I'm gathering parts for my 383 (with tpi) and have run into a slight
problem. I've been told that due to my internally balanced 4340-one
piece rear main seal crank - I will need a "neutrally" balanced flexplate
for my beefed 700R4 tranny. I've seen "externally" balanced flexplates
advertised (this I know I don't need) and "internally" balanced flexplates
advertised. My question is: Is the "internally" balanced flexplate the same
thing as the "neutrally" balanced flexplate? I need to know which to buy
before I take the parts to the machine shop for balancing.
What say you - O nabobs of knowledge ???

 

Yeah, "internally balanced" should be the same as "neutrally balanced". Internally balanced means that all the balancing of the motor is done on the crankshaft throws, so the harmonic balancer and flywheel/flexplate have nothing to do with the balance of the motor, therefore making them "neutral balance".

 

If you have a 1-pc rear main sela motor, go get a 1-pc rear main seal flex plate.

Banish the words "internal balance" and "external balance" from your vocabulary. All they will do, is confuse you, as they have already done.

1-pc RMS motors come from the factory internally balanced, with the last little bit of the internal balance weigh mounted on the flex plate. So they "look", to the eyeball, like they're "external balance", but they're not. Those terms don't refer to which side of the oil seal the balance weight is on.

DO NOT take your flex plate or your crank damper to the shop when your crank is being balanced!!!! What will inevitably happen, is some doof at the shop will do himself the favor - AT YOUR EXPENSE - of sodomizing you by "custom" balancing your flex plate to some IMPROPERLY BALANCED rotating assembly, because it's less work for him, but he can charge you the same money (after all, you would neither know nor care what he hacked on as long as it didn't vibrate, right? so you'd pay the same either way). And then, what are you going to do when your starter tears up your "custom" one? Ah, there's where he did himself the favor: all you can do, is disassemble your motor, and take the moving parts back, and have another "custom" one created AT YOUR EXPENSE to match your IMPROPERLY BALANCED rotating assembly that he sodomized you over.

Instead, have your rotating assembly balanced TO THE STOCK SPEC. That way, you assemble it, and you bolt up a STOCK damper and a STOCK flex plate, and you're good to go.

If your shop tells you they need your flex plate in order to "guarantee" the balance job, or whatever other lame-a$$ excuse they can cook up, find a new shop; and tell them that right then and there. Walk out the door. You don't want them working on your stuff. I mean it.

 

Hey sofa,
Thanx for the scoop on the crank balancing ! You mentioned having the crank balanced to "stock spec". Would that apply to a slightly lighter
( 50#) aftermarket crank with aftermarket h-beam rods ?

**********************************************************
" That's no vernacular - that's a derby! "

Jerome Howard - 1936

 

Yes.

The idea being, that "balancing" the rotating assembly, consists of first making all instances of the same part have the same weight (at each end, in the case of the rods); and then adjusting the counterweights or the rods until the counterweights exactly counterbalance the rods. To set it up to "stock" spec, all they have to do is to install a part with the "stock" moment of inertia (the unit of measure for "balanced-ness"), and make the crank and rods perfect in conjunction with it; then any other stock part that bolts up to it will also match.

What you DO NOT want, is your flex plate or flywheel or damper modified. Because then, they're NOT stock any more, and the rotating assembly WON'T be stock either, and you'll have one-off parts that don't interchange with anything. The simplest way to avoid that is to just not take those parts along. The shop DOES NOT need them in order to balance the motor; and with them absent, your odds of getting the job done right are much higher.

 

I am also at the point of figuring what flexplate i need for my 383 internally balanced engine, its mating up to a 700-r4, and know i need the neutral or zero balance flexplate and i found one on summit : its a quicktime one for a neutral balance engine 400(6.6L) but it comes in two sizes... the 10.75 inch and 11.5. How do I know which one I will need?

 

Even an internally balanced engine that uses a neutrally balanced flexplate and dampener should take everything to a machine shop when the rotating assembly is balanced.

