[68F250]

here it is