Does the crown race need to seat flush with the shoulder of the crown when installed?
I have a new carbon fork (steel steerer) and installed a Chris King crown race on it. The race seated on the fork perfectly straight, but there is an even 1mm or less gap between the bottom of the race and the top of the crown.
Does this fork need to be machined so that the race is seated flush with the crown? or is it ok as is.
Uh, it sounds like you didn't seat the race all the way. If you can't do it yourself, take it to a LBS and have them do it. Otherwise if you install it as is, you may ruin the machined surface on the fork.
I concur with Nick. The crown race seat should have been machined by the fork manufacturer. If it is not seated properly, the headest will eventually loosen and you'll damage the bearing surfaces through Brinelling. Unless you have a proper driver for installation you can damage the crown race seat itself.
However, the drivers are simply enough to make. All you need is length of heavy lead or brass pipe that is just large enough to slip over the steerer column, but not the top edge of the crown race. You could use steel pipe, but a material softer than the crown race is preferred to avoid damaging the crown race. The end of the driver used to set the crown race MUST be cut square. Mark the square cut end so that you always use the proper end for seating the crown race.
When installing the crown race, ensure the fork is supported under the crown. The crown race can be seated by hand if the driver is heavy enough, or it can be done using a hammer, if the driver is longer than the steerer column.
One last item of caution. There are several diameters of crown race and crown race seats. To work properly, the crown race should be an interference fit of 0.1 - 0.2 mm (i.e. the inner diameter of the crown race should be 0.1 - 0.2 mm smaller than the diameter of the crown seat). If the crown race is too small for the crown seat, it can split when being driven and/or you will damage the crown seat. Too big and you'll have a loose heaset that you cannot tighten and will wear out quickly.
IF YOU ARE IN DOUBT OF ANYTHING, TAKE IT TO YOUR LBS!!! IF THEY SCREW UP, IT COMES OUT OF THEIR POCKET, NOT YOURS.
Other than the obvious difference between a 1" and a 11/8" steerer /HS, the major incompatibility is trying to put a 1" ISO 26.4mm CR on a 1" 27.0 JIS crown.
Sidney's statement is true of modern headsets, however, if you are working on vintage equipment there were other standards. French bicycles tended to use 26.5mm and Austrian used 26.7mm. Also, Campagnolo produced 0.5mm undersize versions of their crown races for worn or damaged crown race seats.
Sidney, I have never had a problem with damaging a carbon fibre fork while seating a crown race. You don't have to use much force with the hammer. It's more like light taps than wailing away. However, if you have a better method for seating crown races on carbon fibre forks, I'd like to hear about. That's one of the purposes of this forum, learning new and better ways to do things.