Current bike has a 1" steerer tube with a shim to allow use of a 1 1/8" stem (currently installed).

50mm worth of 1" spacers below the stem.

1 1/8" top cap.

Once I find my sweet spot I will have the steerer tube cut but for now if I want to lower the bars by moving spacers from below to above the stem, should I use the original 1" top cap or the current 1 1/8" top cap?

Just trying to make sure my bars don't come flying off at an inopportune moment!

Thanks in advance!

The top cap has nothing to do with keeping the stem attached to the steerer, that is what the stem bolts do. The top cap is only used to apply preload to the headset; once the adjustment is made and the stem bolts tightened the top cap serves no mechanical purpose and can in fact be removed.

With 1" spacers above the stem use the 1" top cap. BTW, the stem's steerer clamp is what keeps the stem and bars firmly in place. If you tighten it properly, the bars won't come "flying off" no matter which top cap you use.

Thanks guys - that answers my question!

threadless 1" I presume?

Yep, threadless.