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I'm trying to find a front fork. My front axle bolt measures 3/8. The closest forks seem to have a 9mm dropout which is .525mm too small. Should I file down the dropouts to fit the extra .525mm or keep searching for the unicorn, a fork with 3/8 dropout

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Most nominally 9mm forks fit 3/8" without issue. There are exceptions, usually road forks, where it fits a 9mm too snug for a 3/8 to go in. Usually the difference won't be that full .525mm though.

Filing it to fit is always going to be an at-your-own-risk judgment call, but generally it is totally fine on a steel or aluminum dropout. When I do it I'm careful to count strokes, distribute the strokes evenly, and try to apply even pressure, with the goal of altering the dropout and fork alignment as little as possible, and keeping everything symmetrical. It's important to understand that you really want to avoid inadvertently making one dropout deeper than the other one, because the fork can't really be aligned properly if that happens. It's still a good idea to put dropout alignment tools on afterwards, but if you don't have them, just be careful with how you do it and go slow, particularly on aluminum dropouts. I wouldn't tend to recommend doing any such thing to carbon dropouts.

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Your hub has 3/8" (9.52mm) solid axles and most forks are set up for quick releases (9mm). There are some 10mm quick release dropouts as well as 12mm, but they seem to be exclusive to rear dropouts.

The easiest solution would be to get a quick release hub. A new wheel is cheaper than most new forks. A used wheel can often be gotten for free or close to free from a bicycle coop.

If you insist on using 9mm forks with your 3/8 axles, then you really want an engineer to look at your dropouts to tell you if you can dremel out 0.52mm of material in a safe enough way. If your fork tabs are flat BMX style tabs, it might be safe, but most forks these days are cast. I personally wouldn't risk it.

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    0.5mm is not exactly a lot of material to remove. OP could try removing the paint in the dropout and see if that leaves enough space. I've never found a wheel dropout that doesn't fit both 9mm QR and normal bold-up nut axles. – Criggie Jul 24 '17 at 4:44

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