Lucas Murray wrote:
> I'm putting together my Sieber track bike and need to
> know what size bearings my headset takes. Anybody know
> off the top of their head? Its this model.
>
> http://www.blackbirdsf.org/
I don't know the answer, but here's how I'd go about it if you don't get a definitive answer from a real Magistroni guru.
Nine out of 9 headsets use one of three sizes: 3/16", 5/32" or 1/8". (I know it might seem strange that an Italian part would use fractional-inch balls, but they really do, exceptions are exceedingly rare.)
Actually 3/16" is pretty unlikely for that old thing, so maybe you could skip 3/16", then you've narrowed it down to just 5/32" or 1/8". But here's the basic rule (assuming yours has cup/cone bearings -- the following is _not_ true for the v-groove headsets like the Stronglight V4 and some Italians):
With the headset cup clean, devoid of grease and grime, try one ball, the largest size first.
The curvature of the concave surface of the ball race in the cup must be larger than the ball, so the ball touches in only one (theoretical, mathematical) point. If the curve on the cup is larger as it should be, then the ball will be able to travel a tiny bit up and down the curve in the direction perpendicular to the race. If the ball is the same size as the curvature of the cup or larger, then it'll touch in two places and will only be able to move exactly in the direction of the race, like it's locked on a rail, no wiggle room in the perpendicular direction. You need the wiggle room.
If the ball is too large, go down to the next smaller size.
If the ball seems to fit the curvature of the cup properly with some wiggle, then load the race up with grease and a full complement of balls (as many as will fit), as long as that leaves a little extra room, like a fraction of a ball. If it completely fills it up with no extra room, take one ball out. It's safe to have one ball too few, and terrible awful to have one too many, so when in doubt, take one out.
Feel for roughness or fit problems. Roughness often means balls hitting somewhere they're not supposed to (balls too big), or cup and cone coming into contact with each other (balls too small). The bottom of the cup should come down low enough on the cone that there is only a very thin gap (where water and grit can get in). If the balls are too large it'll lift the cup up too high, and on some headsets that'll be noticeable as a larger than normal gap. Similarly, balls too small will let the cup come down too low over the cone, to where the cone looks recessed up into the cup.
Some experience with what a properly-assembled headset looks and feels like will go a long way to setting your mind at ease that you've got the right size.
Mark Bulgier
Seattle, WA USA