So I got a Robin Mather...

1. New steerer to replaced the threaded one on the fork to accept a threadless set-up.
   
   
   
   

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i'd either look at those bonti bars or an adapter in the short term, and then~~ if you like it and want to keep it, I'd probably drop him (or another builder) an email and see if you can covert the forks to an ahead. TBH there looks like there's enough space with all those spacers just to clamp around the threaded part with a shim.

Great minds, etc.

But seriously replacing the steerer tube is surely the only sensible option. It's a lovely frame made by an amazing builder that it sounds like you want to keep.

As neat an idea as having a custom built quill stem is, how much is it likely to cost after painting/chroming? £50? £100? £150? And then what if you want to change something later?