need a short stack height headset so has to be crank bros. which one should i get? mainly do singletrack, trails, xc and all mountain riding, no downhilling or big drops

Do you really need 21mm stack height? CB is the lowest but they aren't cheap and aren't reliable imho. FSA Orbit XL2 is 27.5mm iirc and only £20…still leaves the cash for a low height stem like some of the FSA stems.

pretty sure. steerer form base to top is 185mm, headtube is 125mm which leaves me just over 60mm to play with, any alternative suggestions as to the way to go with it?

Up to you as long as your measurements are accurate as it's close…most stems are 40mm stack height (like Thomson) so that leaves 20mm, given you need a couple of mm gap between top of steerer and top cap that would be fine with a CB headset. Equally go with a 35mm stack height stem (like some FSA), leaves 25mm so with the gap between steerer and top cap an XL2 or similar height headset would fit.

Oh and FWIW I have an iodine C headset…fitted last autumn and it now has surface rust on it and the lower bearings don't feel that smooth.

thanx for that, been trying to rack my brains to work this out, low stack stem it is then 🙂