Just a general question directed to the people that know : I was checking the valve clearances on a '67 250 Scrambler (springer) earlier today, and found that shims from my 450 were loose (side to side) on the top of the valve on the 250.

So, does that mean that 250 valve stems are narrower than those on the 450, or has somebody slipped in some non-standard parts on me I thought all the singles had the same valve stem diameters...

All the 2 valve bikes 250cc & bigger (bevel & belt) use valves with 8mm stems. The 160's had 7mm stems, and I don't know what the 200's, 175's, and earlier bikes used. Is it possible a previous owner installed valves with 7mm stems in your 250? There should be room to get a vernier caliper on the stem to check the dia.
Good Luck, Rocket

Rocket, it turned out that my (lack of) sense of 'feel' was causing the concern

The valves on this bike had wide chamfers on the valve tips, and I was actually rocking the thin shims on the tops of the valves instead of just doing side-to-side movement - my bad

The valve stems are 8mm as you described Ended up having to shave the exhaust shim from .063" down to .052" to get the proper clearance - not much left on that one

Thanks for the pointers