How can I tell if I have an internally or externally balanced small block? I'm in the prediciment of having a small block (with a bad vibration) that I don't know the contents of. I know the block is a 3970010 block from a 1975 pickup and that the heads are original L82 heads from the corvette.

I pulled the balancer this weekend thinking the rubber may have rotted out, causing the vibration. Come to find out, it looks OK but its a balanced part (has holes drilled in it, presumably for balance). I don't know what the inside of the motor looks like (including the flywheel). Any way of telling what i'm working with?

Options: Put back together, drive until it blows apart. Replace damper with new (with the presumption that the balanced part is causing the imbalance). Try to find out if the flywheel is balanced or not (again, if its external balance, the flywheel being wrong could cause imbalance).

Any help greatly appreciated!

 

350's back then were internally balanced. Shouldn't see any balance holes on balancer.

If someone made a 383 out of it...it's likely externally balanced. They typically use 400 balancers. They have a concave section around the outer ring (where timing mark is on balancer) on one side that creates the heavy spot on other part of balancer. The flywheel/flexplate will also need to be an external one. It will have a weight welded to it on the outer edge of flexplate or a cast in heavy weight you can see on the flywheel (on motor side of it).

Does it look like the outer ring slipped on the hub of balancer? Again. really shouldn't be any balance holes on it..so this is indicating someone maybe did a stroker deal or at least used some real light pistons and rods.

Any more info on motor?


JIM

 

It does not look like the outer ring has moved. I thought that may be the case but the rubber is intact and it looks pretty good, except for the holes drilled in it. I have not pulled the trans so I don't know what the flywheel looks like. On the same token, besides being a 350 (according to the former owner of the car, who had it for 27 years), I know nothing about the internals. Therein lies the problem.

Suggestions?

Could a balanced damper be causing such a vibration if the engine is, indeed, internally balanced?

 

If the damper just has holes drilled in it then it was to balance the damper. Those drilled holes make it a neutral balanced damper. That means that someone balanced the damper as a piece part and it will have no effect on the overall balance of the engine.

A 400 external balanced damper will have a larege counterweight on the inside of the ring as viewed from the back of the damper (ie you have to take the damper off and look the the side that faces the timing cover). This offset weight acts like an extra coutnerweight on the crank and therefore makes it an external balance engine. The Flywheel or flexplate will have a similar counter weight on it as well.

All 350's are internal balance. However when the switched to a 1 piece rear main seal on the crank, they removed the funky shape of the rear of the crank. Due to this material removeal they had to add a counter weight to the flywheel/flexplate only. its not a true external balance, but you do have to balance the assembly with the flexplate to get it right.

If you have the engine at #1 TDC and the 0 degree timing mark doesn't line up with line on the balancer then the balancer has slipped. If the balancer ring has slipped those drilled balance holes to make the balancer neutral balanced is now creating an imbalance in the engine.

If the previous owner had the car for 27 years there's a good chance that he has never messed with converting it to a 383. The only real way to tell is to take a head off and measure bore and stroke.

Hope that helps.

 

Even internal OEM type balancers require tweaking to get them right due to variations in casting thicknesses, etc., so don't worry about the small holes - I've seen 'em in quite a few. As usual, Jim is right on with the other stuff but there are external balancers that add a chunk of weight rather than knocking some out as he described. Good luck, hope it's not too tough to get straightened out.

 

I guess, in my opinion, these aren't small holes...not what I would think would be needed to address imbalance due to manufacturing variability. There are three of them, about 3/8" diameter and 1/4"-1/2" depth. I can't imagine GM would have balanced each of these from the supplier in this manner.

If I put #1 at TDC and look at the timing marks...which is the 0 degree mark? Is there a mark on the timing cover or on the crank? How can I tell if the balancer outer ring has slipped?

Thanks!

 

a picture of the back of the balancer would have been good.

My guess is that somebody just slapped any old balancer on and used the wrong one. I would buy a nice new after market balancer from jeg's or summit racing.

running an out of balance motor is just destroying the bearings.

 

Is the car have an auto trans or a manual? The reason I ask is I had a out of balance Torque converter (new rebuild) that was put in with a transmission rebuild that ended up taking out the main engine bearings on another car I have. I had taken the car back and had the trans rebuilt because of the vibration and they swore it wasn't from the torque converter that it was a motor problem. I drove it until the engine gave out. When I installed the replacement motor and it still had the vibration, I new it had to be the converter and sure enough a replacement converter fixed the vibration.

You mention flywheel, so if it is a manual do check the balance of the flywheel and clutch if you can or they could be loose. The vibration will destroy the engine.

 

Good idea....a picture could solve a lot of the questions....

JIM

 

You might want to make sure that you have not got a cracked or broken motor/tranny mount some where ? ujoints all okay ? These things can all cause bad drive train vibrations as well.

Ikwhite

 

Drew. It might help if you provide a bit more informaiton about the vibration. Did the vibration just start? Is the vibration only noticed at certain rpms? Is the vibration constant regardless if idle, or under acceleration? Standard or Auto? Any recent changes?
-Jim

 

I have been a bit too busy lately...hence no pictures yet.

To answer some of the questions...its a 4 speed car. I replaced the trans mount a while back and the motor mounts look OK on visual inspection and are not loose. Two local mechanics have said it is probably the damper or flywheel (then again, I'm not completely faithful in them). I'm well aware of the impact of this vibration...and obviously I'd really like to get it fixed sooner than later. At the same time, the motor has some miles on it and needs valve seals and have various leaks (typical SBC oil leaks) so I don't want to spend tons of money that could go towards a new motor. It has been a problem for about a year now...practically since I got the car. Its a higher RPM problem, though it does shake a little at idle...probably worst above 2500, where the shifter starts shaking.

My question is still whether it COULD be externally balanced and, thus, perhaps a mismatch of parts could be causing an imbalance. If the prevailing opinion is that its probably internally balanced, then I might as well replace the balancer and hope it helps. If not, I have a new balancer for whatever engine I decide to put in the car when mine rattles itself apart.

Any other suggestions? Thanks to all, as usual

 

The vibration gets bad at around 2500 rpm? In EVERY gear? Even in neutral while stopped?

It would be extremely unlikely that your 350 was converted to external balance.

And the harmonic damper is NOT a balancer. It's a tuned-mass damper to damp out vibrations. 3 3/8" diameter drilled holes of varying depths are normal.

 

Yes, all gears, including neutral.

I agree, it is unlikely that its externally balanced but stranger things have happened.

So, assuming the balancer is correct for the engine, what else could be my problem?

 

Drew. I wonder if your clutch plate has failed you. About 30+ years ago when I used to spin high-R's, a portion of the clutch plate facing came off and caused a vibration similar to what you are describing. It still drove okay at first, but then I noticed it didn't hook as solid when I dumped the clutch as it had in the past. Just another idea.
-Jim

 

Have you removed all belts from front of motor and tried it? Often accessories can vibrate.

Does it do it with clutch in and out? Or does it change?


JIM

 

As I was on the way home, I realized I fogot to also mention, an easy check for a bad clutch plate is to push the clutch in, put the trans in 1st, start the engine and see if the vibration goes away. If it does, I'm sure you have a bad clutch plate. The reason for putting it in 1st (actually any gear will work), is to stop the clutch plate from spinning.
-Jim

 

I'm glad you mention the clutch...the thought had crossed my mind but had been put wayyyyy in the back, as I'm not crazy about pulling the trans. I may have to check into that, as it has shown some symptoms.