VW/Porsche factory balanced (crank) good enough?

[BugMan114]

Oh. gotcha. that makes sense. thanks for clearing that up
_________________
1974 Super Beetle: Custom resto in progress
1972 Super Beetle: Daily Driver
1971 Std. Beetle w/ 1929 Mercedes Benz Gazelle kit
1960 Baja Bug
1969 Baja Bug
Sand Rail- Homemade
Sand Rail- FUBAR

Aircooled Airheads

Why the hell do they call it a gland nut. its obviously a big fat bolt!!!

[ALB]

Jake and Nick (or anyone?); Would a crankshaft with one rod journal .005" out of index be difficult to balance? Specifically, once the rod and piston were hung?
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On a lifelong mission to prove (much to my wife's dismay) that Immaturity is Forever!!

[modok]

If the stroke is not the same on all 4 throws than yes, all that fuss to get it balanced to half a centimeter-gram could be wasted when you put it together.
If #3&2 were different stroke than #1&4 this would certainly throw you off, and it is certainly possible for the crank to be ground this way.
Balancing with bob-weights would help compensate, but is silly.
To be honest 5cmg or 1cmg imbalance is not going to make a big difference. If someone says 1cmg is a big deal they are just wanking.
Just cut the factory specs in half and call that hp spec.
Balancing is important no doubt, and things can be waaaay off.
But I don't think you will notice the difference between perfect and almost perfect.

You want reality?
How bout this-- I can see how you want #1&3 pistons to weigh the same, and #4&2 to be the same. But do all four need to be the same?
not really
Flat engines are the only ones that can do this so nobody knows.

[thepadawan]

your crank is in all reality fighting against itself, and that one journal will acutally be the weak spot in the crank. you should send it back, or take it to who ever machined it. its junk.

[modok]

Junk? I don't know about that.
I think realistically all the strokes should be +/- .0015".
It will be hard to get closer than this with common equipment.
So .005 is just a monday morning at the plant.
Unfortunate, but understandable.
Momentum goes up at the square of speed, so balance becomes INCREACINGLY important at higher rpms.
Just don't put that monday crank in a screamer, and you will be ok.

[nsracing]

I will let someone else hammer MODOK.

[nsracing]

Here is the formula for the tolerance.

Tolerance= 2.4 x weight (ounce) / RPM

So: If the crank weighs 70 lbs.

= 2.4 x 70 lbs (16 ounces) / 5000 RPM
= 0.5 ounce inch

So you need at least 0.5 ounce inch tolerance to spin a 70 lb crank 5000 RPM. Obviously, you will not need 0.5 ounce tolerance if you are only going to spin it 3000 RPM. So the balance can be worse.

[modok]

Nice to have NSR with us this evening!
Thats a fine explanation if I ever heard one.
What do you mean have someone else hammer me? You slackin?
I am way out of my element here so feel free.

I'll attempt to figure it out.
Rod big end weighs what, 15 ounces?
I'm gonna say we got two throws .005 short both on the same side.
So when we put rods on it we have 2 big ends 15 oz each .005" farther out.
I am really stretching my imagination here, but is that about 0.15 of imbalance?
That does sound like splitting hairs, I guess it doesn't really matter.

But wait.......................
28 gram = 1 ounces
2.54cm=1 inch
so 70cmg=1oz-in
Factory tolerance for the crank is 12cm-g, so is that 0.17 oz-in?

I may have screwed up, but it looks like our china crank is almost thrown out of factory spec when you put rods on it.
What do you think, did I forget a decimal point or something?

[nsracing]

Okay, I will hammer you right now. 1 ounce inch balance on factory cranks? I DO NOT THINK SO. It is way higher than that... 4x or so.

And what is "cmg"? What unit of measurement is that?

[modok]

I admit I am pretty clueless here, but the manual says crank: max 12 cmg. It is the metric equivalent of inch ounces.
I will check another manual because that seems too small.

[nsracing]

1 ounce = 28.349 grams

So your 12 cmg = 0.423 ounce ...or 12 grams at 12 cm distance

Didn't I tell you it is about 4x??? Who's your daddy?

[modok]

Okay. My ancient Henry Elfrink "1600" manual says factory spec :
8cmg
(11oz.in.)

That makes more sense, whew.
I just totally screwed up the conversion! I multiplied where I should have divided.

[nsracing]

8 grams = 0.28 ounce

You are up too early, MOdok.

and 11 ounces = 311 grams! that is half the weight of the rod.

[modok]

I have polished off half of the Samuel Adams sampler.
Hofeuly all this will make sense in the morning.

[modok]

Wait...................Ol henry forgot the decimal point back in 1969
Crank:
8cmg
(.11 oz.in.) <(I added it for him)

flywheel
5cmg
(.07 oz.in.)

clutch
15cmg
(.21 oz.in.)

So in this manual 1oz.in.=70cmg
funny, thats what I got, but it seems imposiby tight tolerance!
Who makes up these numbers?
Perhaps I can find another older manual.

[modok]

Without Guesswork August 1969- no listing for balance
Without Guesswork October 1971-crank unbalance max 12cmg

There you have it. The Germans say 12cmg, they sure do like their accuracy! Since about 1970. The pride of Germany laughs at your 0.2oz.in.

If you can beat the Germans at their own game you're doin GOOD.

However I would wager 90% of the cranks do not meet these specs, so you should visit NSR immediately to return your engine to 1970's balance specs.

[nsracing]

Now why would the Germans balance the crank to 12cmg (under 0.2 oz)and hang rods and pistons w/ balance tolerance variance of 20 grams???

That does NOT make any sense whatsoever.

All racing circuits to include Formula I, INDY, NASCAR..so on w/ cranks turning 9000+ PRM say 0.2 ounce inch tolerance on crank and 0.1 grams pistons/ rods.

[ALB]

NSR- Thanks for the explanation. Well written and very informative. You and Modok have fun with this one .

Padawan- It's been part of the VW high performance scene since a place called Claude's Buggies started importing stuff from a certain South American country. About 30 years ago I put a Brazilian cast counterweighted crank in a motor and had to cut 3 of the 4 piston tops .010" to get deck heights close. After 15 or 20,000 miles I sold the crank, went with more cam, big valve ported heads, 44IDF's, merged exhaust and a Berg counterweighted crank and surprise, surprise, had to cut the 4th piston. Deck height was within 2 or 3 thou all the way across with that crank. Now we're having the same problems with stuff coming from China, and the Bergs don't make cranks (or much else VW) any more. What a shame.....

But I digress.... my point was that it did run, but was a SHITTY PART. I bought a cheap crank once, and learned a valuable lesson.
_________________
On a lifelong mission to prove (much to my wife's dismay) that Immaturity is Forever!!

[modok]

I am awake and sober now.
I did read the manual correctly, they all say 8-12 cmg.
I admit, it does not make sense for them to balance the crank and flywheel to that level of precision, it's hard to believe.
Perhaps they did it so any pressure plate and flywheel can be bolted on any way you like without throwing it too far out of balance.
Sense or no sense it does explain where the "myth" came from.

[nsracing]

Maybe Jake will chime in on this, but I am yet to see those tolerances you mentioned, Modok.

There gotta be some typo on that. Schenck were the first balancing people...and yes German. CAn you believe 1900s?

There are other balancing machines that can measure way low for rotors built for high speed motors or grinders, or turbos or really small parts, but you will not see them in garages. Costs of these machines can finance an army.