General question, taking an older bike 1x and trying to keep cost down rather than the usual parts fest.

Is there a huge difference between reusing an old double (675) with a ring on the outer and a proper 1x crank like a direct mount, from a view of chain running?

Just different chain lines I believe. I converted a 2×10 into a 1×10 with a double SRAM S1000 crank. Worked ok until someone put the rear mech in the rear wheel. So in the end we upgraded to 1×11 with a 1x crank and I think it just works that little bit better.

You’ll want to space the chainring in a bit. Chainline will be around 52mm whereas cassette is centred on about 44mm. See https://int.oneupcomponents.com/pages/understanding-chainline-for-optimal-1x-conversions-boost-and-non-boost.

Ta! I was just wondering out loud, I’ve run triples as singles in the past but never used a double that way and wondered if the ring might end up too far out. Just wondering out loud really while considering whether to buy a ring or go the whole hog. I’ll give it a go I think.

I’d space it in but it’ll work without it, the difference in chainline really isn’t that much

Direct mount’s an advantage in that you can fit a small ring mind.