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Engine SwapEverything about swapping an engine into your Third Gen.....be it V6, V8, LTX/LSX, crate engine, etc. Pictures, questions, answers, and work logs.
I've been looking at different engines lately and I was just wondering what the difference was between flywheels on the newer and older sbc. Are they interchangeable with each other? Do they all fit the same clutches and will they all fit in the same bellhousings? Do they need to have different starters?
The basic difference between one piece and two piece rear main seal engines is in the balance. Two piece rear main seal crankshafts are internally balanced and use a neutral balance flexplate. One piece rear main seal crankshafts rely on a weighted flexplate or flywheel for balance. Flexplates/flywheels for the two are not interchangeable. Ring gears are either 153 or 168 tooth on either one piece or two piece flexplates. It's the number of teeth(diameter) that affects starter compatibility.
Flexplates/flywheels for the two are not interchangeable.
Because of the balance differences , the two styles have a different bolt pattern to the crank to prevent them being swapped.
Other than that there is no difference as far as related parts go
Two piece rear main seal crankshafts are internally balanced and use a neutral balance flexplate. One piece rear main seal crankshafts rely on a weighted flexplate or flywheel for balance.
Thats backwards. One piece RMS cranks are all internal balance. 2-piece RMS cranks are all external balance.
The basic difference between one piece and two piece rear main seal engines is in the balance. Two piece rear main seal crankshafts are internally balanced and use a neutral balance flexplate. One piece rear main seal crankshafts rely on a weighted flexplate or flywheel for balance.
No, no, NO! The basic difference is the crank flange for mounting the flexplate or flywheel. THAT'S ALL THAT IS IMPORTANT! That is what makes the two non-interchangeable.
Originally Posted by 89_RS
Thats backwards. One piece RMS cranks are all internal balance. 2-piece RMS cranks are all external balance.
Wrong again. They are BOTH internal balance!
The 1-piece rear main moved the external counterweight that was built into the 2-piece rear main crank to the flexplate/flywheel. That does NOT make 1-piece "external" balance (or 2-piece, for that matter). Both use a neutrally balanced harmonic damper (except as noted below), which is really what distinguishes "internal" from "external" balance.
The factory used an additional weight on the harmonic damper and flexplate/flywheel for the 3.75" stroke cranks to achieve rotating assembly balance (factory 3.75" cranks were all 2-piece, and all had that exact same counterweight built into the crank external to the RMS that the shorter stroke cranks had). They did this due to space limitations within the crankcase for counterweights (without using "heavy" metal such as mallorymetal). This is the true "external" balance, and only that.
If the damper is neutrally balanced, the engine is "internal" balance. No ifs, ands, buts, or exceptions.
To explain further, you can't slip a 1-piece rear main seal over the back of the crank flange and that built-in counterweight. The output flange of a 1-piece RMS crank is constant behind the rear main bearing journal to the flange.
To explain further, you can't slip a 1-piece rear main seal over the back of the crank flange and that built-in counterweight. The output flange of a 1-piece RMS crank is constant behind the rear main bearing journal to the flange.
One-piece:
Two-piece:
I'm assuming that if I read the picture left to right it is rear to front on the crank correct?
The 1-piece rear main moved the external counterweight that was built into the 2-piece rear main crank to the flexplate/flywheel.
That does NOT make 1-piece "external" balance (or 2-piece, for that matter).
The essential element here is the weight position was MOVED
The popular misconception arises from aftermarket makers calling anything with a weight on it "external" balance
( as opposed to neutral balance ) which relates to the flywheel / flexplate only; not the actual engine balance style.
The two piece RMS SBC engine is completely balanced with no external counterweights needed. To call the one piece RMS engine internally balanced is misleading and incorrect as it requires a counter balanced flexplate to compensate for the smooth rear flange. If it were truly internally balanced, like its predecessor, it would not need an external counterweight. Yes, the one piece crankshaft takes the same harmonic balancer as the two piece, internally balanced crankshaft. Excuse me forgetting the difference in the bolt pattern.
They did this due to space limitations within the crankcase for counterweights (without using "heavy" metal such as mallorymetal). This is the true "external" balance, and only that.
