A friend is rebuilding the engine in his 2000 motor home. It has a Vortec engine. (I wasn't aware that the Vortec Gen 1 SBC carried on past the years that they were installed in pickups and the like past 1999).
One thing has led to another and now he's acquired a used Eagle stroker kit at a good price. (He's a hotrodder at heart).
In conversation he says the flexplate has a weight on it. To me that indicates externally balanced. My understanding, like with any Gen 1 SBC (excluding the 400), was that they are all internally balanced.
What's up with that?

 

SBCs are all INTERNALLY balanced, except for the 400. (stock ones, that is)

However, due to the 1-pc RMS crankshaft flange's round shape, the rearmost bit of INTERNAL balance weight, which was historically located on the funky-shaped flange at the back of the crank, had to be moved to the flex plate / flywheel. All of the 86-up 1-pc rear-main-seal motors require that.

The fact that the rearmost bit of weight appears to be on a different part from the crank DOES NOT make the motor "externally" balanced. It is still INTERNALLY balanced.

Any crank for 1-pc RMS that is designed to use stock parts such as the flex plate, will also be INTERNALLY balanced; and as such, will require that little bit of weight on the other "external" looking part.

96 - 2000 is the approximate range for the Vortec 5.7, even though the 5.3 LS series started appearing in some applications starting in 99.

 

That explains that. My experience with 1 piece rear main SBC's is limited. All of my junk has been old 70's vintage.
I'll pass that along. I haven't asked him if it's for a 2 or a 1 piece RMS. I hope for his sake he hasn't scored a 2 piece RMS crank for his 1 piece block. If I recall correctly, the blocks are somewhat different where the seal is involved.
Thanks.

 

If it's for a 2000 motor, it's 1-pc.

What the terms "internal balance" and "external balance" ACTUALLY mean, takes a bit of explanation... it DOES NOT mean which side of the rear main seal the weight is on, which side of the crank/wheel parting line it's on, etc. It refers to the ability (or not) to fit the counterweights that are in between the pistons, UNDER the bottom of the pistons when they're at BDC.

 

That in itself must create a lot of confusion.

 

My 588 BBC has an internally balanced crank. The 396/427 BBC was originally internally balanced. When GM came out with the 454, there wasn't enough practical room inside the block for the extra weight just like when they came out with the 400 so they moved it to the balancer and flexplate/flywheel.

That doesn't mean all the big engines are externally balanced. My aftermarket crankshaft has an extra weight added on the inside to make it internally balanced. That just means I need to use a neutral harmonic balancer for a 427 and can use a flexplate from a 350 since it's also neutrally balanced. The downside to the internally balanced crank is the location of the extra weight. When the crank was balanced for the bob weights, it took a bit of work getting the weight in the right place since the extra weight tab on the crank blocked the position they wanted to drill in.

As for your crankshaft, Sofa already explained it perfectly. The extra tab on the flexplate is not a significant amount of weight to change the crank from internally to externally balanced but is still required for that style of crankshaft. A proper externally balanced crank also has a special harmonic balancer which has most of the weight to be externally balanced.

Even with an old 400 block, you can easily get an aftermarket internally balanced crankshaft for it and use a 350's balancer and flexplate. Anybody doing a quick glance at the engine wouldn't see the 400's externally balanced dampener and wouldn't know that it's a 400. Same as my big engine. I could easily say it's a 427 and nobody could tell the difference just by looking at it. Well, other than its a Dart block and nothing on it is OEM.

 

Just as an fyi, the Vortec gen one's came in vans, which were used as motorhome chassis's until 2002. The last couple of years they ran on 411 PCM's just like LS stuff.

 

The 2nd engine I ever assembled was a 400. I remember the unbalanced dampener and flexplate (despite being many years ago).
And know as I think about, I also recall the flexplates used with the 1-piece RMS blocks although I've never assembled one (I think too that it was sofakingdom's explanation) . That's what caught me off guard when my friend mentioned the "blob of weight" on his flexplate.