BMX IS42 Headtube Repair: Heat sink + Post-Weld Reaming


A friend hit me up for a repair. Top tube on one of his BMX frames cracked (4130). As a rule I don’t repair other people’s bikes but . . .he’s a good friend and I may or may not owe him a few big favors. But before I agree to get into this I want to address some question (I really don’t wanna spend $$ on tools I’m never going to use again).

The TT sits about 3-5mm below the bottom of the taper.

  1. Obviously I want to use a heat sink but do I need to fab something that sits where the bearings pop in?
  2. Post weld-reaming. (I think I already know the answer): Do I have to?
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I made some heatsinks when I made my wife’s bike with an IS headtube. I made them with the same contact angle and ID as the bearing seat to keep things a straight as possible. No post weld ream was required, not sure if its because the heatsinks fitted well or because of the distance from the weld to the bearing seat though.

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Thanks. This is helpful. I’m hoping I can get this done w/out investing time/$$ in tools.

If there are any others that have used IS headtubes I’d love your input.

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I’ve used a few is headtubes and it went well, I used a simple heatsink like @Pi_bikes on both ends and have had very minimal distortion. They sit on the cone surface of the bearing seat and are a tight fit on the ID. There’s a good chance you’ll be ok without a reamer especially when the tube is that far below the chamfer. @Neuhaus_Metalworks has a lot of experience with the is headtubes and I recall him mentioning that he usually doesn’t need to ream after welding when using a heat sink