Well, couldn't you say the same thing for the one piece?
Five7 and vettoz are correct; ALL SBCs except the 400, including the 1-pc RMS ones, are internally balanced.
The terms "internal" and "external" balance DO NOT refer to which part the counterweight is attached to. The weight on the flywheel of a 1-pc motor, while it might look "external" to the untrained eye, is actually an INTERNAL weight; specifically, it is exactly the same INTERNAL weight that used to be provided by that funky shaped flange on a 2-pc RMS crank, it just had to be moved to allow the seal to slip over the crank. This DOES NOT make it "external".
Best to AVOID those words altogether and not fall into the confusion.
To answer the OP's question ACCURATELY, the flywheels or flex plates DO NOT interchange, which is why there are 2 different kinds. (duh) The differences are the bolt pattern, the ID of the big hole in the middle, and the presence of the INTERNAL counterweight on the 1-pc ones and absence of it on the 2-pc ones.
Five7 and vettoz are correct; ALL SBCs except the 400, including the 1-pc RMS ones, are internally balanced.
The terms "internal" and "external" balance DO NOT refer to which part the counterweight is attached to. The weight on the flywheel of a 1-pc motor, while it might look "external" to the untrained eye, is actually an INTERNAL weight; specifically, it is exactly the same INTERNAL weight that used to be provided by that funky shaped flange on a 2-pc RMS crank, it just had to be moved to allow the seal to slip over the crank. This DOES NOT make it "external".
Best to AVOID those words altogether and not fall into the confusion.
To answer the OP's question ACCURATELY, the flywheels or flex plates DO NOT interchange, which is why there are 2 different kinds. (duh) The differences are the bolt pattern, the ID of the big hole in the middle, and the presence of the INTERNAL counterweight on the 1-pc ones and absence of it on the 2-pc ones.
Hold on I'm a little confused here. I've been looking at the bolt holes on the 1 piece and 2 piece flexplates and they look exactly the same:
1 piece:
2 piece:
the 2 piece has smaller holes drilled on the outer part and the 1 piece has just a few bigger holes on the outer. That's just a weight balance thing I guess? (it's 2 different brands).
Also I just checked my flexplate that was on my 2 piece 350 and it looks like it has the same bolt pattern as these two?
Here is a 1-piece RMS flexplate showing the weight.
It's often called a "bat wing" weight because of its shape.
Summit improperly lists the flexplate as "external balance". Look up the same p/n on sdparts.com and you'll see "counterweighted", which is more accurate. No mention of "internal" or "external" balance.
The real key is 1- or 2-piece RMS, which both Summit and sdparts properly designate it as 1-piece.
(It is also noteworthy that your pics were of the opposite side of the flexplate, so those views would not show the counterweight.)
And if the flexplate is still in the car you don't even have to take it out and measure it.
A 153 tooth FW takes the starter with inline mounting bolts;
A 168 tooth has the starter with offset bolts
OK, my head is spinning (out-of-balance) after reading all this. I have read several forums on the explanations and remain confused. But here is my question:
I bought a 383 stroked IROC with auto 700R4 tranny. It ran great (smooth also). I decided this winter to do a T56 swap from a 97 Camaro.
First step, found out the flywheel would not fit because it was from a 1-pc rear seal. So I did my homework and found I need the 2pc RMS based on the bolt pattern.
I bought the Ram Clutch #798-2555 alum neutral balance flywheel. Once I bolted it on, I started the car and it felt rough. The dummy (me) then when on to complete the installation and my first trip down the street I knew it was out-of-balance.
Went home and checked my flexplate that was previously installed and it has a counterweight that is about 6" long on it and a single hole opposite 180deg.
So, it looks like my new neutral balance flywheel needs to have a counterweight to match the old flexplate counterweight to get balance back in my life. BTW, it also moved me from the 14" flexplate, to 12.8 flywheel.
So I am guessing in can carry the two somewhere and they will know how to put a counterweight on my flywheel to fix the problem.
Question: short of disassembling the motor, how can you determine what amount of counterweight is needed on the neutral balance flywheel other than using the original flexplate counterweight?
