The Stovebolt Site Forums The Shop Area Engine Shop Driveline vibration above 65

Hi folks.
My 52 3/4 ton GMC pick up will run down the highway at up to about 3500 rpm (as much as I dare). It's got a 235 straight 6 made a few years AFTER this truck was built, but it does well.

My question is concerning a bad vibration that really prevents me from wanting to go faster than 65, even though physically I can. I just avoid it because of the vibration.

I started out thinking the 235 may not be a very well balanced type of motor, but that theory went out the window when I stepped on the clutch at 70 mph and watched the rpms drop to 1000. The vibration is definitely speed-related and not engine-related. This caused me even more concern.

It almost felt like it could be coming from the front wheel bearings. What are the thoughts from some of you more experienced Stovebolt owners? What would be a good way of isolating and fixing the vibration...or is beyond 65 mph the design limit before these things shake themselves apart?
Thanks...

How old are the tires? Have they sat in the same spot for a long time? Bias ply tires are known for flat spots after sitting too long. Does the vibration come up through the steering wheel or can you feel it more in the seat and body? Wheel bearings don't usually cause vibration but tires and drivetrain are the usuals that do.
Scott

Yeah I would check the tires,and the u-joints,and the pinion to make sure the nut hasn't worked loose.
I'm betting the tires have a flat spot,or a bent belt,or just need balanced.

Hmmmmmmm--

Little jimmi 228 replaced with larger displacement Chebby 235 and the whole truck starts vibratin at speed. Prolly a lesson in this for GMC truck owners. Need to be careful when replacin those jimmi motors with the more popular chevy engine--tear up ur runnin gear with the more capable higher speed motor.

I'd check that front end--tie rod ends and king pins--might want to look at those motor mounts too--drive shaft and pinon gearing--beef up and fix whatever you need to--gud motor you got there--good idea to replace those little 228/248 engines with a stout little 235. :p

Did you do the king pins? Bad king pins with flat spotted tires and tie rod ends that are sort of bad all add up to a terrible ride over 50...it does on mine.

But start easy....check the tires. Santa told me she had tires in her sleigh for me this year....and two more rims too....that Santa...she's quite a gal.....

Tires are a year old and are modern steel belted radials. 265/75R-16's I believe. They are mounted on 2002 GMC 8-lug chrome wheels with wheel spacers. So it isn't the tires.

Truth is though, the ancient "schoolbus" tires this thing had before were a lot worse. They vibrated above 50 mph and didn't corner worth a darn.

So there is an improvement. Maybe it's my Kingpins then???

HRD,

You might wanna think twice about running that 235 at 3500 sustained rpms. You're reaching up close to the sustained RPM limit for that engine in stock form.

I'd be scared to death to run my 292 at 3500 rpms for more than a minute or two. Literally!

Keep the revs down and enjoy a long engine life.

-ftyler

Didn't mention the driveline in your post. If it is the old torque tube set up it might be a worn bushing in the tube. Prefer it be something else frankly.

Jack the rear up and run it up to vibrating speed...see if it will do it sitting still. that should tell you if it is the driveline.

After 50 years, it's entirely possible that something could be bent - like the driveshaft or an axle. I'd check the wear parts first, then the hard parts if the problem is still there. My old daily driver Talon had a driveline shimmy that I couldn't get rid of for anything, so I feel your pain.

BTW, 3500 RPM sustained sounds like a lot for a 235. There's a reason why the old '50s drag racers dropped GMC engines into Chevy cars...

If it isn't the tires, then I'd definitely check out the joints on the driveshaft.

I drove a '84 oldsmobile, rwd one time... between 55-60mph or so it would start shaking like the devil... then it'd be fine... my pop crawled under there and the joint on the driveshaft towards the rear going into the rear axle was all beat up - it was just slap worn out. Replaced it, went back to driving it, no more problems.

It is something to check anyway,

-W

My '55 1st has a serious shake between 50 and 55. Its nice and smooth up to 50 and nice as smooth past 55.

New radials on new wheels less than 10,000 miles ago. New shackles and pins this past weekend. With the exception of the drag link, all the steering seems tight. The kingpins are good and tight. Wheels bearings were all replaced a couple years ago.

I feel it in the body and the doors shake bad enough I either speed up or slow down. I also feel it in the steering, but have never decided if its because the wheels are shaking, or if the body is shaking the steering column.

When I was a kid my dad had a Ford f-250 4x4 that would do the same thing, and replacing the parts I'd associate with the kingpens fixed it. He owned that thing long enought that I replaced them a couple times.

Another thing which might not apply if you don't have them on your tires are those Port-A-Walls (also called toilet seats or falsies) which are fake white walls which fit under the wheel lip. I have them on my truck and they are getting old. One is split and I often envision them lifting away from the tires as I drive. Maybe not but worth a try if you have them. Mine are comung off in the Spring, too ratty.

That's strange...3500 sustained rpm will blow up a 235 Chevy straight 6???

In 1953, Chevy put the 235 in the first Corvette. Same engine, but different cam and induction. Still, same basic engine, same rotating assembly.
They were rated at 150 hp @ 4200 rpm, and in '54 and '55, 155 hp @ 4200 rpm.

Not a high revver, however, it appears that they are designed to live above 3500 rpm. I don't know why that would blow it up in the truck but not in a C1 Corvette. :confused: