How to measure headset bearings

[αdαmsκι]

I would like to replace the bearings in a headset.  A bit of a search through the yacf vault gave me this:

Headsets are more likely to be 5/32 or 3/16.  3/32 is tiny, and normally only found in Shimano SPD pedals.

Can I replace the current bearings with either a set of 5/32'' or 3/16'' bearings or do they need to be the same size as the ones being replace?  And if I do need to replace like for like, how do I easily tell if they are 5/32'' or 3/16''?  Or is it a case of trying to measure a bearing with some decent callipers and watching them get fired across the room as I screw up?

Ta.

[bikenerd]

Take the old bearings to a (good) LBS.  Say "I'd like a packet of these please" and they'll produce them from under the counter and charge you £1.50 or so.
The loose ball headsets I have are 1/8", so it looks like the bearing size can vary quite a bit.

[αdαmsκι]

Take the old bearings to a (good) LBS.  Say "I'd like a packet of these please" and they'll produce them from under the counter and charge you £1.50 or so.

Fair enough 

[Biggsy]

You must replace with the same size.

Measure with calipers or buy a little guage with holes with it.

Insist on grade 100, or preferably grade 25.  Balls of a lower grade (higher grade number) are not so smooth and uniform in size.

[rogerzilla]

I can tell by looking 

I serviced some SPDs the other week and 3/32" bearings are the Devil's work.