I haven't done it personally, but I am in the same boat as you and wanted to extend my short threaded steerer into a ahead one.
Theoretically this product exists: http://bbbcycling.com/bike-parts/headset-parts/BHP-21 which can be had on wiggle for around $20.
Which gives you something like this:
The good thing about this is that internal bolt you see is threaded, so you can preload the stem with a top cap. If it was me, I would make sure the lower stem bolt is clamping onto the original steerer at least.
I've been using an adapter like this for years and still have all my teeth. Really good and easy solution in my opinion.
@b3ans12345 started
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I haven't done it personally, but I am in the same boat as you and wanted to extend my short threaded steerer into a ahead one.
Theoretically this product exists:
http://bbbcycling.com/bike-parts/headset-parts/BHP-21 which can be had on wiggle for around $20.
Which gives you something like this:
The good thing about this is that internal bolt you see is threaded, so you can preload the stem with a top cap. If it was me, I would make sure the lower stem bolt is clamping onto the original steerer at least.