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I have a fork with a 1" steerer tube, and a frame with a 1.5" head tube.

The steerer tube is slightly short, and when inserted into the frame, is flush with the top of the head tube. The fork is threaded.

My question: Is it possible to extend and adapt this fork to threadless, and what kind of headset (reducing, normal?) would I need? (the frame I bought said the headset size was 1 1/8 traditional press-in)

Or is all this just stupid, and I should just buy a new fork with a 1 1/8 steerer?

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Great! Thanks for both of your answers. I've put in an order for a new fork. I figure all those modifications were beating around the bush. Safety first. Thanks! – Knarf Jan 6 '13 at 20:34
up vote 8 down vote accepted

I would get a fork with a steerer that fits the frame. The expense and risk of modifying your existing fork won't be worth it. There were some mountain bike forks with replacement steerer tubes but they didn't allow for changes of size or type of steerer. They were 1 1/8" threadless and so was the replacement. You also want to match the fork rake/angle that the frame maker designed into the frame.

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Short answer, buy a new fork that fits.

Long answer, you are not able to safely extend the length of the fork, and trying will result in injury. There are headsets available to deal with the size difference on the steerer tube, but if the steerer is that short, you won't be able to mount a stem and handlebar.

In addition, if it is a threaded 1" fork, the threaded portion of the steerer would not be strong enough to support a threadless stem design. The clamping force from the stem will crack the steerer through one of the cut threads.

So, buy a new fork that fits. :)

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