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RE: [T3] Crank/Rod Balancing

On 8 Mar 2002, at 7:17, Erkson, Toby wrote:

> Just to further clarify, you simply can't add/remove material on the object
> haphazardly because the item will become unbalanced.  You want the part to be in
> balance/harmony/ying to the yang :)  When parts start moving in an engine, they
> move FAST, and suddenly a tiny weight difference becomes a BIG difference.  It's
> all physics and mind-twisting stuff like that with plenty of math to scare
> little children away.

Yeah, I realized I should have answered better. Here's how things are done, 
when they are done the best possible way:  

Pistons are weighed and weight removed from the heavier ones until they all 
weigh the same. This is just a simple static balance job.  

Rods are weighed end for end and weight removed end for end until they all 
match. This is just a slightly complicated static balance job.  

Rotating parts are done on a spin balance machine as follows (this is Berg's 
procedure, but few places do it this nicely. To do this right, the crank must 
be spun about the axis determined by its main bearings, so every time you 
mount these parts, you have to dial in the position carefully.)  

mount the crank, spin it up, dynamically balance it.  

add the flywheel, spin them up, dynamically balance them together.  

add the fan, spin them up, dynamically balance them together.  

add the pulley, spin them up, dynamically balance them together.  

add the pressure plate, spin them up, dynamically balance them together.  

mount the clutch disk and balance it dynamically by itself.  

Again, note that dynamic balancing is much better than static, but that few 
of us have access to dynamic balancing machines. I'm afraid this really is a 
job for the pros, and it's an important job to do if you expect your engine to 
survive at high rpms. If you're thinking of doing this careful a job on an 
engine, you should also be installing a counterweighted crank here.   

If you don't want to put this much work into your engine, then you should at 
least know that when VW assembled your engine, the flywheel, pressure 
plate, and crank were each marked (with paint marks, which are usually 
gone by now) as to where their heavy sides were, so the engine assemblers 
put things together with all the heavy sides staggered. The ONE thing you 
should do when you take a virgin engine apart is to mark the relative 
positions of the flywheel and crank. I stamp a mark on the face of the 
flywheel and another at the bottom of the gland nut hole so I can put them 
back together the same way.  

I don't bother with the pressure plate unless I think I might be able to reuse 
it. If you want to mark it, just stamp matching marks near the outer edge of 
the pressure plate.  

Berg stamps a "V", but I like to use an "A".  ;-)  Both make good pointers.  

If you send your parts to Berg for balancing, they will either follow your 
marks, or make their own (if you didn't) so you know how to assemble them.  

Jim Adney
Madison, WI 53711-3054

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