Internal or External Balance on 383?
 

I've been reading Lingenfelter's book, J F on Modifying Small-block Chevy Engines. In it he says:

"What many Chevy fans may not know is that all '86-and-later Chevy small blocks are also externally balanced engines requiring specific flywheel/flexplates and torsional damper."

So, do I go with an internal or external crank in my 383 stroker project? Thanks for helping a willing-to-learn rookie!

Interested in this as well. I am also doing my homework before I buy.

Disclamer: these statements are what I have been told and read.

I've been doing some research on this and from what I have found so far by calling some speed shops and scat is that if you buy the internally balanced crank for a 1 piece rear main you can reuse your stock 350 balancer and flexplate (still recommended to have the assembly balanced though, I'm confused on this). I believe you can only get these for the 6 inch rods though. If you go with the externall balanced crankshaft, you have to buy 400 flexplate and balancer, and it will work with 5.7 inch rods. And of course you have to have that balanced. I was told by not only the machine shop I am talking with to do my short block but also speed shops I have called to inquire on parts that even if the rotating assembly is advertised as balanced and ready to drop in, you should balance it anyway. Machine shop I'm using said half the stuff they get in that claims to be already balanced, when put on the balancer(or whatever they do), is not really, or is barely within spec.

Whether the quote is accurate or not, the information is erroneous. Because of the change in the shape of the crankshaft rear flange, as revised for the, new for 1986, one piece rear crankshaft seal, the rear of the 5.7 L crankshafts became externally balanced for the first time. The neutral balance at the front of the engine is unaffected.

You can choose to internal or external balance as you can afford or prefer, but a truly 100% neutral, internal balanced, one piece crank engine will require a unique, non-commercially available, specially neutrally balanced flywheel or flexplate.

RACE ON!!!

Depends on what trans you have. Your profile says you have an 89.
89's and later could be had with an automatic or a 6 speed known as a ZF6. If you have the ZF, you have what's known as a dual mass flywheel (unless at some point someone swapped it out for a single mass conversion or an altogether different trans).
The DM FW needs to be taken along with the dampener of your choice with the reciprocating assembly to the balancer. As mentioned above, it's wise to get everything checked and balanced regardless.
As an example, in my case I had a DM and converted to a single mass.
The new engine was full internal neutral balance, the FW was neutral and the harmonic dampner was neutral. Took it all to the balancer to make sure. This way I can use any harmonic dampener and/or FW I want at any time as long as both are neutral. With the L98 ZF combo, you are limited to only one (very expensive) DM FW and the correct weighted dampener.

CFI_EFI:

You have discussed this issue several times. If flexplates are not commercially available, then what do people do with an internally balanced 383 stroker in a post 1986 one piece rear seal engine? I would appreciate your comments.

Mark

I just when through about 10 phone calls to Summit, Jegs, and a few flywheel manufacturers. I have an internally balanced 383. What I needed was a 153 tooth, 1pc seal, internally balaced flywheel. Problem is that no car, to my knowledge, came from chevy like that. If it is internally balanced, its a 2 pc seal. 1pc=external. The solution was a Hayes flywheel for an externally balanced motor comes neutral balanced. It will be here on Friday and its going straight to the machine shop to make sure its balanced.

Perhaps, I am being too subtle in using the term NEUTRAL balance. There is a difference between internal/external balance and a truly neutral balance. The engine can be internally balanced to one piece seal, imbalance specs, or it can be internally, balanced to TRULY neutral balance specs. Since no production one piece seal engine was ever truly neutrally balanced, there is no neutral balanced flywheel or flexplate that will bolt to the one piece seal crankshaft flange.

As I discovered doing some leg work for conv90, most (all?) internally balanced one piece seal rotating assemblies are internally UNBALANCED to factory imbalance specs, so they will take the production, one piece seal, bolt pattern, unbalanced flywheel or flexplate.

If one desires a truly, neutral balanced, one piece seal rotating assembly, he will have to have a one piece seal bolt pattern flywheel or flexplate custom, neutral balanced to match up.

RACE ON!!!

As CFI stated the thing is essentially internally balanced just not neutral balanced.
I sent the entire rotating assy and FW to the machinist to get it balanced. All balancing was done with the rotating assy, no drilling was done to the FW or balancer (really just a damper for the LT1)

The reason I did it this way is exactly for the reason CFI-EFI stated, I wanted to be able to use a standard FW (same would apply with a flex plate)

So you were able to use your stock flywheel? and balancer? I'm about to order my stuff and this is the last thing I have to work out.

I would balance internally and externally. :thumbs:

Yes,
The only diference is I have a Fidanza FW but it is a direct replacement for the DM

You can use any FW that will work just bring it with the rotating assy whaen you get it ballanced.

I internally balanced my 383 in my 84 - 95 roller block. I used a 6 3/4 harmonic balancer (Summit Brand) and removed the counter weights on the flex plate to make it neutral. Then the machine shop balanced the complete assy. The externally balanced assy's use a 8 inch balancer and weighted flex plate. The 8 inch balancer would have required some grinding on the crossmember which I did not want to do. IMHO - Internally balanced is always better than external.

You could have just as easily used the 1995, or even the 1984, damper on the front with no modifications.

You now have the situation, out back, that I described above. If you ever need to change the flexplate, you will have to have a one piece seal bolt pattern, flexplate, custom, neutral balanced.

RACE ON!!!

I had my rotating assembly all neutral balanced. the ati damper i ordered was neutral balanced. I went with a mcleod twin disk with an aluminum flywheel which is neutral balanced as well. = balanced 388.

:cheers:

You are correct regarding the balancer. I went with a SFI flexplate (thicker) and it was not a big deal which was the plan all along. Hopefully my starter will not chew it up. So far so good.....