Bought an old on one hardtail frame and building it up. Is my first build an slightly unsure on what measurements I need to know when buying a set of cranks. Also unsure on how to get the correct chain line as there are spacers in the bottom bracket 2 one side 1 the other and again have no idea why. Any help greatly appreciated

So far as the spacers are concerned, follow the instructions that come with the BB. What you need know for picking a BB are how wide the shell is (this is easily found by flipping the frame upside down and putting a ruler across the bottom of the BB and measuring the width of the metal. That’s going to be 68 or 73mm.

Only other things you need to know are what cranks you plan to fit. Mostly unless it’s SRAM/Truvativ, Campag or any of the press fit stuff you want a standard Shimano HT2 (hollowtech2) BB which will be Threaded with an English Thread (describes pitch and handedness). There are French and Italian thread standards but you’ll almost never see them on a MTB and relatively rarely on road now either.

Thanks for reply mate. The frame come with external bb fitted. Are like shimano cranks just a standard size then? Seen this boost stuff which just totally confused me. How do I get correct chain line also as there are 3 spaces inside the bb, 2 drive side 1 the other

Assuming the previous owner had it right, just fit any hollow tech 2 chainset

Leave the spacers as they? are.

It must be a 68mm BB shell. If it was a 73mm shell you would only use one spacer and that would be on the driveside.

That spacer can be removed to fit a chain device.

Don’t worry about boost that’s a relatively new standard, can’t imagine an old On One frame using it.

Cheers guys. Just trying to understand what’s going on really in terms of widths ect. Measured and the bb shell is 68cm like you said. Do I keep the spacers as they are then or do all the spacers go drive side? Do I need to worry about chain line or is that adjusted using the derailleur high and low screws?

So if I was to buy a shimano 3 speed or 2 speed crankset and convert to single ring would this work ok?

68cm like you said

I hope not!!
Converting to single ring will be fine. You can use the spacers to get the chain line right. Keep the old ones just in case.

I’d recommend you leave the spacers as they are, if you do move them watch for the crank/chainstay clearance.

.So if I was to buy a shimano 3 speed or 2 speed crankset and convert to single ring would this work ok?

Yes