Interesting, so it's an external balanced crank like a 400 would have? I thought the trend nowadays was to buy a "stroker" crank that was internal balance.
yep, I was too pumped about getting this thing running and I didn't even think about this being a 383 stroker instead of a 350.... so now, I have bought the wrong flywheel twice ($79, $435 .... ). I am going to try to find a machine shop who will put a counterweight on a neutal balance flywheel. We will see if I am successful. If now, I have another flywheel for sale if someone is interested.
Interesting, so it's an external balanced crank like a 400 would have?
I thought the trend nowadays was to buy a "stroker" crank that was internal balance.
The lower cost stroker kits use a 3.750" crank based on a 400 crank casting (w/ 350 journals ) so they still need the true "external " balance parts a 400 does
I didn't even think about this being a 383 stroker instead of a 350...
I am going to try to find a machine shop who will put a counterweight on a neutal balance flywheel.
If you have 2 pce RMS crank with a counterweight weight then it should be 400 balance ? ( unless someone did a weird hybrid balance job )
No, no, NO! The basic difference is the crank flange for mounting the flexplate or flywheel. THAT'S ALL THAT IS IMPORTANT! That is what makes the two non-interchangeable.
Wrong again. They are BOTH internal balance!
The 1-piece rear main moved the external counterweight that was built into the 2-piece rear main crank to the flexplate/flywheel. That does NOT make 1-piece "external" balance (or 2-piece, for that matter). Both use a neutrally balanced harmonic damper (except as noted below), which is really what distinguishes "internal" from "external" balance.
The factory used an additional weight on the harmonic damper and flexplate/flywheel for the 3.75" stroke cranks to achieve rotating assembly balance (factory 3.75" cranks were all 2-piece, and all had that exact same counterweight built into the crank external to the RMS that the shorter stroke cranks had). They did this due to space limitations within the crankcase for counterweights (without using "heavy" metal such as mallorymetal). This is the true "external" balance, and only that.
If the damper is neutrally balanced, the engine is "internal" balance. No ifs, ands, buts, or exceptions.
I was wondering if you could please have a look at this thread I started and help sort it all out and explain why there are 2 different flywheels for the same application... the neutral balanced and counter balanced and shed some light on situation. TIA
If that's the thread I think it is, I already did.
Same info as is already in this post. Which is, all the info there is on the subject.
Put all of that crap about "internal", "external", "neutral", "counterblanced", etc. etc. etc. OUT OF your vocabulary. Forget you had ever heard ANY of it.
You still need a STOCK (as in, configuration... not necessarily "GM" from the stealership) flywheel for 86-92 Camaro/Firebird V8. Heavy or light, take your pick. There is ONLY ONE "stock" balance configuration; and that's what you need. Any part that conflates the issue by piling ANY OF those other words on top, STAY AWAY FROM. They will only get you into trouble.
Old thread and correct me if I'm wrong but i was under the impression if any weight needed bolted or attached to the crank to balance it was considered external, internal meant all the weight needed to balance the crank was made into the crank. As a kid I thought internal meant inside the crankcase. I see the 1pc advertised internal + external, front half internal back half external. Is this correct?
As a kid I thought internal meant inside the crankcase
If I may be allowed to quote a source of wisdom far beyond my own level:
"When I was a child I thought as a child... when I became a man I put away childish things"
People think all sorts of odd things when they are kids. Doesn't somehow obligate reality and the world at large to conform to their "thinking" until they learn better.
Balance isn't all that hard to understand if you put away childish things and LEARN. I'll post this again, since it seems to get lost in the depths of history after awhile.
Consider the simplest possible engine: a 1-cyl, like, say, a lawn mower. You have one piston and rod. Imagine that we're looking at it with the crank horizontal and the piston moving vertically, for clarity. Obviously this is gonna vibrate like all hell without a counterweight, at any kind of RPMs. So let's stick the simplest possible counterweight on it (leave aside that it needs to be different weights at TDC vs BDC for the moment). This counterweight would be along the crankshaft one way or the other, to the "side", so to speak, of the rod journal. Let's size it such that if the piston was halfway between TDC & BDC, the whole thing would be "statically" balanced; i.e. when at rest, the piston would tend neither to fall because it was heavier, nor rise because the counterweight is heavier. Would this cure the vibes?
