1972 GM 350 throbbing engine vibration

[Franks 72]

I have a 72 Suburban with a stock GM 350 (1st oversize) with a strange vibration. Vibration is not really severe but is annoying and varies with engine RPM. I have disconnected accessory belts (alternator/water pump belt , PS belt with no change. I am wondering about the harmonic balancer. Engine was completely rebuilt with the only change being 1st overbore ,slight cam and new aftermarket harmonic balancer ,aftermarket polyurathane engine mounts. When replacing the harmonic balancer I was told by the parts guys that the original 8" balancer was not available anymore and would need a smaller version which is aprox 6 1/2 ". In retrospect I wonder if this is correct and is this the source of my troubles or will harder rubber in the engine mounts translate into more vibration being passed to the frame.
Any advice would be appreciated.

[The Rocknrod]

Ya the old "they don't make em anymore" for an 8" harmonic balancer doesn't wash. I found them on eBay, Summit etc for mine.
For instance, here's a whole page of them on eBay (new and used) - https://www.ebay.com/sch/i.html?_fro...y+350&_sacat=0
If it's surging check for vacuum leaks.
If not you can find an 8" balancer - If that's the problem.

[Bigdav160]

8" balancer is easy to find. Throbbing type vibrations are usually two vibrations that resonate together.

I assume it didn't do that before the rebuild? You could have some type of serious internal balance problem.

[Franks 72]

Thanks for the speedy responces !
After rebuild I didnt notice it. I will start with 8" balancer.
Stay tuned for a resolution (I hope)
Thanks again !

[GASoline71]

Those polyurethane motor mounts will transfer a lot more vibration to the frame than stock rubber ones do.

Gary

[garyd1961]

The size of the balancer is not critical but whether it's weighted or neutral is critical. Sounds like you may have a weighted balancer or flywheel where you need a neutral one. I believe that block casting calls for a neutral balancer and flywheel unless the crank has been changed.

[Rick Bollinger]

garyd1961 that's what I was thinking too. Fly wheel flexplate changed?

[LONGHAIR]

The block has absolutely nothing to do with balancers. It only matters to the crankshaft itself. The only one that uses the counter-balanced one is the 400. All of the other cranks use the regular neutral balanced units.
It's not likely that this is the problem here though. The vibration from the wrong balancer would be pretty intense.
I suppose that the lightweight one could do something, as could the aftermarket pistons from the over-bore kit?
Of course the poly mounts can't cause a vibration, but they can sure magnify one.
Transmission too. Clutch/flywheel and even torque converter/flexplate can cause vibrations. There are counterweighted versions of these for the 400 cranks too.

[garyd1961]

Originally Posted by LONGHAIR

The block has absolutely nothing to do with balancers. It only matters to the crankshaft itself. The only one that uses the counter-balanced one is the 400. All of the other cranks use the regular neutral balanced units.
It's not likely that this is the problem here though. The vibration from the wrong balancer would be pretty intense.
I suppose that the lightweight one could do something, as could the aftermarket pistons from the over-bore kit?
Of course the poly mounts can't cause a vibration, but they can sure magnify one.
Transmission too. Clutch/flywheel and even torque converter/flexplate can cause vibrations. There are counterweighted versions of these for the 400 cranks too.

Actually I got this confused with another thread where the OP stated he had an XXXXX010 block. I caught my error but left my reply because all 72 350s came with a neutral balancer and flexplate. As I said unless the crank has been changed it should be a neutral balancer and flex plate. The latter 350s with a one piece rear main seal did come with a weighted flexplate. So the block can tell you what balancer and flexplate came with it from the factory. Of course if it has an aftermarket crank it could come with weighted balancer and flexplate or neutral balancer and flexplate or any combination there of.

[paul mora]

Be sure to check the flex plate. It is most likely the problem. I had a friend that drove it for a year before he would listen to me and make the change.

