I am building a 383 671 blower motor and i dont understand the difference in crank balancing. I am having a hard time trying to pick out a crank because i dont know the difference between them. does it have to do with the harmonic balancer, and does it make a difference on a blower motor

thanks alot
Marc

Externally balanced engines (ie. stock 400 sbc) use the harmonic balancer and the flywheel/flexplate to help balance the crankshaft. Thus, when the engine is balanced, all three pieces should be balanced together.

Internal, or neutral balance, mean that the harmonic balancer and flexplate/flywheel are evenly balanced (ie. stock 350 sbc). The balancing of the engine is accomplished by the crankshaft coutnerwieghts.

You can build a 383 several ways. You can turn down a 400 crank (external balance). You can buy an externally balanced stroker crank that has the longer stroke and samller main journals. Or, you can buy the same crank as in internally balanced unit. Select your balancer and flywheel/flexplate accordingly.

I'll defer to others on which is the best choice for a blower.

so either way the crank has to be balanced?
thanks alot very inlightening,

If you want to make power, the whole assembly, pistons, rods, crank, etc should be balanced. This is besides your choice of internal or external balance. Its 2 different things

You order your crank, whatever you want, with the correct balancer and flywheel.

You then have the choice if you want the whole assembly balanced or not.

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My Pontiac 400 was balanced when I built it in the early 80's, I took the clutch and flywheel off the other day and noticed it probably could use replacing. But if I do, will this throw the balance off? They drilled a lot of holes in the backside of it getting it lighter and balanced. Can I get just the flywheel balanced?
I don't really want to disassemble the engine that's for sure.

Hard to say.....Did they use the flywheel weight to balance the engine originally? Looks like it.

Is the 400 Pontiac externally balanced.

I always figured that they balanced the recipricating assembly (engine parts) as a unit and that the flywheel etc was neutral.............unless, they are externally balanced where that would come into play.

We have probably totally confused the original poster now..............

Time for MachineShopTom to step in.............................over to you Tom.

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I'm curious, too.

My builder insisted that I give him the flywheel & balancer I intended to use for my 383. He dynamically balanced the crank accordingly. I went with an SFI approved balancer in hopes of preventing future headaches (in case the balancer slipped---like my 400's did right before it proceeded to start eating my main bearings).

Back to the original post, I have been told not to blindly trust the "balanced" rotating assemblies that you can buy. I was lead to believe that the 383 rotating assembly should always be dynamically balanced. It can't hurt - especially if you are putting a blower on top.

I always thought a balanced engine was where all parts are static balanced and then the assembly was balanced as a whole.
When we balance rolls at work we static balance them then dynamic balance them to a certain RPM.

When I built my engine I know the pistons and rods where weighed and matched separately, the crank and flywheel was supposedly balanced separately and then the crank, balancer and flywheel were assembled and balanced as a unit.

What constitutes an internal vs external balance?
I have a hard time believing anytime you change a flywheel you'd have to remove the crank and have everything re-balanced. How is someone suppose to know what type balance they have?
It's very confusing

Hang in there... you just have two different ideas going on here. First, you can have an engine that is externally or internally balanced. Second, you can balance any engine (like you would balance a tire).

Internally balanced means that the crankshaft counterweights alone balance the engine. The harmonic balancer and flywheel are just along for the ride. They spin like perfectly balanced tires on either end of the crankshaft.

Externally balanced means that the crankshaft journals alone do not balance the engine - you must have the flywheel & harmonic balancer attached to act as counterweights for the crankshaft. It is just a design difference.

You can tell the difference between the int & ext balance flywheels & harmonic balancers by looking at their backsides. If I recall correctly, the external balanced ones have an area scalloped out on one side (they are not perfectly symetrical on the backside). There are probably exceptions to this.

You can BALANCE any rotating assembly. For example, my piston/rod combos were each individually weighed and machined to be within a couple grams of each other. This is static balancing. Then, my externally balanced crank had the harmonic balancer & flywheel bolted on each end. My machinist spun them at a pretty high RPM on a machine that worked much like a tire-balancing machine. It told him where to drill out the crank journals (which journal & how much). That is dynamic (moving) balancing.

When you put the statically balanced pistons/rods on the dynamically balanced crank, you have a balanced rotating assembly.

An external balance uses a harmonic damper and flywheel that have a heavy spot on them to assist in balancing the rest of the assembly. A heavy weight at a larger distance from the center of rotation than the crankshaft counterweights themselves allows for adding or subtracting less weight on the crankshaft itself.

Advantages = less expensive/ easier balancing procedure.
Disadvantages = The weight hung on either end of the crankshaft to balance the center of the crankshaft can/ does cause crankshaft twisting forces that might be detremental in high performance applications, even if a proper damper is used to reduce harmonics. = possible broken crankshaft. Other theories are bearing damage or loss of power due to crankshaft twist.

For our intents and purposes, it makes little difference.

A competent machine shop when balancing any engine of either type will NOT alter the damper or flywheel specification weight, only the crankshaft counterweights. i.e. So that any spec external flywheel will interchange exactly.

When my externally balanced 400 started to act up, it began shaking. The harmonic balancer had slipped. Thus, it was like an out-of balance tire going down the road. You know, whomp, whomp, whomp... technical terms...

The end result was the the front main bearings were worn all the way around. The middle bearings were worn on the bottom. The rear bearing was worn on the top. The rotating assembly was shaking itself to death... The cylinder walls were wearing unusually as well.

Had I had an internally balanced engine, the slipped harmonic balancer would have just meant that my timing marks were off. It would have run about the same.

That is why I personally plan on having my rotating assembly rebalanced if I ever swap flywheels. But, that is just my experience...

Interesting. I thought that the balance portion of the damper was locked solid to the crankshaft, that is, independent of the outer rubber mounted damper oscillation ring. Maybe we should go take a look at a 400 damper to be sure.

I think that what you experienced is not a "balance" failure but rather an elastnomer failure that cause the damper ring to slip and thus causing the damper to fail in its job of preventing crankshaft oscillations which in turn destroyed your bearings.

Harmonic dampers are incorrectly commonly termed harmonic balancers. On any engine, even an internally balanced engine they do "balance" but their main purpose is to dampen oscillations of crankshaft twist and whip.

Xntrik - have a close look at the bottom of the pic for this balancer:
http://store.summitracing.com/partd...15&autoview=sku

See the scalloped edge?

I can't argue with that. Either way, I'm playing it safe

You are correct on that one. We always have used aftermarket dampers that do not use elastnomer rings for any performance engine. ATI etc. It has been a long time since I messed with a stock 400. Thinkig back I think it was 98 since I saw a 400 using a stock type damper......