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I'm looking to attach a front mudguard to my Cannondale fat bike and could do with some kind replacement that would function as a star fangled nut, some kind of expanding bung. The internal diameter measures about 34.7mm. Does anyone have any ideas what would work.
The Topeak Defender fits with a bung in the bottom of the steerer, is that what you mean?

Thanks fudge, think those SKS bungs will work, I've ordered some up. It might be worth checking the shed Monty I'm sure I've got the remains of one of those Topeak Defender out there.
I had a shock blade on my lefty bike and the different sized bungs provided with the mudguard were not large enough. I ended up taking the largest that came with the kit and using electrical tape on the inside of the steerer tube to pad it out so the bung could be used. Worked like a charm for about 3 years, no issues at all. And it survived some action on DH trails.