NO. Because at any kind of RPMs, the piston will be tending to make the end of the crankshaft that it's hooked up to, fly off in one direction, while the counterweight will tend to make its end fly off in the opposite direction. The crank will try to "wobble", FURIOUSLY. Clearly we have to do better. We're going to have to "dynamically" balance it: that is, make it so that when in motion, it still has no (or at least, minimal) tendency to vibrate in any mode.
So, let's make the first improvement to our counterweight. Let's split it in half, and put one half on each side of the rod journal, and try again. NOW what happens?
Right: while there's still no way to statically balance the whole assy at more than one point in its rotation, AT LEAST the crank doesn't "wobble" anymore. While still not "perfect" - the lawn mower will still vibrate somewhat - it's AHELLUVALOT better than the first scenario. OK we're good now, we can sell lawn mowers, and customers won't bring them back because they make their hands go numb within the first 5 minutes of trying to use them.
OK, so now let's take a step up in complexity. Our mktg dept says we aren't making enough power yet; our customers want a faster lawn mower, we gotta step it up. Let's add a 2nd cylinder. Just for funsies, let's put this cylinder right next to the first one, beside it along the crankshaft, and with its rod & piston directly opposed to the first one. (#2 piston at TDC when #1 is at BDC & vice-versa) Using the same logic as above, even though this would statically balance all by itself with no counterweight at all, its dynamic balance would be AWFUL. So, what to do? Aha, let's take the same approach we did before: let's give #1 a counterweight that balances it as perfectly as possible with half of it on each side of the #1 rod journal, and do the same for #2. Make sense so far? Once again, it's not going to be perfect, because the exact correct counterweights for each one are going to be different when the piston is at TDC vs when it's at BDC; but at least we have something useful.
But now we have 2 counterweights in the middle, which since the pistons are identical and opposed, are also identical and opposed. Do we really need those? Well, not really, with one important caveat: if we leave them off, the "unbalance" that this creates, will be constantly trying to BEND the crankshaft. If it was flexible, it would "wobble" again, except instead of the whole thing "wobbling" together, it would flex in the middle like a jumprope for 2 kids at the same time with the middle held still. Can we tolerate this? Well, maybe; we'd need to make sure the crank is strong enough to withstand this bending action, and maybe put an upper RPM limit on it, aka "redline". OK, we're good now, off to the factory we go to make millions.
Alright, now that we have that much accomplished, let's take the next step up in complexity, because this thing is kinda yyyuuuuuuujjjje, and if we could somehow move the cyls closer together, it would be ALOT eeeeezzzzyer to package it onto a lawn mower if it was more compact. Let's make a "V" motor, with the cyls at 90° to each other. That shrinks it pretty drastically. Butt now, those counterweights in the center don't cancel out anymore; we still need a CW at each end to balance out half of #1's imbalance, opposite its rod journal, and one opposite #2 to cancel out half of its imbalance at the other end, right? BUTT: these are ALSO no longer opposite each other on the crank; they're at 90° too. So what happens to the ones in the middle? They don't just cancel out anymore, do they? Well, we can combine those 2 into a single, butt we'll end up with some amount of weight remaining that DOESN'T completely cancel out; this new weight will be aligned opposite neither #1 or #2 rod journal, but rather, halfway in between. The correct amount of weight will still fit between the journals, so this motor is INTERNALLY balanced. All the CWs are INTERNAL to the rod throws.
Are we good so far? Take your time and get your brain wrapped all the way around this, because it gets more complicated as we go along.