[LONGHAIR]

Originally Posted by garyd1961

Actually I got this confused with another thread where the OP stated he had an XXXXX010 block. I caught my error but left my reply because all 72 350s came with a neutral balancer and flexplate. As I said unless the crank has been changed it should be a neutral balancer and flex plate. The latter 350s with a one piece rear main seal did come with a weighted flexplate. So the block can tell you what balancer and flexplate came with it from the factory. Of course if it has an aftermarket crank it could come with weighted balancer and flexplate or neutral balancer and flexplate or any combination there of.

Wasn't the one piece rear main seal a change that happened in the mid 80s? I'm thinking '86 off the top of my head. Anyway, by then it would have been called 5.7 liter, wouldn't it? It would have taken a completely different balancer too.
The counterweighted cranks are indexed to the flywheel/flexplate too. It would be hard to get that mixed up.

[garyd1961]

Originally Posted by LONGHAIR

Wasn't the one piece rear main seal a change that happened in the mid 80s? I'm thinking '86 off the top of my head. Anyway, by then it would have been called 5.7 liter, wouldn't it? It would have taken a completely different balancer too.
The counterweighted cranks are indexed to the flywheel/flexplate too. It would be hard to get that mixed up.

The one piece rear seal blocks are still a gen 1 sbc and the balancer is the same as all other 350s. You can put 283 heads on a one piece seal motor and it will run just fine. The only real difference in the blocks is the rear seal and they are machined for a roller cam. BTW I never said the block determined the balancer, I said you could tell by the block what crank should have come in it.

[Wrenchbender Ret]

The SBC 400 engines used a har. balancer & flywheel/flexplate with balancing weights built in. Sometimes someone will put one of these on the internal balanced engines, 85 on back. It will cause a serious vibration problem. A customer came in with one years ago. I removed the flywheel cover & could see the weight on the flexplate. I went up through the opening & cut off the weight. That solved the problem & saved removing the trans. The fly./flex. on an 86 up 1 piece rear seal engine will not fit on the earlier engines.
George

[tdangle]

I had a vibration in my 350, never could really figure it out. Had a trans rebuilt and when they (at my request) tested full throttle 1-2 shift at 5K + broke the end of the water pump. They put a new pump on and it did it again, this time actually breaking the housing of the pump in half. Ended up that I had neglected to install the bushing in the clutch fan that centered it on the water pump. Once I replaced it vibration all gone. Oh and 5K shifts no longer break the water pump

[Ironangel]

Exactly which balancer was installed? With the false claim that 8" balancers are unavailable I'd start my search there. I'd also check that flexplate verifying that is non-weighted neutral flexplate. I'm also wondering what if anything they did to the crank and new pistons before reassembling the short block? Was the crank polished and balanced with the new piston bob-weights? A high frequency vibration (faster than the rpm) that follows the rpm is likely internal "if" that new balancer checks out...

[burnin oil]

Originally Posted by garyd1961

The one piece rear seal blocks are still a gen 1 sbc and the balancer is the same as all other 350s. You can put 283 heads on a one piece seal motor and it will run just fine. The only real difference in the blocks is the rear seal and they are machined for a roller cam. BTW I never said the block determined the balancer, I said you could tell by the block what crank should have come in it.

Not all 1 piece rear seals had provisions for roller cams. Just FYI.

OP. Check the ujoints also. I once had on eat the needles and actually wore the shaft on an angle. I picked up a vibration and then a knock while driving. You would have sworn it was a rod bearing. Junk the small balancer for the correct 8" model and you will probably need the timing tab also since they match balancer size. The 400 balancers are around 8", ie large, so the external balance theory on that should be out the window. I see a cracked or bent flexplate being an issue. If you are unsure of the flexplate origin the 400 model is easy to spot. It has a piece of sheet steel welded to it that is a couple inches long. Pry the flexplaye all the way around and look for the wieght. Also a great chance to see if the flexplate got bent without pulling the trans.