[Manolito]

The O-ring from a fuel injector that I tried was just a bit too fat. I could not press the sensor back down in the hole.

I think have searched every size chart on the web. Standard inch-unit sizes will not fit, not even the military or aerospace specs.

That leaves us with metric O-rings.

I think that the original was a M3x8
The Bosch fuel injector O-ring is an M3.5x8

So the choice is -
1) Buy M3x8 and replace it if/when it shrinks like the original did.
2) Find a very soft (softer than #70) M3.5x8 and force it in.
3) Wrap Teflon tape around the original O-ring.

I have not yet found anyone who sells O-rings in small numbers retail who stocks either of the above sizes.
O-ring selections that are available in kits don't include either if these sizes.

I have sent out a few emails, but for a small lot like I would be buying, I doubt that any vendors will respond.

Most O-ring compounds are compatible with antifreeze. You just need one that is still pliable at sub-zero temperatures and will tolerate 220F or so. Buna N or Neoprene should be inexpensive and work fine.

IF I find one, I won't post a part number until I have actually tried it and confirmed that it works.