OK, so our next step is, our customers LOVE our lawn mowers, they make plenty of power and run smooth; butt since time is money, now they want to be able to mow twice as many acres per hour. So, what to do? Suddenly we need twice as much power. Well, that seems straightforward enough; let's just bolt 2 of these things together. OK fine, butt, why do we need 2 complete crankcases, 2 individual crankshafts, 2 blocks, 2 cams etc.? We don't really, do we? So let's make a 4-cyl. Furthermore, let's make them in a V again, with 2 cyls on one side and 2 on the other; and just for Ss & Gs, let's make the crank have 4 journals, one every 90°. Well guess what... to balance this new V4 thing, we have to do exactly the same thing that we did for the V2: between each pair of rod journals, there will be a counterweight of some intermediate size less than what it would take to completely balance 2 rod & piston assys, and offset to somewhere in between the directly opposite points of each of those pairs of journals. Our weight system is still INTERNAL: our CWs still fit in the place along the crank that opposes the weight they're supposed to balance against (the rods & pistons), as close as possible to that weight. Our engine is still INTERNALLY balanced: the CWs are right where they belong for minimal forces trying to tear the crank apart, and they fit between the rod journals and underneath the pistons.
Sales are great, customers LOVE our lawn mower, butt customers being what they are, they're GREEDY. They want to double their acres per hour YET AGAIN. We need more power Scotty. What to do? Simple enough: let's double the # of cyls again. Let's make a V8. Butt weight: do we REALLY need 8 crank journals? Couldn't we make each rod journal longer, and attach 2 pistons to each one, with one associated with the left bank of 4 cyls and one with the right? That makes sense, let's do that. What about counterweights? Well, we'll do the same thing, to minimize stress in our crankshaft: we'll find the weight that each rod journal's pair of rods needs, which since they're at 90° to each other (not opposed and not inline either), will be some other amount of weight; and since each of these new CWs except for the 2 end ones will still be between 2 cyl pairs, they'll have to be combined like we've been doing all along since we went to the V configuration. And we're still INTERNALLY balanced. Our CWs are still located along the crank right where they need to be to minimize harmonic vibes in the crank, which of course is, INTERNAL to the rod journals.
The customers still aren't satisfied. They want to mow still MORE acres per hour. OK fine, we need more power yet again; so what to do? Well, obviously, we can increase the displacement of our engine, by increasing its bore and/or stroke. Butt, what are the limits to that? how far can we go before we have to redesign a whole new engine (IOW our "small block" is no longer big enough)? We can increase the bore as far as we dare until the walls between the cyls are as thin as we think we can get away with; and we can increase the stroke. How far can we go with the stroke? Well, it's obvious that the stroke can't be greater than the height of the cyl casting in the block; butt can we even go THAT long with it? What are the limits?
Remember that the counterweights are somewhat opposite the rod journals. (duh) Not exactly opposite, but regardless, when the pistons are at TDC (all the way up) the CWs are at BDC (all the way down); and when the pistons are all the way down, next to the crank at BDC, uh-oh... the CWs are all the way UP. Pistons down, CWs up. What's eventually gonna happen as we increase the stroke? You guessed it: the CWs have to be bigger; AT THE SAME TIME AS the pistons come down farther. Eventually we're gonna get to a point where the CWs hit the bottom of the pistons, right??? What are we gonna do now?
Well, we can get creative; we could drill out the rod throw so that it weighs less, we could increase the rod length so that the small end of the rod is farther up in the piston and the piston therefore can be shorter which might leave us some more room, and maybe acouple of other tweeeeks; butt sooner or later, as we increase the stroke, we no longer have enough room in between the rod journals, to fit enough CW to properly balance the motor. (unless of course we make the crank longer to allow thicker CWs which then means we have to make the block bigger, or non-homogeneous, i.e. of different materials with different weights in different places, i.e. use "heavy metal" in the CWs, i.e. $$$$$$$, or SOMETHING unpleasant like that) What to do? The ONLY solution we have left is to shave some metal off of the "top" of the INTERNAL counterweights, and move it someplace that is NOT directly opposite the rods that it's supposed to be balancing; IOW, this will have to be weight that is NOT INTERNAL to the crank. It must be something else.
