Engine Balance Question
 

I have been led to believe forever that Pontiac engines were internally balanced, now that I am getting ready to balance and assemble my short block I researched parts and discovered there is more to this than I thought. I have not purchased a flexplate or balancer yet so my question leans on what flexplate to purchase - internal or external balance?

I know, in order to answer that you need more information on my short block - I have a .030 over early 75 (481988) 400 block with a stock early 1970's 400 crank. I L2262F30 Speed Pro forged pistons (heavy I know) and Eagle 6.625" 5140 I-Beam rods. My understanding from information I could find is the pistons are close in weight to the stock cast pistons and the rods are slightly lighter than stock cast rods.

When complete, this will be in my 1974 Grand Prix mainly for cruising and shows. The car will have a 200-4R overdrive and 3.73 gears (maybe 3.42 gears if I eliminate the torque converter lock-up. If I planned on running it hard, I would use a better forged piston but feel the L2262F's should be plenty strong for how I plan on using the car.

My gut says I should use the external balance flex plate, no matter what I do I will have it professionally balanced. I just want to avoid adding heavy metal to the counterweights if possible - it really add to the cost of the balance job! Thanks. for your responses up front. Tom

Balancing:
Pontiac flywheels and flex plates have an asymmetrical bolt pattern so that the flywheel will bolt on the crank only in one position.
There is a slight external unbalance built into factory parts to aid in the internal balance of the engine.
This amounts to about 6 inch-ounces which if not compensated for will show up as a pronounced vibration over 2500 rpm.

Harmonic Balancers:
The harmonie balancer is designed to dampen torsional vibrations in the
crankshaft.
Without it the crankshaft may break.
The harmonie balancer is not designed to compensate for engine balance and can only affect it if the balancer itself is out of balance.
Pontiac does not use any of the ' 'unbalanced balancers" like the 454 Chevy units, so Pontiac balancers may be interchanged freely.
It is critical that the balancer you select be in good condition; the rubber should be free of cracks and evenly spaced all the way around.
Normally, the heavier the balancer is, the higher the engine can be wound without the crank failing from the torsional vibrations which increase greatly with high speeds and heavy loads .
The rule of thumb is to use the most massive balancer you have available.
The best unit is the early Super Duty model, but requires the use of the early four bolt water pump and timing cover.
The stock balancer is adequate for all but the most severe use.
All 1968-76 harmonic balancers are 6.75" diameter and virtually the same.
The Ram Air IV balancer is the same as the stock harmonic balancer except
for closer tolerance on the balance.
Interchange problems only arise when using the 1968+ balancer with earlier timing covers, water pumps and power accessories.
However, all 1969-76 timing covers are identical so you will bypass any alignment problems by staying with the late model parts.
Beware of aftermarket timing tapes designed for the 1967 and earlier balancers which were 5.25" diameter.
Timing the engine with one of these tapes on the late balancer will result in a
retarded setting.
With the 1968+ balancer, for instance, 360 total advance would be 2 1/8" clockwise from the zero degree pointer.

Post #2 is loaded with good information for you to digest. Based on the information provided, this engine will be getting a flex plate, not a flywheel. So IMO, It really doesn't matter which way you go for balance. (full internal vs weighted flex plate). I prefer an internal balance as that gives you full flexibility for the engine in the future. (future use as a stick shift engine with aftermarket flywheel and so forth). Also, your desire to avoid adding heavy metal to the crankshaft may be flawed. That decision may or may not matter what flex plate you use because weight may need to be added to the front of the crankshaft as well which has no compensation on the front. Since a re-balance is in the plan, have it spun up and see where you land going for an internal balance. Honestly, if a little weight needs to be added, the cost is about the same as messing with a weighted flexplate. Almost every one I have used needed some weight removed or a little more welded on to get the assembly within 3 grams which is our default for a street engine. Hope this helps.

It also helps to understand how the factory balanced the motor in relation to the slight intensional unbalance.

The factory did a preliminary balance job and then once each short block was assembled ( yes with the cam even) the short block was then spun up by a darn big electric motor and the final balancing job was done.

Is that supposed to say 36 degrees total advance?
Thanks for the great info!

Yes, he meant 36 max ( with 68 and up iron heads ) total mechanical only ( no vacuum advance at full throttle) and depending on your vehicles rear gearing and if your auto or 4 speed your going you want that total advance in by 70 mph in high gear.

The stiffer your rear gears the faster you can bring in the total advance.
Here’s what pre 1971 motors could come set for if the car was a ram air with 3.90 rear gears so you will need to temper it for your car.

With a low compression motor and especially if your a tad over Cam’ed your going to want to run more then 10 degrees initial and your vacuum advance can only be 8 to 10 of added part throttle timing .

If the balancing will be done by anyone other than a Pontiac specific shop, it is the safest to neutral balancing it and buying an aftermarket neutral flywheel. I have ran into to many Pontiac’s that are balanced wrong for the factory flywheels. You are not the only person that has thought Pontiac’s are all internally balanced. As Pontiac’s have been built less, I think the issue has gotten worse.

This brings up an important issue for me. I have an assembly that I had done years ago. I can't for the life of me remember which flex plate I included when the machine work was done. I have the factory flex plate that was balanced with another project a long time ago, and I have an aftermarket internal, and an aftermarket external balance flex plate. The shop that did the work used to be reputable but work I had them do more recently led me to believe otherwise. Is there a safe way to go or do I take everything back to a shop? Sorry if I'm taking away from the OP.

If you sent it with a flywheel and don’t recall which type it was, I think you’re probably out of luck on being able to tell which type it had. I have been around a ton of Pontiac’s that were internal balanced but they used the OEM flywheel thinking it was internal balanced. To be more than just a guess you will probably need to have the crank spun on a balancer again with the correct bob weights. I wouldn’t think it would be very expensive to do though. For neutral balance it does need to be checked with a flywheel. If it comes up being off and needs an adjustment on the back counter weights when spun without the flywheel it was likely balanced for the external flywheel.

Yes, got a zero to much there.
You can take the circumference divided by 10 to get 36° of the total circumference which is 360°.
F.e. you can also divide 360° by 12 and have 30° if needed.
Circumference of 6.75" is 21.195" divided by 12 is 1.75" clockwise from the original mark on damper.