My neutrally balanced flexplate was off by 8gm.

Internal/external balancing and rotating assembly balance have nothing in common.

 

So I have and "internally balanced" 1-piece rms 357 w/ stock rods and and a stock crank (from a different engine) but aftermarket pistons. I have the summit racing internally balanced dampener. I just purchased and Edge racing 3400 10" for a 700r4.

Andre said to just purchase a flexplate for the same year engine and tranny I have(1987). I am unclear though if I have a 10.5" or a 10.75" bolt pattern. Will my stock flexplate work for my new motor? It is a 1987 305 tpi.

Anymore info on this subject will be helpful.

Thanks

 

I'm not sure who Andre is, but tell him thanks from jmd for leading you towards stock-compatibility parts.

All 1986 to 2003 305 and 350 flexplates interchange with regard to balance.

Yes, your stocker works.

 

I know this is a cheap one but will it do the trick or break with my combo? It is basically a stock replacement. I wouldn't mind spending an extra $20 to get something a little better but they all seem to have the 10.75 bolt pattern for LT1's and not the 10.5 bolt pattern for my 87'.

http://www.jegs.com/i/Pioneer/787/FR...ductId=1524865

Yeah I would prefer to just buy one now I guess rather than use my stocker bc the engine is still at the machine shop. This way he can finish installing the flexplate and I can put the burden of having everything properly balanced on him rather than me. This along with my distributor and base timing adjusted and this will be a drop in deal I would think.

Could someone please state that it doesn't mater if the flexplate description describes the application as "internal" or External" they are both the same. I am just a little unclear on this. Is this just a marketing gimmick when these terms are used on 1-piece RMS engines?

 

My 383 assembly is "internally" balanced and requires a neutral balanced flexplate. All 1-pc RMS flexplates have a counterweight built in so to use one I needed to drill out the counterweight. When I switched to a T56 I had the clutch and flywheel neutral balanced after removing the bolt-on counterweight. I would double check to see if you need a neutral or counterweighted flexplate. If you use the wrong one there will be a very noticeable engine vibration that will quickly destroy main bearings.

 

Hello 91 1LE,

I too have an internally balanced 383 sbc. I am changing to a LT1 T56. Isn't the stock LT1 flywheel neutral for and internally balanced engine? Do I need to have it checked? Also do I need to have it checked with the clutch assembly?

thanks

 

If the original one isn't broken, your machine shop can look it over and you could use it.

All stock 1986-2003 production 305 and 350 engines are internally balanced, with an external weight on the flywheel / flexplate. So yes, "external" and "internal" tend to confuse the issue.

Good point. I'm not sure why people go for a setup that requires a non-stock compatible custom part when they could have balanced it to an 86-up stock one.

Assuming it's an 86-up engine, it could be internally balanced to a stock 86-up SBC/LT1 flexplate which has a weight on it. The stock 86-up LT1 / SBC flywheels/flexplates are not neutral balanced and are part of a balanced rotating assembly.

If your current flexplate / flywheel works, figure out what it is and match the balance of the new flywheel to it. For example, if it's a stock balance 86-up SBC/LT1 flexplate /flywheel, then the 93-97 LT1 flywheel will work fine.

 

Trust me, this wasn't my choice. I bought a complete forged rotating assembly that was factory balanced. After paying my engine builder to check/rebalance the assembly he discovered that it required a neutral balance flexplate. At first I agreed with what you said about using stock compatible parts but then I realised it doesn't matter. Since my engine is "self-contained" balance-wise, anything I bolt to it (clutch/flywheel/flexplate) needs a neutral balance which any machine shop can handle.

 

True that. So it's a zero overbore 383 basically? Few 377s out there; had to ask.

 

Exactly what it is. I just call it a 383 so I don't have to answer a bunch of questions! And most people who have heard of a 377 assume it's a destroked 400. After 120k miles my block showed no signs of wear so we honed it and ordered pistons that were .002 oversize to curb any cooling issues with an overbored block.