There's some discussion on the shift fork in this thread, including a suggestion from the Doc to use a modified 3 synchro fork as they are more readily available. That's what was done to my 4 synchro box rebuild by a very reputable Mini mechanic.

viewtopic.php?f=2&t=96523

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ex-NSW Police 1970 MK II Cooper S
VMCI #43

I've heard of using a modified three synchro, did they modify a brand new fork or a good used one?
I have a few 3-sync gearboxes, thinking I'll pull some apart and see how the forks are for wear and how they fit

At the time I believe NOS or new repro were readily available so preferable of course. Don't know about now. Maybe search the usual suppliers.

_________________
ex-NSW Police 1970 MK II Cooper S
VMCI #43

I stripped two 3 synchro gearboxes on the weekend, and both forks were better than the 4 syncro one.
I had a look at how to modify the fork to clear the case and reverse selector, and this the seems like the way to go

I have a NOS fork if you need one.

I'll message you.
What does the new one measure inside? My best used fork is around 11.9mm where it runs on the sides of the gear. I Think the gear is around 11.5mm
I'm at work so can't double check now.

Mine is 11.5mm