In English we have a word that means "not internal". This word, conveniently enough, is EXTERNAL. Somehow this new thing will have to be EXTERNALLY balanced. Note that this new terminology says NOTHING WHATSOEVER about exactly HOW, physically, we locate this weight; all that the word "external" means in this context is, that the counterweight is no longer in the correct location to dynamically balance the crankshaft, i.e. no longer between the rod journals, i.e. no longer "internal". It MIGHT still be on the crankshaft somewhere; it's just NO LONGER UNDER THE PISTONS. Butt since the crank is no longer properly balanced, the stress within the crank from it trying to bend itself, DRAMATICALLY increases; which means we've got to beef it up by making it thicker or using higher-$$$$ metal, or lower the "redline", or SOMETHING. For this reason, EXTERNAL balance is considered inherently inferior to INTERNAL balance.
The 400 is THE ONLY small block Chevy that's "externally" balanced. ALL OTHER SBCs - 262.5, 265, 267, 283, 302, 305, 307, 327, & 350 - are 'internally" balanced. In all of those smaller motors the crank CWs are ROUND; there's enough room under the pistons to fit the proper amount of CW. All butt the 400, whose CWs are NOT round. Here's a pic of a 400 crankshaft showing the amount of metal they had to shave off of the CW to keep the pistons from hitting it.
Note the "flat cut" on this CW: the CW is round for part of its circumference, but then has a "flat" area cut into it. You can walk into a warehouse of SBC cranks and spot a 400 one from 100 yards away because this feature is so distinctive. ALL of the counterweights on a 400 crank have this feature, because they are ALL under pistons, and lack enough room to be round.
Last edited by sofakingdom; 10-27-2023 at 12:28 PM.
Now: if it was possible to locate the weight that had to be moved from the INTERNAL CWs to somewhere EXTERNAL, i.e. no longer under any pistons, butt keep it inside the motor (like, say, create a super unbalanced crank timing sprocket), we could have an EXTERNALLY balanced motor, that uses the same crank damper as all the others, and is thus VISIBLY IDENTICAL from the outside. This would still be an EXTERNALLY balanced motor, but the EXTERNAL unbalance would still be INSIDE the oil pan and INVISIBLE. OTOH since we're not kids anymore, we don't "think" that something isn't there just because we can't see it, or that for something to exist, we have to be able to see it.
As it happens, this isn't practical (a VERY different thing from "possible") in the SBC, without either other changes, or GOBS of $$$$$, or something; so the factory took the eeeeeezy way out and let it be visible at the front of the engine. They located the EXTERNAL weight at the front of the crank, on the crank damper. To the untrained (a kid's) eye, this makes this weight appear visibly "external". Butt it DOES NOT HAVE TO BE VISIBLE. The "external" weight COULD THEORETICALLY BE within the confines of the engine's exterior features. Here's a 400 crank damper.
They "unbalanced" the damper by leaving metal OFF OF the inertia ring, as opposed to ADDING MORE metal to the other side. Butt, since the inertia ring isn't metalically connected to the crank, these things are NOTORIOUS for getting vibes when the inertia ring spins on the hub, as they INEVITABLY DO (taking the timing mark along with it, you should never forget). Butt remember, this weight COULD BE inside the oil pan if there was a place to put it in there, and the motor would STILL be "externally" balanced.
We have the same situation at the other end of the crank, except in reverse. In the short-stroke SBCs - 265, 283, 302, 307, & 327 - ALL of the CW fits inside the oil pan, and the rear flange of the crank is round and "visibly" balanced. BUTT in the long-stroke SBCs - 262.5, 267, 305, 350, & 400 - some of the rearmost CW DOESN'T fit inside the pan, without making other changes. It theoretically COULD; that is, it's "possible"; it's just not "practical". The solution they chose was, to put it on the flywheel flange. That funky flange you're used to seeing on the old 2-piece rear main seal motors is ACTUALLY a crank CW. Even though it is physically "external" to the oil pan, it is still INTERNALLY balanced (other than 400), because none of the INTERNAL weight between the rod journals, had to be moved out of that place and to somewhere NOT under a piston and directly opposite the weight it is CW'ing. This weight is located EXTERNALLY (not under a piston), but the motor is still INTERNALLY balanced. Here's a pic of that type of flange. This one happens to be a 400 crank - note the 509 casting # which is a 400 block - butt the other long-stroke cranks have the same thing. That funky flange is A COUNTERWEIGHT, external to the oil pan. Butt since the CWs that are supposed to be INTERNAL still are, even motors like this, with the rearmost CW located EXTERNALLY to the rod journals & pistons, are still INTERNALLY balanced (except of course, the 400). In fact I would say that this style of flange, even though part of INTERNAL balance in all of those except the 400, is every bit as EXTERNALLY VISIBLE as the one on the 400 damper in that pic. Would you not agree?
