The Stovebolt Site Forums The Shop Area Driveline Driveline vibration

So after taking apart the torque tube on my 51 3100, to try and address a leak at the slide on the torque tube, the speedometer not working ,a leak on the bottom of the rear bearing retainer and a leaking PTO cover the truck has developed a vibration after I put it all back together.
It only seems to be under load at higher rpms in fourth gear.
Prior to taking it apart it ran and drove perfectly fine.
I took it back apart and everything looks to be in good condition.
When I took it apart the first time I did see the trunnion was in backward. I'm basing that on the slots in the trunnion were facing to the rear of the truck in stead of the front.
Am I correct that the slots should be to the front to better pick up lubrication from the transmission?
The only part that I was not sure of after taking it apart the first was the position of the rear trunnion bearing and bushing.
I disassembled the trunnion from the front yoke and reinstalled it correctly facing the slots towards the front.
My question is which picture with trunnion bushings installed
Is correct picture #1 or picture #2.
When I reassembled it I did it as in picture #1.
Seemed to move around alot and only stay center by the torque tube ball.
Thanks for any help.

I believe your #1 photo is the correct position for the bushings. It will allow the cross to move around, but the two yokes should be positioned by the bushings in the transmission and torque tube. If they were installed in the #2 position, there's a possibility of the cross hitting the ends of the bushings (the ends with the taper.)

Generally when a u-joint is disassembled, it should be put back together in the same position it was originally. In other words, the same trunnion bushing on the same yoke ear and on the same end of the cross. But if it was previously assembled incorrectly then all bets are off.

I'm not understanding when you mention slots on the trunnions. If you mean the faces where the bolts go thru, the trunnion bushings should fit into the recesses of the front yoke. If they were backward, there would be a gap between the bushing and the cutout in the front yoke. I believe the mating surface should be lined up with the center of the bushings.

KL ,
I'm pointing to the slots I'm referring to with a screw driver in the photo.
I should have taken a picture when I was taking it apart.
I believe after closer examination that the trunnion bushing caps are worn fairly severely.
Since I hadn't had a vibration before I took it apart I figured that the trunnion was ok.
So I ordered a new trunnion today. Hopefully that will eliminate the vibration.
I'm also waiting on new speedometer drive and driven gears that GDads helped me track down.
He found them both NOS on ebay.

Oh! those slots. I don't think it would really matter whether they're toward the front or back. It's probably like a washing machine inside the u-joint ball when going down the road. Oil is going to get everywhere.

If you turned the cross (what you're calling a trunnion) around, then you moved the worn surface to the opposite side. That might have initiated the vibration.

Since you have it apart, a u-joint rebuild kit is a good idea.

I will let you know how I make out. Probably be a week or so before all the parts arrive.

Yesterday all the parts arrived to reassemble my torque tube and get my speedometer operational.
I ordered the new u- joint from Classic Truck parts. The speedometer drive gear and the driven gear on the output shaft of the transmission were located on Ebay
with the help of Gdads51(Dan) and John Milliman.
Dan was able to locate the part number for the gears and located them on Ebay. The drive gear came in a NOS box. It was obviously new. And it had the correct part number on the box. The driven gear that connects to the cable however was not NOS. Even though it was advertised as such with four other Similar gears. I contacted the seller and he claims that it was a mistake and has sent out the gear pictured with the NOS tag attached. The gear was tight and had a piece of broken off cable stuck in the end of it. I was able to get the piece of cable out. And after some GM Heat Valve Lubricant. Which I had for 30 yrs was applied to the gear and housing cleaned out the shaftand now spins freely. Not sure you can still buy the Heat valve lube but back in the day the stuff was awesome.
So before I reassembled the speedometer drive I compared what I had taken out to what was supposed to be in it. After seeing what was correct and what was installed it was obvious that things had been miss matched somewhere downvthe line. Not sure what my driven gear is from but it is pitched the opposite direction from the gear I removed and the drive gear that I removed was 6 tooth as opposed to the 4 tooth one that it calls for. Also I discovered the part numbers for the speedometer gears are listed in the rear axle section pg 4-48 of the Chevrolet Truck Shop Manual.
So after it was all said and done. The speedometer worked great. And no more vibration.

So glad to hear that you were able to resolve the issues with your speedo. Sad to hear you had an issue with the driven gear - booo, but that you were able to get past them - Yay!!! I just checked and it seems that seller has sold, or maybe pulled from the sale, all the remaining driven gears. Thanks for adding the note about the speedo gear info in the Shop Manual. I missed that when researching before. It will make it much easier to find for the next time this question pops up.

Just received the second one from that vendor in the mail today. This one is perfect. And also obviously NOS. Gear shows no wear. Cable drive clean. Sent the one pictured this time.

Nice job Jeff.
I’m trying to find the right combination I need to get the same parts for my SM 420 4 speed.