I'm no expert but when I made my radial, I first drilled & reamed the valve guide hole so it became the 'standard' to which the valve stem was lapped for proper sliding fit because it gets turned in a lathe anyway. The laps are not sophisticated, just 2 pieces of metal tacked together before drilled & reamed for lapping hole. You could tack bond them with paper spacer to make the clearance gap before drilling, or just glue them & then sand the faces a bit after drilling/separating. I have not used titanium but I would think diamond paste (cheap stuff from Aliexpress) would cut it? I highly recommend rigging up a vacuum tester to test the fit of valve / seat for leaks.
I really good instrument for lapping work like is an indicating micrometer (stock photo but mine is similar). You can quickly take measurement up & down the shaft to accurately hit target dimension & adjust lapping accordingly. Its a feel thing. The mic is repeatable & easier to interpret vs a digital with lots of 0.000000 in display. I bought mine used quite inexpensively. There seem to be more metrics than imperial for some reason, but with the analog needle/range its really not a big deal. Be super diligent about cleaning any lapping compound before measuring, don't want that on your micrometer anvils.
View attachment 165888