BUTT WEIGHT, THERE'S MORE! The newer 1-pc RMS motors, ALL of which are long stroke, can no longer have that funky shape flange, even though they need some CW at the flywheel end of the crank; the flange has to be ROUND again, so that a seal can go over it. Which means then, they had to take that little EXTERNALLY located bit of INTERNAL weight and move it to the flywheel (see five7's pic up there); which, to the untrained (a kid's) eye, "looks" EVEN MORE "external". IT IS NOT. The engine is still INTERNALLY balanced, because the weights that belong in between the rod journals, are STILL INTERNAL, in between the rod journals.
Butt the general marketplace for these things, while perhaps not "dominated" by kids who haven't put away childish things yet, is certainly FLOODED with them; to the point that many if not most vendors who make these parts have GIVEN UP trying to use the correct terminology, and just kinda go with the flow. (flood) Unfortunate that they are overwhelmed (flooded over) with ignorance, butt such is the human race. In practice this means that those "ternal" words mean DIFFERENT THINGS to DIFFERENT PEOPLE and even at DIFFERENT TIMES depending on the context. One mfr or machine shop or customer may define them properly while another may still be blighted by the ignorance or have succumbed to it out of frustration, or may even allow them to be mixed, as in an "internally balanced" motor that needs an "externally balanced" flywheel, so then what's the whole thing called?. Danger danger danger. Thus, mixing & matching parts or procedures from different sources, using those words, is a recipe for MISMATCH and DISASTER.
The facts remain then:
- ALL factory SBCs except the 400 are INTERNALLY balanced, including the 1-pc RMS motors with the "bat wing" weight on the flywheel or flex plate
- The factory 400 and ONLY the factory 400 (among SBCs) is EXTERNALLY balanced, because the INTERNAL CWs can't be made large enough, because they'd hit the bottom of the pistons if they were
- The thing that makes the 400 EXTERNALLY balanced is the inability to fit the required amount of CW in the space available between rod journals on the crank without resorting to $$$$$ things like Mallory metal or titanium rods
- A 383 that is built using a factory 400 crank, or an aftermarket one using a crank with factory 400 balance, is EXTERNALLY balanced
- A 383 built with a 1-pc RMS crank (which is ONLY an aftermarket thing: no such thing was ever available in a factory motor in a vehicle although GM does in fact supply OTC motors, essentially aftermarket even though sold by GM, in this configuration nowadays) COULD BE either internally OR externally balanced: you'd have to ask the crank mfr and then wade through the FLOOD of ignorance about what they actually mean when they say those words
- A 400 or a 383, or for that matter even larger engines using very long strokes (427, 434, 454, even CIDs near 500 are readily possible) COULD BE built with internal balance, using aftermarket parts such as longer / lighter rods and/or shorter / lighter pistons and/or heavy-metal inserts in the CWs, which then could reduce either the CW physical size need or the piston thickness or both, enough to provide enough space under the pistons to accommodate the necessary CW
- It's best to avoid the terms "internal" and "external" ALTOGETHER in the interest of clarity and unambiguity, and use words like "stock balance" for whichever type of setup you're building ("stock 400", "stock 305/350 1-pc RMS", etc.)
- And last butt not least, a factory 1-pc RMS 305 / 350 motor is INTERNALLY balanced, even though the rearmost INTERNAL CW is located on the flywheel
Are we clear on this? Then you just grew up. You no longer have to "think like a kid", because now you know better and understand what the terms REALLY mean.
Last edited by sofakingdom; 10-27-2023 at 12:21 PM.
12-08-2011, 02:25 PM
Nevermind. 
)
You no longer have to "think like a kid", because now you know better and understand what the terms REALLY mean. 
