Flywheel balancing wobble issues

[Phil West]

Hi all,

On a recent trip to the balancers to have a crank assembly dynamically balanced the guy rang me to say he couldn't balance the assembly as he picked up an immediate serious flywheel wobble.  So I picked up the whole lot to investigate.  He pointed out the place that was causing the wobble - there was a larger gap in one place between the endfloat shims and flywheel mating surface (see attached pic).

Using feeler gauges I started with say a 5 thou feeler, it went in all the way around (tight at one point).  So my smallest gap was 5 thou.  Then going up on the feeler gauges I could find the loosest spot at 12 thou.  So overall there is a 7 thou variation at the mating surface.

Crank is a new Scat 69mm, standard 8 dowel (8mm dowels).  Bearing is a new original VW bearing.  Endfloat shims deburred.  I have rotated the bearing and shims 180 degrees and still found the tight/loose spots in the same place i.e. it's not the bearings or shims that is the problem.  Also I have swapped out both the bearing and shims to ensure they are not the issue.  300ftlbs torque on the gland nut each time for consistency of testing.

Next I tried three other flywheels of different manufacture, some new, some old.  I have found differences on all flywheels.  The best I had so far was 4 thou tightest and 8 thou loosest i.e. still a 4 thou 'wobble'.

The bearing (thrust face) seems to sit perfectly flat on the crank vertical surface so I'm fairly sure it's not that.

So - has anyone else seen this sort of wobble that prevents their assembly being balanced?  It seems my balancer wants a tiny difference (in gap size around the whole crank) before he can even think of starting balancing as it just creates a huge wobble at the outer edge of the flywheel if there's a few thou difference.  My initial thoughts are that maybe the dowels are pulling the flywheel slightly off, combined with the fact that flywheel mating surfaces aren't machined that accurately?  Any thoughts on how to solve the issue appreciated.
Cheers
Phil

[Phil West]

These pics are for the best result I had.  4 thou tightest and 8 thou loosest gap.
However:
Just tried the flywheels on another crank.  The one that had a 7 thou wobble now only has a 3 three thou wobble i.e. seems to fit better on another crank.  So it seems the crank and flywheel may contribute to the wobble.  Maybe it is the dowels on the new crank that are doing it?

[Jee Ent.]

What have you done to that flywheel to get it that hot.. Welded it? When thick steel gets as blue as that, there will be distorsion for sure. Remachine the whole thing or leave it to the recyclers.

The O-ring could also mess up your readings.. if you have it in there. The non vw cranks has to little radius on the end for the O-ring to slide over. And sometimes some of the rubber gets in between the crank and the flywheel.

[andy198712]

do you have any small burrs or raises on the end of your crank on the mating surface with the flywheel?

[Dougy Dee]

Do you get the same anomaly when you remove the dowels?
I believe the SCAT dowels are actually a true 5/16 s opposed to VW 8mm.
Not a lot of difference but...

[1168]

i had the same problem on a 74mm crank, i swapped, checked different fly wheels etc. the problem the chamfer on the end of the crank had it turned on a lathe to vw spec. problem solved .

[Phil West]

What have you done to that flywheel to get it that hot.. Welded it? When thick steel gets as blue as that, there will be distorsion for sure. Remachine the whole thing or leave it to the recyclers.

The O-ring could also mess up your readings.. if you have it in there. The non vw cranks has to little radius on the end for the O-ring to slide over. And sometimes some of the rubber gets in between the crank and the flywheel.

Hi, actually that was the fourth and last flywheel I tried, just an old one which I wanted to get some more test readings from.  The other flywheels aren't blue like that.  None of the flywheels had orings in them for the testing.

[Phil West]

Dowels mic up at 0.3146" so 7.99mm which would suggest these are 8mm rather than 5/16" at 7.93mm.  This crank came with the dowels installed already.  From past experiences getting the dowels out from cranks has not been easy - as a minimum the dowels will be damaged and there's always the chance of damaging the crank dowel holes.  They are really tight!  So I didn't want to start trying to get all the dowels out - unless there's a trick to it that makes things easy.  However I've got another Scat crank that came without the dowels installed so I'll try measuring the flywheels on that crank when I get a chance.

The crank mating surface itself looks really good - nice and smooth, flat, no evidence of any burrs.

[69Stu]

I have just had this issue with a new crank and flywheel. With it all bolted up, and dowel pins in place there was approximately 20 thou of runout on the flywheel. I also found the same issue with loose and tight spots between the bearing thrust face and the nose of the flywheel.
My machine shop found that by bolting the flywheel to the crank with the dowel pins removed the runout went away.
The conclusion was that the dowel pins in the end of the crank weren't in square.
All of the dowel pins holes in the crank were redrilled and replaced with 11/32 oversize pins.
Problem sorted.

[Phil West]

Thanks for the input.  I used 400 wet/dry to polish the crank, and each dowel pin.  Then I mic'd the dowel pins and flywheel pin holes, although the holes were a tiny bit larger than the pins I used an 8.1mm drill to slightly enlarge the holes.  After which I used the 400 to polish the flywheel mating surfaces and pin holes.  When I reinstalled the flywheel, hey presto, still a nice tight fit but there was no remaining difference when I used the feeler gauges (well, maybe a thou diff. all the way around).  So it looked like the flywheel was going on ok.  I took the whole lot back to the balancer and he had no issues at all, he was able to balance the assembly easily - no more wobble.
Cheers
Phil