guillaumeber

Hello!

I'll make it simple, it's getting laaaate here and I need some sleep! Hopefully I will wake up tomorrow to some usefull answers

I'll be talking about 1967-1969 engines, chevy sbc

I have read over the board here that some engines were internally balanced (balanced crank) and that the harmonic balancer and flywheel were independently balanced. So the whole unit was completly balanced, as 3 independent parts.

I also read that some engines were balanced as an assembly; those 3 parts bolted together and then balanced as a whole.

That would mean that you can't really change only one of the 3 parts (crank, flywheel, balancer), without royally screwing the balance of the engine bottom.

How can I know what I have? Can I switch those parts without worying to screw something?

I would like to hear more on that topic, what are people doing when changing a bad crank for example?

thanks a lot guys! you are my source for thrustful infos!

g.b.

Solid LT1

All Vette motors from that era are internally balanced. The 1970 Impala introduced the 3.75" stroke 400 small block which was externally balanced and so are many 383 stroker motors. If you have a 327/350 Vette small block its internally balanced. The front harmonic balancer on a externally balanced small block has a counterweight that is easily identified just Google it.

L88Plus

Balancer and flexplate/flywheel are neutral balanced, any good quality pieces that will fit will work.
Do you think GM actually used the specific balancer and flexplate that went on a particular engine when they factory balanced it? They "balanced" cranks by the hundreds and stacked 'em. Of course, factory specs for balance aren't what they are for what we now call a "performance" engine.
If factory bobweight was within a pound or so, send 'er down the line

guillaumeber

Is it the same for all Chevy engines from that era? I know my engine was originally a 68 327 from an unknown car. The crank in it will be a 350 crank which should be neutral too I guess!

REELAV8R

The flywheel or flexplate and the harmonic balancer are neutral, the crank is not. It has a certain amount of "bob weight" to offset the weight of the piston, con rod and rings.
If you change the crank it will have a different bob weight than the previous crank. It may be close enough and you'll be fine, if not then the only remedy is to get it balanced with one of the pistons, con rod and rings or try to have the new crank made the same as the old crank in balance. The higher the RPM you use the more important it becomes.

1Fordman

Small blocks 350 down are internal, 400 is external.

Big block 427 down are internal, 454 up are external.

As Solid LT-1 said when making stroker motors things change.

my 76 ray

Here are pictures of two harmonic balancers. One is for an externally balanced engine and the other is for an internally balanced engine.




The balancer on the left is for an externally balanced engine. Notice the machined out area. The balancer on the right if for an internally balanced engine. The area is not machined.





The balancer on the left is for an externally balanced engine. Notice the machined out area. The balancer on the right if for an internally balanced engine. The area is not machined.

As REELAV8R mentioned, they are manufactured to a spec that is fine for a production engine.