First off, I don't know what I'm doing or what i'm talking about...

I just tore down my engine and I'm trying to figure out what I'm going to do with my bottom end.

The crank is already balanced. If I replace my pistons/connecting rods will I have to get the engine re-balanced again?

or, in other words, do they only balance the crank when they balance an engine?


one more...

Whats the difference between internally and externally balanced?

Thanks in advance

 

A proper balance includes the crank, all the pistons, rings, rods, balancer and flex-plate/flywheel. If a stick, I balance the pressure plate at the same time.

 

If it is stock factory it is not precision balanced. Most people replace pistons without even thinking about rebalancing.
Yes a precision balance requires absolutely everything to be balanced as a unit.
For a stock rebuild just go ahead and replace the pistons and have the rods resized. No need to rebalance.
Personally I would spend the money and have it done.

 

I'm no expert, but I did stay at a Holiday Inn Express last night.

The crank is balanced to offset the weight of the pistons/rods/wristpins.

For a completely balanced motor they will wiegh all of the pistons and remove material from the heavier ones until they all match the wieght of the lightest. Same thing with the piston rods, but they will remove material from either (or both) the big and small ends to make them all balance the same (i.e. all rods same weight, and same center of gravity).
Then the crank is fitting with weights that match the combined weight of your piston/rod assemblies and spun on a balancing machine. Then material is removed (or sometimes added) from the weights on the crank.

Internally balanced means that all of the offset weights for balancing the engine are internal, or inside the oil pan.

An externally balanced engine has more weight in the damper and the flywheel. This means if you have an externally balanced motor they will need the damper and flywheel along with the crank when they balance it

 

>First off, I don't know what I'm doing or what i'm talking about...

None of us do. It's all a facade.


>I just tore down my engine and I'm trying to figure out what I'm going >to do with my bottom end.

Save that for your proctologost.... focus on your engine for now.


>The crank is already balanced. If I replace my pistons/connecting rods >will I have to get the engine re-balanced again?

>or, in other words, do they only balance the crank when they balance >an engine?

You have to have it all re-balanced because the weight changes with new/different parts.

>one more...

>Whats the difference between internally and externally balanced?

Internally is when you have your act together, and can deal with daily stress on a regular basis, with little or no guidance from outside sources.

Externally is when you need to have help with it all because you cannot cope.


>Thanks in advance


Oh.... you meant the engine...

In a nutshell-
Internal means the crank etc was designed to be balanced by dealing with the components inside the block.

Externally means the parts outside the block such as front damper/balancer and flexplate/flywheel must be part of the balancing process.

Add connecting rod bearings to the mix.... I think they were left out of the recipe.

 

thanks all for the replies, and the hard time (especially from Tom454, I know you're just kidding)

With all of this in mind, I think I'm going to dish out the extra bucks and and buy a pre-balanced 383 kit from Summit.

http://store.summitracing.com/defaul...ESP-B13004E030

does that look like a reasonable kit?

once again, thanks for the help. I wouldn't even have thought about tearing my engine apart if it wasn't for this forum...

 

Common Bobweight formula for chevy V8:
TWICE one big end rod mass PLUS TWICE one rod bearing set mass (two half-shells) PLUS sum of one small end rod, one piston, one ringpack, one wristpin (& its locks if any) masses PLUS about 4 grams of "clinging" oil = Bobweight for chevy V8 journal.

The flexplate/flywheel/pressureplate and damper is considered part of the crankshaft itself; but must be known to machinist if external or internal-neutral.

Balance Data street 388" sbc w/ 5.7” Pink Rods (grams)
Big End of Rod 832
Small End of Rod 178
Rod Bearings (2 half-shells) 92
Piston, Pin & 2 Wire Locks 711
Rings 58
Oil 4
BOBWEIGHT = 1875

 

Pay attention to the dish vs flat top kit variations. You want to approach this methodically and settle on what's going on the top end as well. This drives your compression ratio along with the geometry of the piston surface plays a major part. Just remember, heads can be replaced, the bottom end you live with til it's rebuilt or blows up. Pick your piston carefully.