I have a 1998 vortec 350 with lt1 hyperutectic pistons. my question is which flex plate do i need? i asked the guy who built it and im pretty sure he said all 1 pc rear main blocks use an external balance, but i looked on summit and it only shows internal balance flexplates for 1 pc rear mains.....


I really dont wanna grenade my new motor for something dumb like the wrong flex plate.. PLEASE HELP!

 

You need a 153-tooth, one-piece-rear-main-seal flexplate.

It will have a weight on it, but that doesn't make it "external balance". That weight takes the place of a built-in weight on every SBC crank that was external to the block from 1955 to 1985. If that weight on the outside of the block made all of those engines "external balance", then the 1-piece RMS engines are too, I suppose.

The factory used an unbalanced flexplate/flywheel and harmonic damper on the 3.75" stroke 400 SBC's, which were often referred to as "external balance". That, and the 377/383 4" bore, 3.75" stroke non-factory engines using a similar crank are the only true "external balance" engines - and guess what -they have that weight external to the block just like the shorter stroke SBC's (although, there are 1-piece rear main seal 3.75" stroke aftermarket cranks that also need the unbalanced or weighted damper and a differently weighted flexplate/flywheel).

Okay, the factory crate 3.80" stroke engine may also be "external balance" - I don't know off the top of my head.

We've been over this many, many times before, it just seems to keep coming up. Don't let words confuse the issue, Summit will list a "normal" 1-piece RMS flexplate as "internal balance". That's what you need, a "normal" 1-piece RMS flexplate.

 

Would this one from JEGS be a good choice??
http://www.jegs.com/i/JEGS/555/60150/10002/-1/750886|10537

 

555-60153

 

thanks alot, buddy

 

Some of the Summit descriptions have "Rear main seal: 1-piece", "Engine balance type: Internal", "Flexplate balance type: External". While that doesn't totally clear up the confusion, at least describing it that way, you'll know you don't need a weighted damper with a flexplate thus described.

There shouldn't be any 1-piece RMS flexplate without a weight is the point I'm trying to make.

 

ok the damper is the large stock 8" so i should be good right?

 

Should be. As long as it's neutrally balanced.

 

Sorry to ask but what do you mean by RMS flexplate.

 

"Rear Main Seal".

 

Thanks five7kid