How to adjust fork and crown race?

Question

When I removed the fork from my Marin Four Corners 2018 I discovered the headset is not as specified. Among other problem, the crown race is some noname piece not fitting any bearing (not the one which was installed, and not the one which should be installed). The only thing it fits is a fork.

And here lies the problem, the crown race seat (on fork) has diameter 30.2mm instead of 30.1mm. The installed crown race has internal diameter 30.0 mm. Those dimensions are non-standard and so when I install the new crown race (the one it should be installed according to spec) I cannot do this because its ID is 29.9mm -- the difference is too big.

So how to adjust one to the other? One approach would be to buy a tool like Park Tool CRC-1 and strip crown race seat, but for home usage it means one time job so it is rather expensive way. The other would be to strip crown race (no perfect, because I would be altering the well made piece) but how to do it in reliable way? I.e. I don't want to scrap metal in one place, but do it evenly.

I prefer to do it myself, but I know the popular answers for bike problems is "take it to your LBS". Out of curiosity and because of tool prices I checked this option, and my LBS does not have such tools as well. They strip from crown race.

Answer 1

Of the numbers you give, the 30.2 doesn't really stand out. A lot of crown race seats measure about that. An interference fit of 0.1mm at the crown race is seen as about optimal to avoid distorting the part and make installation go easy, but the number 0.2mm for the cups is common and I think some manufacturers just extend "0.2 = right" to the whole thing, or have a tolerance range of 0.1-0.2.

Getting 29.9mm for the race is uncommon and that number does stand out a little as being small.

The only fully recommendable approach to where you're at here is get the fork properly milled down to 30.1.

Another approach: if this a headset that, as many do now, takes a more or less universal integrated headset replacement bearing with a 36 or 45 degree contact face, you could put a different one in its place. They're not very expensive and pretty much the only thing that differentiates crown races of this type is the sealing and whether they have a split. If your crown seat is tending to making things tight, you could get a split one and have it go on by hand.

If you just needed to make what you have work, you could try carefully sanding out the 0.1mm from the race. That will go pretty fast on aluminum. Use a fine grit to slow down the material removal and make it harder to wind up with peaks and valleys, and slowly rotate it with even pressure. If you have it, you could then install it with high-strength loctite, which will fill any valleys that do get created and move it back into alignment.

Personally I wouldn't recommend doing it freehand on a fork, especially if it's steel. The stakes are a little too high if things went wrong, and they could.