I'm REALLY confused on this... A stock LT1 flywheel has a counterweight since EVERYWHERE I look it says that the LT1 is externally balanced. I never see anywhere saying it's internally balanced... Yet when people to T56 swaps to Gen I blocks, tehy use LT1 flywheels all the time... are you guys rebalancing them or what? I ask because I've got a neutral balanced (from the machine shop... should be balanced perfectly) rotating assembly that should stay in balance as long as I have a neutrally balanced flywheel attached to it. Well I bought a flywheel... and since it's a PITA finding an internally balanced flywheel for an LT1 (ignoring the confusion to this point... as perhaps they're the same?) , it's an allegedly externally balanced flywheel. Now I hear that the weight on teh back of the flywheel is required to keep a neutrally balanced engine... neutrally balanced. It's part of a counterweight that used to be on the back of 2 pc RMS cranks. But I dont understand... if my rotating assembly is balanced... then why do I even need the counterweight at all? How can a counterweight attached to the flywheel be part of an internally balanced engine? WTF is the point of distinguishing between teh two if they've both got random weights attached in places outside of the engine? I'm beyond confused here... And I need to know since I'm about to put this motor in tomorrow, and I'd like to have the flywheel/clutch/bellhousing bolted to it when I drop it in, but I'm not gonna torque down teh flywheel bolts until I know what to do about this counterweight that's on it. I took it by my machine shop, and they told me if I just pop the weight off, I should be fine. So I unbolted it and carried on... and now I hear that it's OUT of balance if I dont bolt it back on... because it's part of an internal/neutral balanced system... None of it makes any damn sense.

Was your rotating assembly balanced with, or without a flywheel? If it was internally balanced with a FW, you need to know if that FW had a weight or not. If it wasn't balanced with a FW, then you have to accommodate the weight or not, depending if your setup is 1pc or 2pc RMS.

http://www.carcraft.com/techarticles...er_cranks.html

You also talk of "LT1 flywheels" and Gen I blocks. Since GenI blocks were all 2-pc RMS, and stock LT1s were all 1-pc RMS, I'm not sure what you're trying to do since the stock LT1 FW will not fit on a 2pc-RMS GenI motor..

Correction: '87 and later Gen I blocks used a 1 piece seal, '86 and earlier used the 2 piece.

Not all 1G sbc's were 2 piece rear main seal motors. Infact most 3rd gen F body V8's were 1 piece rms since the change occoured in the mid 80's.

I believe that the LT-1 fw will fit any 1 piece rms 1G motor.

EDIT: beat me to it prime lol, and with actual dates no less.

ls six is correct

LOL.. Yes you are ALL correct... Anyone have the answer for the original poster?

What did you end up doing? Basically if you had the rot-asm balanced with the FW, you have to use whatever setup that FW had. (weight or no weight).

Otherwise if you had it balanced without the FW, the balancer will assume the weight will be there, and your rot-asm will be out of balance without it...

Small block two piece rear main seal cranks have bolt circles on a larger diameter and have a kind of "divot" or maybe offset on the crank flange. This does a very small amount of crank balancing.
One piece rear main seal cranks have a smaller bolt circle and are perfectly round, no offset or divot. So there is a very small amount of balance weight spot welded onto the flexplate on a one- piece rear main seal engine. It is a lot smaller than the piece attached to either a 400 or 454 flywheel. I don't believe that they used any balance offset on the harmonic balancer like they did on the 400 and 454's.

A little late... but here's what I decided to do.... against the wishes of one of the mods here who was trying to convince me otherwise.

Everything I can find states teh LT1 is externally balanced. All aftermarket LT1 flywheels are externally balanced.

My rotating assembly is internally balanced, and my machine shop personally balanced it neutral before the motor ever went together, without a damper or flywheel. So I need a neutrally balanced flywheel and damper. The damper is fine... but since my flywheel has a counterweight... I took it to the machine shop and had them look at it, and tehy told me I should just remove the weight and it should be neutrally balanced then.

One of the mods was saying that they were in fact mistaken, and that counterweight is part of what used to be on the 2pc RMS crankshafts.

I am honestly not sure which way I should have gone, but I decided to remove it. My logic is that my rotating assembly is neutrally balanced without a flywheel... so my flywheel should be neutrally balanced also, and given that LT1's are externally balanced (Gen I 1pc RMS blocks are, from what I can tell, mostly all internally balanced, along with my crankshaft) I figured that was the correct move for me.

I have a feeling that it wont make THAT much of a difference even if I chose wrong.

And a picture to keep everyone entertained...

We have a definitive answer.

There's a horrendous vibration around 1500 RPMs without the counterweight. With the counterweight it runs nice and smooth.

There ya go!

The machine ship that did the balancing didn't need your flywheel to balance it, they used an '87-up flexplate they keep on hand for such things. This is the way the '87-up cranks get balanced. For internal balance, that's referring to whether or not you need a 400 damper, which, for the LTx, is not an option. Thus, you get the front 2 counterweights on the crank, the ines on each side of the first rod journal, being extra heavy.

Trying to revive this thread a little. I just bought a 383 kit for my car and I swapped out the 700r4 for a t56. When getting this balanced, will the stock balancer work or do I need a different one? Scat tech said it was neutral in the front and external in the rear. Is the lt1 flywheel external balanced? It's a 1 RMS and summit advertised it was internal balanced. Can it be both internal and external?

off topic sorry infernal how do you like that tranny cross member???

That is a sweet looking crossmember!

I love it... the exhaust tucks up amazingly. It only dips below the frame rail height as my dual 2.5's merge into a single 3.5 collector and hits the muffler. Ground clearance from the crossmember and forward is incredible. Whenever I get around to lowering the car though I'm gonna have to get it run over the axle like stock though probably unless I can find a much smaller muffler.





I'm pretty sure you need an external balance flywheel. You'll need to buy a flywheel specifically for a T56 and 2 pc RMS/external balance application. They exist. Expect to spend aroudn $400 though.

Mcleod offers a 2pc rear seal LT-1 style flywheel. PN-460370.