The Stovebolt Site Forums The Shop Area Driveline Chasing a vibration in my ‘51 3100

I’ve been chasing a vibration down for a while now. A bit of background. I have a 1951 3100 with a 1954 235 and a sm420 transmission. I put a new clutch disc and pressure plate on. It also has a 1957 chevy truck open rear end that I changed from 390 gears to 338 gears as well as rebuilt with all new seals and bearings. I had a new driveshaft made and balanced. The symptoms I have are as follows:
1. When starting in 1st gear ( not granny) it seems fine. Shifting to 2nd is ok until I pick up speed and then just a slight vibration. Shifting to 3d is a little vibration until it gains some speed and it vibrates pretty strongly.
2. I live in a house that is up a road 2.5 miles from town and is higher in elevation. When I go to town ( downhill) the vibration is not as bad. Coming back uphill when the engine is under load, the vibration is more noticeable, especially in 3d.
3. Coming uphill with curves in the road, I notice the vibration is less when I’m taking a left curve, more when I take a right hand curve.
4. When I put the clutch in, the vibration stops. When I rev the engine with the clutch in, it seems smooth and no vibration.

I am looking for guidance to solve this issue. I know the collective knowledge here at Stovebolt and would appreciate any and all help. What am I missing?

Have you checked the transmission mount?

The symptoms you're describing are consistent with a vertical misalignment between the transmission output shaft and the pinion flange on the rear end. It might be a good idea to check both with an adjustable bubble level.
Jerry

The first thing I would check is the universal joints in the driveline. They can be checked pretty easily with a screwdriver to check for movement between the cross and yokes. If you find any, change out the u-joints. Especially if it was previously smooth and vibration free.

If you can this, open the hood so you can see the engine while setting in the truck.

Start it up and put in second gear,

While holding the brakes, let out the clutch and see if the engine torques overs, engine may die and that's OK.

Then try it in reverse,

Two quick test of the motor mounts, it should torque over some, but not much.

It does appear to be a " under load " vibration. Driveline angles, u-joints, motor mounts could all be suspects. Does it vibrate as bad on level ground ? Is it the speed or the torque being applied?

The left and right turns don't make much sense, but not driving it myself I don't know.

Put the rear up on sturdy jack stands and slowly run it up to vibration speed. Then have a helper do the same while you watch under the truck from a safe distant ( NOT UNDER IT ! ).

I replaced the clutch in my buddies 68 C-10 6cyl 3 spd and had a bad vibration and ended up having to have the clutch and flywheel assembly balanced. It took if I remember right was around 43 grams off the flywheel to smooth it out. Funny thing was it didn't vibrate with the old clutch assembly.

So you're saying if going 40 mph and you disengage clutch and rev the engine, the vibration is not there? If yes, that to me that points to a mis-aligned clutch disk; or possibly a mis-aligned transmission to bellhousing and/or engine connection. As FAST55 said, could also be a out of balance clutch/flywheel; but I'd say the mis-aligned disk would be the first thing to check. If you can safely put the rear axle on jack stands; do that and slowly engage the cultch and see if the vibration returns as the clutch engages. I doubt it's driveline/rear end related; as these components are still turning when coasting with clutch in and no vibration. Is the SM420 front main shaft bearing in good condition; if sloppy that could cause a vibration under power. Worn pilot bearing a possibility as well. Hope this helps.

Worn u-joints will do the same thing. Vibrate under load and not when unloaded (clutch disengaged.) It's easy to check u-joints, which is why I suggested starting there.

Thanks for all the suggestions. Since it’s COLD and windy here and an unheated barn is it’s resting place ,I plan to slowly go through the list sometime later when it doesn’t feel like a meat locker!:). Those of you with heated spacious work spaces have my envy. Any other possibilities, let me know. Always willing to learn.

Usually bad u-joints show more in lower gears, but good call to check these first as its a lot quicker to check these than pull the tranny first thing.

Just a thought-how deep should the pilot bushing be seated? I replaced one and measured the depth of the crank hole and the transmission pilot and tried to match them. The result was that the pilot bushing was flush with the face of the crank hole. Should it have be recessed slightly?

Snowman - The Shop Manual shows the bushing installed not quite all the way bottomed out in the crank recess. If yours is situated like the one in the shop manual, it should be right where it needs to be.

That depth maybe so you get a puller behind it and get it back out.

So update- was driving down to town and noticed the vibration was getting WAY worse so I stopped and turned around and headed back. As I pulled into the driveway, I heard a loud bang and then the familiar clanging of a dropped driveshaft. As I looked under the truck I saw the rear u joint had come free. The nuts had come off of the u bolt. The u joint seems ok but I didn’t want to take any chances. I replaced the u joint with new nuts and washers ( I checked the threads on the u bolts and they were in good shape). A quick test drive later, I realized that the increasing vibration was caused by the loose nuts/ possible u joint binding? It seemed to cure 96% of the vibration. When I looked at the u joints, I didn’t look closely at the rear nuts. When I wiggled them, they seemed tight. Oh well, another lesson learned. On to find the last small vibration (s).

See, I told you.

Glad you got it fixed up.

I think this has been a very good thread from start to finish and it would be great to hear the rest of the story when you identify the remaining issue. Finding loose nuts on the ujoints brings to mind a practice that I find helpful on critical components is to "mark" each fastener after tightening / torgueing with a paint marker. I use green or yellow. It helps me to make sure that I've did each fastener and because I make a "line" across the head or nut to the attached surface I can see if it's "moved" or came loose later.