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If you are replacing a like-for-like chainring, choosing the chainring should be relatively straight-forward. If you are aiming to match chainrings for a specific crank, chainrings have a couple of things to consider. Firsly the number of 'arms' on the right hand crank arm usually 5 or 4. The bolt circle diameter (BCD) is the measurement of a circle diameter which passes through the centre of all 4 or 5 bolt holes. If you are unsure google your bike's crank BCD.

The number of teeth on the chainring also needs a little thinking about, particularly if you are running 1X transmission.

No. Chainging from 2X to 1X, using an existing 2X or 3X chainring will result in continually oily fingers! 1X chainrings alternate between narrow and wide teeth. These alternating teeth fit into the narrow and wide (alternating) openings in the chain - providing much more contact, securing the chain onto the chainring, even on the largest or smallest sprocket.

When converting to a 1X set up, you will also need shorter (1X or singlespeed) chainring bolts. 

Yes, a smaller chainring will help. It might be more cost effective to replace your bike's cassette instead. Check your rear derailleur's maximum sprocket size. For example, changing from a 25 sprocket to a 28 biggest sprocket with a 39 chainring would give a similar gear to 34 X 25.